/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.hardware.camera2;

import android.annotation.RequiresPermission;
import android.annotation.NonNull;
import android.annotation.Nullable;
import android.content.Context;
import android.hardware.ICameraService;
import android.hardware.ICameraServiceListener;
import android.hardware.CameraInfo;
import android.hardware.camera2.impl.CameraMetadataNative;
import android.hardware.camera2.legacy.CameraDeviceUserShim;
import android.hardware.camera2.legacy.LegacyMetadataMapper;
import android.os.IBinder;
import android.os.Binder;
import android.os.DeadObjectException;
import android.os.Handler;
import android.os.Looper;
import android.os.RemoteException;
import android.os.ServiceManager;
import android.os.ServiceSpecificException;
import android.util.Log;
import android.util.ArrayMap;

import java.util.ArrayList;

/**
 * <p>A system service manager for detecting, characterizing, and connecting to
 * {@link CameraDevice CameraDevices}.</p>
 *
 * <p>You can get an instance of this class by calling
 * {@link android.content.Context#getSystemService(String) Context.getSystemService()}.</p>
 *
 * <pre>CameraManager manager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);</pre>
 *
 * <p>For more details about communicating with camera devices, read the Camera
 * developer guide or the {@link android.hardware.camera2 camera2}
 * package documentation.</p>
 */
public final class CameraManager {

    private static final String TAG = "CameraManager";
    private final boolean DEBUG = false;

    private static final int USE_CALLING_UID = -1;

    @SuppressWarnings("unused")
    private static final int API_VERSION_1 = 1;
    private static final int API_VERSION_2 = 2;

    private static final int CAMERA_TYPE_BACKWARD_COMPATIBLE = 0;
    private static final int CAMERA_TYPE_ALL = 1;

    private ArrayList<String> mDeviceIdList;

    private final Context mContext;
    private final Object mLock = new Object();

    /**
     * @hide
     */
    public CameraManager(Context context) {
        synchronized(mLock) {
            mContext = context;
        }
    }

    /**
     * Return the list of currently connected camera devices by identifier, including
     * cameras that may be in use by other camera API clients.
     *
     * <p>Non-removable cameras use integers starting at 0 for their
     * identifiers, while removable cameras have a unique identifier for each
     * individual device, even if they are the same model.</p>
     *
     * @return The list of currently connected camera devices.
     */
    @NonNull
    public String[] getCameraIdList() throws CameraAccessException {
        synchronized (mLock) {
            // ID list creation handles various known failures in device enumeration, so only
            // exceptions it'll throw are unexpected, and should be propagated upward.
            return getOrCreateDeviceIdListLocked().toArray(new String[0]);
        }
    }

    /**
     * Register a callback to be notified about camera device availability.
     *
     * <p>Registering the same callback again will replace the handler with the
     * new one provided.</p>
     *
     * <p>The first time a callback is registered, it is immediately called
     * with the availability status of all currently known camera devices.</p>
     *
     * <p>{@link AvailabilityCallback#onCameraUnavailable(String)} will be called whenever a camera
     * device is opened by any camera API client. As of API level 23, other camera API clients may
     * still be able to open such a camera device, evicting the existing client if they have higher
     * priority than the existing client of a camera device. See open() for more details.</p>
     *
     * <p>Since this callback will be registered with the camera service, remember to unregister it
     * once it is no longer needed; otherwise the callback will continue to receive events
     * indefinitely and it may prevent other resources from being released. Specifically, the
     * callbacks will be invoked independently of the general activity lifecycle and independently
     * of the state of individual CameraManager instances.</p>
     *
     * @param callback the new callback to send camera availability notices to
     * @param handler The handler on which the callback should be invoked, or {@code null} to use
     *             the current thread's {@link android.os.Looper looper}.
     *
     * @throws IllegalArgumentException if the handler is {@code null} but the current thread has
     *             no looper.
     */
    public void registerAvailabilityCallback(@NonNull AvailabilityCallback callback,
            @Nullable Handler handler) {
        if (handler == null) {
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalArgumentException(
                        "No handler given, and current thread has no looper!");
            }
            handler = new Handler(looper);
        }

        CameraManagerGlobal.get().registerAvailabilityCallback(callback, handler);
    }

    /**
     * Remove a previously-added callback; the callback will no longer receive connection and
     * disconnection callbacks.
     *
     * <p>Removing a callback that isn't registered has no effect.</p>
     *
     * @param callback The callback to remove from the notification list
     */
    public void unregisterAvailabilityCallback(@NonNull AvailabilityCallback callback) {
        CameraManagerGlobal.get().unregisterAvailabilityCallback(callback);
    }

    /**
     * Register a callback to be notified about torch mode status.
     *
     * <p>Registering the same callback again will replace the handler with the
     * new one provided.</p>
     *
     * <p>The first time a callback is registered, it is immediately called
     * with the torch mode status of all currently known camera devices with a flash unit.</p>
     *
     * <p>Since this callback will be registered with the camera service, remember to unregister it
     * once it is no longer needed; otherwise the callback will continue to receive events
     * indefinitely and it may prevent other resources from being released. Specifically, the
     * callbacks will be invoked independently of the general activity lifecycle and independently
     * of the state of individual CameraManager instances.</p>
     *
     * @param callback The new callback to send torch mode status to
     * @param handler The handler on which the callback should be invoked, or {@code null} to use
     *             the current thread's {@link android.os.Looper looper}.
     *
     * @throws IllegalArgumentException if the handler is {@code null} but the current thread has
     *             no looper.
     */
    public void registerTorchCallback(@NonNull TorchCallback callback, @Nullable Handler handler) {
        if (handler == null) {
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalArgumentException(
                        "No handler given, and current thread has no looper!");
            }
            handler = new Handler(looper);
        }
        CameraManagerGlobal.get().registerTorchCallback(callback, handler);
    }

    /**
     * Remove a previously-added callback; the callback will no longer receive torch mode status
     * callbacks.
     *
     * <p>Removing a callback that isn't registered has no effect.</p>
     *
     * @param callback The callback to remove from the notification list
     */
    public void unregisterTorchCallback(@NonNull TorchCallback callback) {
        CameraManagerGlobal.get().unregisterTorchCallback(callback);
    }

    /**
     * <p>Query the capabilities of a camera device. These capabilities are
     * immutable for a given camera.</p>
     *
     * @param cameraId The id of the camera device to query
     * @return The properties of the given camera
     *
     * @throws IllegalArgumentException if the cameraId does not match any
     *         known camera device.
     * @throws CameraAccessException if the camera device has been disconnected.
     *
     * @see #getCameraIdList
     * @see android.app.admin.DevicePolicyManager#setCameraDisabled
     */
    @NonNull
    public CameraCharacteristics getCameraCharacteristics(@NonNull String cameraId)
            throws CameraAccessException {
        CameraCharacteristics characteristics = null;

        synchronized (mLock) {
            if (!getOrCreateDeviceIdListLocked().contains(cameraId)) {
                throw new IllegalArgumentException(String.format("Camera id %s does not match any" +
                        " currently connected camera device", cameraId));
            }

            int id = Integer.parseInt(cameraId);

            /*
             * Get the camera characteristics from the camera service directly if it supports it,
             * otherwise get them from the legacy shim instead.
             */

            ICameraService cameraService = CameraManagerGlobal.get().getCameraService();
            if (cameraService == null) {
                throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                        "Camera service is currently unavailable");
            }
            try {
                if (!supportsCamera2ApiLocked(cameraId)) {
                    // Legacy backwards compatibility path; build static info from the camera
                    // parameters
                    String parameters = cameraService.getLegacyParameters(id);

                    CameraInfo info = cameraService.getCameraInfo(id);

                    characteristics = LegacyMetadataMapper.createCharacteristics(parameters, info);
                } else {
                    // Normal path: Get the camera characteristics directly from the camera service
                    CameraMetadataNative info = cameraService.getCameraCharacteristics(id);

                    characteristics = new CameraCharacteristics(info);
                }
            } catch (ServiceSpecificException e) {
                throwAsPublicException(e);
            } catch (RemoteException e) {
                // Camera service died - act as if the camera was disconnected
                throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                        "Camera service is currently unavailable", e);
            }
        }
        return characteristics;
    }

    /**
     * Helper for opening a connection to a camera with the given ID.
     *
     * @param cameraId The unique identifier of the camera device to open
     * @param callback The callback for the camera. Must not be null.
     * @param handler  The handler to invoke the callback on. Must not be null.
     *
     * @throws CameraAccessException if the camera is disabled by device policy,
     * too many camera devices are already open, or the cameraId does not match
     * any currently available camera device.
     *
     * @throws SecurityException if the application does not have permission to
     * access the camera
     * @throws IllegalArgumentException if callback or handler is null.
     * @return A handle to the newly-created camera device.
     *
     * @see #getCameraIdList
     * @see android.app.admin.DevicePolicyManager#setCameraDisabled
     */
    private CameraDevice openCameraDeviceUserAsync(String cameraId,
            CameraDevice.StateCallback callback, Handler handler)
            throws CameraAccessException {
        CameraCharacteristics characteristics = getCameraCharacteristics(cameraId);
        CameraDevice device = null;

        synchronized (mLock) {

            ICameraDeviceUser cameraUser = null;

            android.hardware.camera2.impl.CameraDeviceImpl deviceImpl =
                    new android.hardware.camera2.impl.CameraDeviceImpl(
                        cameraId,
                        callback,
                        handler,
                        characteristics);

            ICameraDeviceCallbacks callbacks = deviceImpl.getCallbacks();

            int id;
            try {
                id = Integer.parseInt(cameraId);
            } catch (NumberFormatException e) {
                throw new IllegalArgumentException("Expected cameraId to be numeric, but it was: "
                        + cameraId);
            }

            try {
                if (supportsCamera2ApiLocked(cameraId)) {
                    // Use cameraservice's cameradeviceclient implementation for HAL3.2+ devices
                    ICameraService cameraService = CameraManagerGlobal.get().getCameraService();
                    if (cameraService == null) {
                        throw new ServiceSpecificException(
                            ICameraService.ERROR_DISCONNECTED,
                            "Camera service is currently unavailable");
                    }
                    cameraUser = cameraService.connectDevice(callbacks, id,
                            mContext.getOpPackageName(), USE_CALLING_UID);
                } else {
                    // Use legacy camera implementation for HAL1 devices
                    Log.i(TAG, "Using legacy camera HAL.");
                    cameraUser = CameraDeviceUserShim.connectBinderShim(callbacks, id);
                }
            } catch (ServiceSpecificException e) {
                if (e.errorCode == ICameraService.ERROR_DEPRECATED_HAL) {
                    throw new AssertionError("Should've gone down the shim path");
                } else if (e.errorCode == ICameraService.ERROR_CAMERA_IN_USE ||
                        e.errorCode == ICameraService.ERROR_MAX_CAMERAS_IN_USE ||
                        e.errorCode == ICameraService.ERROR_DISABLED ||
                        e.errorCode == ICameraService.ERROR_DISCONNECTED ||
                        e.errorCode == ICameraService.ERROR_INVALID_OPERATION) {
                    // Received one of the known connection errors
                    // The remote camera device cannot be connected to, so
                    // set the local camera to the startup error state
                    deviceImpl.setRemoteFailure(e);

                    if (e.errorCode == ICameraService.ERROR_DISABLED ||
                            e.errorCode == ICameraService.ERROR_DISCONNECTED ||
                            e.errorCode == ICameraService.ERROR_CAMERA_IN_USE) {
                        // Per API docs, these failures call onError and throw
                        throwAsPublicException(e);
                    }
                } else {
                    // Unexpected failure - rethrow
                    throwAsPublicException(e);
                }
            } catch (RemoteException e) {
                // Camera service died - act as if it's a CAMERA_DISCONNECTED case
                ServiceSpecificException sse = new ServiceSpecificException(
                    ICameraService.ERROR_DISCONNECTED,
                    "Camera service is currently unavailable");
                deviceImpl.setRemoteFailure(sse);
                throwAsPublicException(sse);
            }

            // TODO: factor out callback to be non-nested, then move setter to constructor
            // For now, calling setRemoteDevice will fire initial
            // onOpened/onUnconfigured callbacks.
            // This function call may post onDisconnected and throw CAMERA_DISCONNECTED if
            // cameraUser dies during setup.
            deviceImpl.setRemoteDevice(cameraUser);
            device = deviceImpl;
        }

        return device;
    }

    /**
     * Open a connection to a camera with the given ID.
     *
     * <p>Use {@link #getCameraIdList} to get the list of available camera
     * devices. Note that even if an id is listed, open may fail if the device
     * is disconnected between the calls to {@link #getCameraIdList} and
     * {@link #openCamera}, or if a higher-priority camera API client begins using the
     * camera device.</p>
     *
     * <p>As of API level 23, devices for which the
     * {@link AvailabilityCallback#onCameraUnavailable(String)} callback has been called due to the
     * device being in use by a lower-priority, background camera API client can still potentially
     * be opened by calling this method when the calling camera API client has a higher priority
     * than the current camera API client using this device.  In general, if the top, foreground
     * activity is running within your application process, your process will be given the highest
     * priority when accessing the camera, and this method will succeed even if the camera device is
     * in use by another camera API client. Any lower-priority application that loses control of the
     * camera in this way will receive an
     * {@link android.hardware.camera2.CameraDevice.StateCallback#onDisconnected} callback.</p>
     *
     * <p>Once the camera is successfully opened, {@link CameraDevice.StateCallback#onOpened} will
     * be invoked with the newly opened {@link CameraDevice}. The camera device can then be set up
     * for operation by calling {@link CameraDevice#createCaptureSession} and
     * {@link CameraDevice#createCaptureRequest}</p>
     *
     * <!--
     * <p>Since the camera device will be opened asynchronously, any asynchronous operations done
     * on the returned CameraDevice instance will be queued up until the device startup has
     * completed and the callback's {@link CameraDevice.StateCallback#onOpened onOpened} method is
     * called. The pending operations are then processed in order.</p>
     * -->
     * <p>If the camera becomes disconnected during initialization
     * after this function call returns,
     * {@link CameraDevice.StateCallback#onDisconnected} with a
     * {@link CameraDevice} in the disconnected state (and
     * {@link CameraDevice.StateCallback#onOpened} will be skipped).</p>
     *
     * <p>If opening the camera device fails, then the device callback's
     * {@link CameraDevice.StateCallback#onError onError} method will be called, and subsequent
     * calls on the camera device will throw a {@link CameraAccessException}.</p>
     *
     * @param cameraId
     *             The unique identifier of the camera device to open
     * @param callback
     *             The callback which is invoked once the camera is opened
     * @param handler
     *             The handler on which the callback should be invoked, or
     *             {@code null} to use the current thread's {@link android.os.Looper looper}.
     *
     * @throws CameraAccessException if the camera is disabled by device policy,
     * has been disconnected, or is being used by a higher-priority camera API client.
     *
     * @throws IllegalArgumentException if cameraId or the callback was null,
     * or the cameraId does not match any currently or previously available
     * camera device.
     *
     * @throws SecurityException if the application does not have permission to
     * access the camera
     *
     * @see #getCameraIdList
     * @see android.app.admin.DevicePolicyManager#setCameraDisabled
     */
    @RequiresPermission(android.Manifest.permission.CAMERA)
    public void openCamera(@NonNull String cameraId,
            @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler)
            throws CameraAccessException {

        if (cameraId == null) {
            throw new IllegalArgumentException("cameraId was null");
        } else if (callback == null) {
            throw new IllegalArgumentException("callback was null");
        } else if (handler == null) {
            if (Looper.myLooper() != null) {
                handler = new Handler();
            } else {
                throw new IllegalArgumentException(
                        "Handler argument is null, but no looper exists in the calling thread");
            }
        }

        openCameraDeviceUserAsync(cameraId, callback, handler);
    }

    /**
     * Set the flash unit's torch mode of the camera of the given ID without opening the camera
     * device.
     *
     * <p>Use {@link #getCameraIdList} to get the list of available camera devices and use
     * {@link #getCameraCharacteristics} to check whether the camera device has a flash unit.
     * Note that even if a camera device has a flash unit, turning on the torch mode may fail
     * if the camera device or other camera resources needed to turn on the torch mode are in use.
     * </p>
     *
     * <p> If {@link #setTorchMode} is called to turn on or off the torch mode successfully,
     * {@link CameraManager.TorchCallback#onTorchModeChanged} will be invoked.
     * However, even if turning on the torch mode is successful, the application does not have the
     * exclusive ownership of the flash unit or the camera device. The torch mode will be turned
     * off and becomes unavailable when the camera device that the flash unit belongs to becomes
     * unavailable or when other camera resources to keep the torch on become unavailable (
     * {@link CameraManager.TorchCallback#onTorchModeUnavailable} will be invoked). Also,
     * other applications are free to call {@link #setTorchMode} to turn off the torch mode (
     * {@link CameraManager.TorchCallback#onTorchModeChanged} will be invoked). If the latest
     * application that turned on the torch mode exits, the torch mode will be turned off.
     *
     * @param cameraId
     *             The unique identifier of the camera device that the flash unit belongs to.
     * @param enabled
     *             The desired state of the torch mode for the target camera device. Set to
     *             {@code true} to turn on the torch mode. Set to {@code false} to turn off the
     *             torch mode.
     *
     * @throws CameraAccessException if it failed to access the flash unit.
     *             {@link CameraAccessException#CAMERA_IN_USE} will be thrown if the camera device
     *             is in use. {@link CameraAccessException#MAX_CAMERAS_IN_USE} will be thrown if
     *             other camera resources needed to turn on the torch mode are in use.
     *             {@link CameraAccessException#CAMERA_DISCONNECTED} will be thrown if camera
     *             service is not available.
     *
     * @throws IllegalArgumentException if cameraId was null, cameraId doesn't match any currently
     *             or previously available camera device, or the camera device doesn't have a
     *             flash unit.
     */
    public void setTorchMode(@NonNull String cameraId, boolean enabled)
            throws CameraAccessException {
        CameraManagerGlobal.get().setTorchMode(cameraId, enabled);
    }

    /**
     * A callback for camera devices becoming available or unavailable to open.
     *
     * <p>Cameras become available when they are no longer in use, or when a new
     * removable camera is connected. They become unavailable when some
     * application or service starts using a camera, or when a removable camera
     * is disconnected.</p>
     *
     * <p>Extend this callback and pass an instance of the subclass to
     * {@link CameraManager#registerAvailabilityCallback} to be notified of such availability
     * changes.</p>
     *
     * @see #registerAvailabilityCallback
     */
    public static abstract class AvailabilityCallback {

        /**
         * A new camera has become available to use.
         *
         * <p>The default implementation of this method does nothing.</p>
         *
         * @param cameraId The unique identifier of the new camera.
         */
        public void onCameraAvailable(@NonNull String cameraId) {
            // default empty implementation
        }

        /**
         * A previously-available camera has become unavailable for use.
         *
         * <p>If an application had an active CameraDevice instance for the
         * now-disconnected camera, that application will receive a
         * {@link CameraDevice.StateCallback#onDisconnected disconnection error}.</p>
         *
         * <p>The default implementation of this method does nothing.</p>
         *
         * @param cameraId The unique identifier of the disconnected camera.
         */
        public void onCameraUnavailable(@NonNull String cameraId) {
            // default empty implementation
        }
    }

    /**
     * A callback for camera flash torch modes becoming unavailable, disabled, or enabled.
     *
     * <p>The torch mode becomes unavailable when the camera device it belongs to becomes
     * unavailable or other camera resources it needs become busy due to other higher priority
     * camera activities. The torch mode becomes disabled when it was turned off or when the camera
     * device it belongs to is no longer in use and other camera resources it needs are no longer
     * busy. A camera's torch mode is turned off when an application calls {@link #setTorchMode} to
     * turn off the camera's torch mode, or when an application turns on another camera's torch mode
     * if keeping multiple torch modes on simultaneously is not supported. The torch mode becomes
     * enabled when it is turned on via {@link #setTorchMode}.</p>
     *
     * <p>The torch mode is available to set via {@link #setTorchMode} only when it's in a disabled
     * or enabled state.</p>
     *
     * <p>Extend this callback and pass an instance of the subclass to
     * {@link CameraManager#registerTorchCallback} to be notified of such status changes.
     * </p>
     *
     * @see #registerTorchCallback
     */
    public static abstract class TorchCallback {
        /**
         * A camera's torch mode has become unavailable to set via {@link #setTorchMode}.
         *
         * <p>If torch mode was previously turned on by calling {@link #setTorchMode}, it will be
         * turned off before {@link CameraManager.TorchCallback#onTorchModeUnavailable} is
         * invoked. {@link #setTorchMode} will fail until the torch mode has entered a disabled or
         * enabled state again.</p>
         *
         * <p>The default implementation of this method does nothing.</p>
         *
         * @param cameraId The unique identifier of the camera whose torch mode has become
         *                 unavailable.
         */
        public void onTorchModeUnavailable(@NonNull String cameraId) {
            // default empty implementation
        }

        /**
         * A camera's torch mode has become enabled or disabled and can be changed via
         * {@link #setTorchMode}.
         *
         * <p>The default implementation of this method does nothing.</p>
         *
         * @param cameraId The unique identifier of the camera whose torch mode has been changed.
         *
         * @param enabled The state that the torch mode of the camera has been changed to.
         *                {@code true} when the torch mode has become on and available to be turned
         *                off. {@code false} when the torch mode has becomes off and available to
         *                be turned on.
         */
        public void onTorchModeChanged(@NonNull String cameraId, boolean enabled) {
            // default empty implementation
        }
    }

    /**
     * Convert ServiceSpecificExceptions and Binder RemoteExceptions from camera binder interfaces
     * into the correct public exceptions.
     *
     * @hide
     */
    public static void throwAsPublicException(Throwable t) throws CameraAccessException {
        if (t instanceof ServiceSpecificException) {
            ServiceSpecificException e = (ServiceSpecificException) t;
            int reason = CameraAccessException.CAMERA_ERROR;
            switch(e.errorCode) {
                case ICameraService.ERROR_DISCONNECTED:
                    reason = CameraAccessException.CAMERA_DISCONNECTED;
                    break;
                case ICameraService.ERROR_DISABLED:
                    reason = CameraAccessException.CAMERA_DISABLED;
                    break;
                case ICameraService.ERROR_CAMERA_IN_USE:
                    reason = CameraAccessException.CAMERA_IN_USE;
                    break;
                case ICameraService.ERROR_MAX_CAMERAS_IN_USE:
                    reason = CameraAccessException.MAX_CAMERAS_IN_USE;
                    break;
                case ICameraService.ERROR_DEPRECATED_HAL:
                    reason = CameraAccessException.CAMERA_DEPRECATED_HAL;
                    break;
                case ICameraService.ERROR_ILLEGAL_ARGUMENT:
                case ICameraService.ERROR_ALREADY_EXISTS:
                    throw new IllegalArgumentException(e.getMessage(), e);
                case ICameraService.ERROR_PERMISSION_DENIED:
                    throw new SecurityException(e.getMessage(), e);
                case ICameraService.ERROR_TIMED_OUT:
                case ICameraService.ERROR_INVALID_OPERATION:
                default:
                    reason = CameraAccessException.CAMERA_ERROR;
            }
            throw new CameraAccessException(reason, e.getMessage(), e);
        } else if (t instanceof DeadObjectException) {
            throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                    "Camera service has died unexpectedly",
                    t);
        } else if (t instanceof RemoteException) {
            throw new UnsupportedOperationException("An unknown RemoteException was thrown" +
                    " which should never happen.", t);
        } else if (t instanceof RuntimeException) {
            RuntimeException e = (RuntimeException) t;
            throw e;
        }
    }

    /**
     * Return or create the list of currently connected camera devices.
     *
     * <p>In case of errors connecting to the camera service, will return an empty list.</p>
     */
    private ArrayList<String> getOrCreateDeviceIdListLocked() throws CameraAccessException {
        if (mDeviceIdList == null) {
            int numCameras = 0;
            ICameraService cameraService = CameraManagerGlobal.get().getCameraService();
            ArrayList<String> deviceIdList = new ArrayList<>();

            // If no camera service, then no devices
            if (cameraService == null) {
                return deviceIdList;
            }

            try {
                numCameras = cameraService.getNumberOfCameras(CAMERA_TYPE_ALL);
            } catch(ServiceSpecificException e) {
                throwAsPublicException(e);
            } catch (RemoteException e) {
                // camera service just died - if no camera service, then no devices
                return deviceIdList;
            }

            for (int i = 0; i < numCameras; ++i) {
                // Non-removable cameras use integers starting at 0 for their
                // identifiers
                boolean isDeviceSupported = false;
                try {
                    CameraMetadataNative info = cameraService.getCameraCharacteristics(i);
                    if (!info.isEmpty()) {
                        isDeviceSupported = true;
                    } else {
                        throw new AssertionError("Expected to get non-empty characteristics");
                    }
                } catch(ServiceSpecificException e) {
                    // DISCONNECTED means that the HAL reported an low-level error getting the
                    // device info; ILLEGAL_ARGUMENT means that this devices is not supported.
                    // Skip listing the device.  Other errors,
                    // propagate exception onward
                    if (e.errorCode != ICameraService.ERROR_DISCONNECTED ||
                            e.errorCode != ICameraService.ERROR_ILLEGAL_ARGUMENT) {
                        throwAsPublicException(e);
                    }
                } catch(RemoteException e) {
                    // Camera service died - no devices to list
                    deviceIdList.clear();
                    return deviceIdList;
                }

                if (isDeviceSupported) {
                    deviceIdList.add(String.valueOf(i));
                } else {
                    Log.w(TAG, "Error querying camera device " + i + " for listing.");
                }

            }
            mDeviceIdList = deviceIdList;
        }
        return mDeviceIdList;
    }

    /**
     * Queries the camera service if it supports the camera2 api directly, or needs a shim.
     *
     * @param cameraId a non-{@code null} camera identifier
     * @return {@code false} if the legacy shim needs to be used, {@code true} otherwise.
     */
    private boolean supportsCamera2ApiLocked(String cameraId) {
        return supportsCameraApiLocked(cameraId, API_VERSION_2);
    }

    /**
     * Queries the camera service if it supports a camera api directly, or needs a shim.
     *
     * @param cameraId a non-{@code null} camera identifier
     * @param apiVersion the version, i.e. {@code API_VERSION_1} or {@code API_VERSION_2}
     * @return {@code true} if connecting will work for that device version.
     */
    private boolean supportsCameraApiLocked(String cameraId, int apiVersion) {
        int id = Integer.parseInt(cameraId);

        /*
         * Possible return values:
         * - NO_ERROR => CameraX API is supported
         * - CAMERA_DEPRECATED_HAL => CameraX API is *not* supported (thrown as an exception)
         * - Remote exception => If the camera service died
         *
         * Anything else is an unexpected error we don't want to recover from.
         */
        try {
            ICameraService cameraService = CameraManagerGlobal.get().getCameraService();
            // If no camera service, no support
            if (cameraService == null) return false;

            return cameraService.supportsCameraApi(id, apiVersion);
        } catch (RemoteException e) {
            // Camera service is now down, no support for any API level
        }
        return false;
    }

    /**
     * A per-process global camera manager instance, to retain a connection to the camera service,
     * and to distribute camera availability notices to API-registered callbacks
     */
    private static final class CameraManagerGlobal extends ICameraServiceListener.Stub
            implements IBinder.DeathRecipient {

        private static final String TAG = "CameraManagerGlobal";
        private final boolean DEBUG = false;

        private final int CAMERA_SERVICE_RECONNECT_DELAY_MS = 1000;

        // Singleton instance
        private static final CameraManagerGlobal gCameraManager =
            new CameraManagerGlobal();

        /**
         * This must match the ICameraService definition
         */
        private static final String CAMERA_SERVICE_BINDER_NAME = "media.camera";

        // Camera ID -> Status map
        private final ArrayMap<String, Integer> mDeviceStatus = new ArrayMap<String, Integer>();

        // Registered availablility callbacks and their handlers
        private final ArrayMap<AvailabilityCallback, Handler> mCallbackMap =
            new ArrayMap<AvailabilityCallback, Handler>();

        // torch client binder to set the torch mode with.
        private Binder mTorchClientBinder = new Binder();

        // Camera ID -> Torch status map
        private final ArrayMap<String, Integer> mTorchStatus = new ArrayMap<String, Integer>();

        // Registered torch callbacks and their handlers
        private final ArrayMap<TorchCallback, Handler> mTorchCallbackMap =
                new ArrayMap<TorchCallback, Handler>();

        private final Object mLock = new Object();

        // Access only through getCameraService to deal with binder death
        private ICameraService mCameraService;

        // Singleton, don't allow construction
        private CameraManagerGlobal() {
        }

        public static CameraManagerGlobal get() {
            return gCameraManager;
        }

        @Override
        public IBinder asBinder() {
            return this;
        }

        /**
         * Return a best-effort ICameraService.
         *
         * <p>This will be null if the camera service is not currently available. If the camera
         * service has died since the last use of the camera service, will try to reconnect to the
         * service.</p>
         */
        public ICameraService getCameraService() {
            synchronized(mLock) {
                connectCameraServiceLocked();
                if (mCameraService == null) {
                    Log.e(TAG, "Camera service is unavailable");
                }
                return mCameraService;
            }
        }

        /**
         * Connect to the camera service if it's available, and set up listeners.
         * If the service is already connected, do nothing.
         *
         * <p>Sets mCameraService to a valid pointer or null if the connection does not succeed.</p>
         */
        private void connectCameraServiceLocked() {
            // Only reconnect if necessary
            if (mCameraService != null) return;

            Log.i(TAG, "Connecting to camera service");

            IBinder cameraServiceBinder = ServiceManager.getService(CAMERA_SERVICE_BINDER_NAME);
            if (cameraServiceBinder == null) {
                // Camera service is now down, leave mCameraService as null
                return;
            }
            try {
                cameraServiceBinder.linkToDeath(this, /*flags*/ 0);
            } catch (RemoteException e) {
                // Camera service is now down, leave mCameraService as null
                return;
            }

            ICameraService cameraService = ICameraService.Stub.asInterface(cameraServiceBinder);

            try {
                CameraMetadataNative.setupGlobalVendorTagDescriptor();
            } catch (ServiceSpecificException e) {
                handleRecoverableSetupErrors(e);
            }

            try {
                cameraService.addListener(this);
                mCameraService = cameraService;
            } catch(ServiceSpecificException e) {
                // Unexpected failure
                throw new IllegalStateException("Failed to register a camera service listener", e);
            } catch (RemoteException e) {
                // Camera service is now down, leave mCameraService as null
            }
        }

        public void setTorchMode(String cameraId, boolean enabled) throws CameraAccessException {
            synchronized(mLock) {

                if (cameraId == null) {
                    throw new IllegalArgumentException("cameraId was null");
                }

                ICameraService cameraService = getCameraService();
                if (cameraService == null) {
                    throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                        "Camera service is currently unavailable");
                }

                try {
                    cameraService.setTorchMode(cameraId, enabled, mTorchClientBinder);
                } catch(ServiceSpecificException e) {
                    throwAsPublicException(e);
                } catch (RemoteException e) {
                    throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                            "Camera service is currently unavailable");
                }
            }
        }

        private void handleRecoverableSetupErrors(ServiceSpecificException e) {
            switch (e.errorCode) {
                case ICameraService.ERROR_DISCONNECTED:
                    Log.w(TAG, e.getMessage());
                    break;
                default:
                    throw new IllegalStateException(e);
            }
        }

        private boolean isAvailable(int status) {
            switch (status) {
                case ICameraServiceListener.STATUS_PRESENT:
                    return true;
                default:
                    return false;
            }
        }

        private boolean validStatus(int status) {
            switch (status) {
                case ICameraServiceListener.STATUS_NOT_PRESENT:
                case ICameraServiceListener.STATUS_PRESENT:
                case ICameraServiceListener.STATUS_ENUMERATING:
                case ICameraServiceListener.STATUS_NOT_AVAILABLE:
                    return true;
                default:
                    return false;
            }
        }

        private boolean validTorchStatus(int status) {
            switch (status) {
                case ICameraServiceListener.TORCH_STATUS_NOT_AVAILABLE:
                case ICameraServiceListener.TORCH_STATUS_AVAILABLE_ON:
                case ICameraServiceListener.TORCH_STATUS_AVAILABLE_OFF:
                    return true;
                default:
                    return false;
            }
        }

        private void postSingleUpdate(final AvailabilityCallback callback, final Handler handler,
                final String id, final int status) {
            if (isAvailable(status)) {
                handler.post(
                    new Runnable() {
                        @Override
                        public void run() {
                            callback.onCameraAvailable(id);
                        }
                    });
            } else {
                handler.post(
                    new Runnable() {
                        @Override
                        public void run() {
                            callback.onCameraUnavailable(id);
                        }
                    });
            }
        }

        private void postSingleTorchUpdate(final TorchCallback callback, final Handler handler,
                final String id, final int status) {
            switch(status) {
                case ICameraServiceListener.TORCH_STATUS_AVAILABLE_ON:
                case ICameraServiceListener.TORCH_STATUS_AVAILABLE_OFF:
                    handler.post(
                            new Runnable() {
                                @Override
                                public void run() {
                                    callback.onTorchModeChanged(id, status ==
                                            ICameraServiceListener.TORCH_STATUS_AVAILABLE_ON);
                                }
                            });
                    break;
                default:
                    handler.post(
                            new Runnable() {
                                @Override
                                public void run() {
                                    callback.onTorchModeUnavailable(id);
                                }
                            });
                    break;
            }
        }

        /**
         * Send the state of all known cameras to the provided listener, to initialize
         * the listener's knowledge of camera state.
         */
        private void updateCallbackLocked(AvailabilityCallback callback, Handler handler) {
            for (int i = 0; i < mDeviceStatus.size(); i++) {
                String id = mDeviceStatus.keyAt(i);
                Integer status = mDeviceStatus.valueAt(i);
                postSingleUpdate(callback, handler, id, status);
            }
        }

        private void onStatusChangedLocked(int status, String id) {
            if (DEBUG) {
                Log.v(TAG,
                        String.format("Camera id %s has status changed to 0x%x", id, status));
            }

            if (!validStatus(status)) {
                Log.e(TAG, String.format("Ignoring invalid device %s status 0x%x", id,
                                status));
                return;
            }

            Integer oldStatus = mDeviceStatus.put(id, status);

            if (oldStatus != null && oldStatus == status) {
                if (DEBUG) {
                    Log.v(TAG, String.format(
                        "Device status changed to 0x%x, which is what it already was",
                        status));
                }
                return;
            }

            // TODO: consider abstracting out this state minimization + transition
            // into a separate
            // more easily testable class
            // i.e. (new State()).addState(STATE_AVAILABLE)
            //                   .addState(STATE_NOT_AVAILABLE)
            //                   .addTransition(STATUS_PRESENT, STATE_AVAILABLE),
            //                   .addTransition(STATUS_NOT_PRESENT, STATE_NOT_AVAILABLE)
            //                   .addTransition(STATUS_ENUMERATING, STATE_NOT_AVAILABLE);
            //                   .addTransition(STATUS_NOT_AVAILABLE, STATE_NOT_AVAILABLE);

            // Translate all the statuses to either 'available' or 'not available'
            //  available -> available         => no new update
            //  not available -> not available => no new update
            if (oldStatus != null && isAvailable(status) == isAvailable(oldStatus)) {
                if (DEBUG) {
                    Log.v(TAG,
                            String.format(
                                "Device status was previously available (%b), " +
                                " and is now again available (%b)" +
                                "so no new client visible update will be sent",
                                isAvailable(oldStatus), isAvailable(status)));
                }
                return;
            }

            final int callbackCount = mCallbackMap.size();
            for (int i = 0; i < callbackCount; i++) {
                Handler handler = mCallbackMap.valueAt(i);
                final AvailabilityCallback callback = mCallbackMap.keyAt(i);

                postSingleUpdate(callback, handler, id, status);
            }
        } // onStatusChangedLocked

        private void updateTorchCallbackLocked(TorchCallback callback, Handler handler) {
            for (int i = 0; i < mTorchStatus.size(); i++) {
                String id = mTorchStatus.keyAt(i);
                Integer status = mTorchStatus.valueAt(i);
                postSingleTorchUpdate(callback, handler, id, status);
            }
        }

        private void onTorchStatusChangedLocked(int status, String id) {
            if (DEBUG) {
                Log.v(TAG,
                        String.format("Camera id %s has torch status changed to 0x%x", id, status));
            }

            if (!validTorchStatus(status)) {
                Log.e(TAG, String.format("Ignoring invalid device %s torch status 0x%x", id,
                                status));
                return;
            }

            Integer oldStatus = mTorchStatus.put(id, status);
            if (oldStatus != null && oldStatus == status) {
                if (DEBUG) {
                    Log.v(TAG, String.format(
                        "Torch status changed to 0x%x, which is what it already was",
                        status));
                }
                return;
            }

            final int callbackCount = mTorchCallbackMap.size();
            for (int i = 0; i < callbackCount; i++) {
                final Handler handler = mTorchCallbackMap.valueAt(i);
                final TorchCallback callback = mTorchCallbackMap.keyAt(i);
                postSingleTorchUpdate(callback, handler, id, status);
            }
        } // onTorchStatusChangedLocked

        /**
         * Register a callback to be notified about camera device availability with the
         * global listener singleton.
         *
         * @param callback the new callback to send camera availability notices to
         * @param handler The handler on which the callback should be invoked. May not be null.
         */
        public void registerAvailabilityCallback(AvailabilityCallback callback, Handler handler) {
            synchronized (mLock) {
                connectCameraServiceLocked();

                Handler oldHandler = mCallbackMap.put(callback, handler);
                // For new callbacks, provide initial availability information
                if (oldHandler == null) {
                    updateCallbackLocked(callback, handler);
                }

                // If not connected to camera service, schedule a reconnect to camera service.
                if (mCameraService == null) {
                    scheduleCameraServiceReconnectionLocked();
                }
            }
        }

        /**
         * Remove a previously-added callback; the callback will no longer receive connection and
         * disconnection callbacks, and is no longer referenced by the global listener singleton.
         *
         * @param callback The callback to remove from the notification list
         */
        public void unregisterAvailabilityCallback(AvailabilityCallback callback) {
            synchronized (mLock) {
                mCallbackMap.remove(callback);
            }
        }

        public void registerTorchCallback(TorchCallback callback, Handler handler) {
            synchronized(mLock) {
                connectCameraServiceLocked();

                Handler oldHandler = mTorchCallbackMap.put(callback, handler);
                // For new callbacks, provide initial torch information
                if (oldHandler == null) {
                    updateTorchCallbackLocked(callback, handler);
                }

                // If not connected to camera service, schedule a reconnect to camera service.
                if (mCameraService == null) {
                    scheduleCameraServiceReconnectionLocked();
                }
            }
        }

        public void unregisterTorchCallback(TorchCallback callback) {
            synchronized(mLock) {
                mTorchCallbackMap.remove(callback);
            }
        }

        /**
         * Callback from camera service notifying the process about camera availability changes
         */
        @Override
        public void onStatusChanged(int status, int cameraId) throws RemoteException {
            synchronized(mLock) {
                onStatusChangedLocked(status, String.valueOf(cameraId));
            }
        }

        @Override
        public void onTorchStatusChanged(int status, String cameraId) throws RemoteException {
            synchronized (mLock) {
                onTorchStatusChangedLocked(status, cameraId);
            }
        }

        /**
         * Try to connect to camera service after some delay if any client registered camera
         * availability callback or torch status callback.
         */
        private void scheduleCameraServiceReconnectionLocked() {
            final Handler handler;

            if (mCallbackMap.size() > 0) {
                handler = mCallbackMap.valueAt(0);
            } else if (mTorchCallbackMap.size() > 0) {
                handler = mTorchCallbackMap.valueAt(0);
            } else {
                // Not necessary to reconnect camera service if no client registers a callback.
                return;
            }

            if (DEBUG) {
                Log.v(TAG, "Reconnecting Camera Service in " + CAMERA_SERVICE_RECONNECT_DELAY_MS +
                        " ms");
            }

            handler.postDelayed(
                    new Runnable() {
                        @Override
                        public void run() {
                            ICameraService cameraService = getCameraService();
                            if (cameraService == null) {
                                synchronized(mLock) {
                                    if (DEBUG) {
                                        Log.v(TAG, "Reconnecting Camera Service failed.");
                                    }
                                    scheduleCameraServiceReconnectionLocked();
                                }
                            }
                        }
                    },
                    CAMERA_SERVICE_RECONNECT_DELAY_MS);
        }

        /**
         * Listener for camera service death.
         *
         * <p>The camera service isn't supposed to die under any normal circumstances, but can be
         * turned off during debug, or crash due to bugs.  So detect that and null out the interface
         * object, so that the next calls to the manager can try to reconnect.</p>
         */
        public void binderDied() {
            synchronized(mLock) {
                // Only do this once per service death
                if (mCameraService == null) return;

                mCameraService = null;

                // Tell listeners that the cameras and torch modes are unavailable and schedule a
                // reconnection to camera service. When camera service is reconnected, the camera
                // and torch statuses will be updated.
                for (int i = 0; i < mDeviceStatus.size(); i++) {
                    String cameraId = mDeviceStatus.keyAt(i);
                    onStatusChangedLocked(ICameraServiceListener.STATUS_NOT_PRESENT, cameraId);
                }
                for (int i = 0; i < mTorchStatus.size(); i++) {
                    String cameraId = mTorchStatus.keyAt(i);
                    onTorchStatusChangedLocked(ICameraServiceListener.TORCH_STATUS_NOT_AVAILABLE,
                            cameraId);
                }

                scheduleCameraServiceReconnectionLocked();
            }
        }

    } // CameraManagerGlobal

} // CameraManager