This class encapsulates scrolling. You can use scrollers ({@link Scroller} * or {@link OverScroller}) to collect the data you need to produce a scrolling * animation—for example, in response to a fling gesture. Scrollers track * scroll offsets for you over time, but they don't automatically apply those * positions to your view. It's your responsibility to get and apply new * coordinates at a rate that will make the scrolling animation look smooth.
* *Here is a simple example:
* * private Scroller mScroller = new Scroller(context);
* ...
* public void zoomIn() {
* // Revert any animation currently in progress
* mScroller.forceFinished(true);
* // Start scrolling by providing a starting point and
* // the distance to travel
* mScroller.startScroll(0, 0, 100, 0);
* // Invalidate to request a redraw
* invalidate();
* }
*
* To track the changing positions of the x/y coordinates, use * {@link #computeScrollOffset}. The method returns a boolean to indicate * whether the scroller is finished. If it isn't, it means that a fling or * programmatic pan operation is still in progress. You can use this method to * find the current offsets of the x and y coordinates, for example:
* *if (mScroller.computeScrollOffset()) {
* // Get current x and y positions
* int currX = mScroller.getCurrX();
* int currY = mScroller.getCurrY();
* ...
* }
*/
public class Scroller {
private final Interpolator mInterpolator;
private int mMode;
private int mStartX;
private int mStartY;
private int mFinalX;
private int mFinalY;
private int mMinX;
private int mMaxX;
private int mMinY;
private int mMaxY;
private int mCurrX;
private int mCurrY;
private long mStartTime;
private int mDuration;
private float mDurationReciprocal;
private float mDeltaX;
private float mDeltaY;
private boolean mFinished;
private boolean mFlywheel;
private float mVelocity;
private float mCurrVelocity;
private int mDistance;
private float mFlingFriction = ViewConfiguration.getScrollFriction();
private static final int DEFAULT_DURATION = 250;
private static final int SCROLL_MODE = 0;
private static final int FLING_MODE = 1;
private static float DECELERATION_RATE = (float) (Math.log(0.78) / Math.log(0.9));
private static final float INFLEXION = 0.35f; // Tension lines cross at (INFLEXION, 1)
private static final float START_TENSION = 0.5f;
private static final float END_TENSION = 1.0f;
private static final float P1 = START_TENSION * INFLEXION;
private static final float P2 = 1.0f - END_TENSION * (1.0f - INFLEXION);
private static final int NB_SAMPLES = 100;
private static final float[] SPLINE_POSITION = new float[NB_SAMPLES + 1];
private static final float[] SPLINE_TIME = new float[NB_SAMPLES + 1];
private float mDeceleration;
private final float mPpi;
// A context-specific coefficient adjusted to physical values.
private float mPhysicalCoeff;
static {
float x_min = 0.0f;
float y_min = 0.0f;
for (int i = 0; i < NB_SAMPLES; i++) {
final float alpha = (float) i / NB_SAMPLES;
float x_max = 1.0f;
float x, tx, coef;
while (true) {
x = x_min + (x_max - x_min) / 2.0f;
coef = 3.0f * x * (1.0f - x);
tx = coef * ((1.0f - x) * P1 + x * P2) + x * x * x;
if (Math.abs(tx - alpha) < 1E-5) break;
if (tx > alpha) x_max = x;
else x_min = x;
}
SPLINE_POSITION[i] = coef * ((1.0f - x) * START_TENSION + x) + x * x * x;
float y_max = 1.0f;
float y, dy;
while (true) {
y = y_min + (y_max - y_min) / 2.0f;
coef = 3.0f * y * (1.0f - y);
dy = coef * ((1.0f - y) * START_TENSION + y) + y * y * y;
if (Math.abs(dy - alpha) < 1E-5) break;
if (dy > alpha) y_max = y;
else y_min = y;
}
SPLINE_TIME[i] = coef * ((1.0f - y) * P1 + y * P2) + y * y * y;
}
SPLINE_POSITION[NB_SAMPLES] = SPLINE_TIME[NB_SAMPLES] = 1.0f;
}
/**
* Create a Scroller with the default duration and interpolator.
*/
public Scroller(Context context) {
this(context, null);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. "Flywheel" behavior will
* be in effect for apps targeting Honeycomb or newer.
*/
public Scroller(Context context, Interpolator interpolator) {
this(context, interpolator,
context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. Specify whether or
* not to support progressive "flywheel" behavior in flinging.
*/
public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
mFinished = true;
if (interpolator == null) {
mInterpolator = new ViscousFluidInterpolator();
} else {
mInterpolator = interpolator;
}
mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
mFlywheel = flywheel;
mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
}
/**
* The amount of friction applied to flings. The default value
* is {@link ViewConfiguration#getScrollFriction}.
*
* @param friction A scalar dimension-less value representing the coefficient of
* friction.
*/
public final void setFriction(float friction) {
mDeceleration = computeDeceleration(friction);
mFlingFriction = friction;
}
private float computeDeceleration(float friction) {
return SensorManager.GRAVITY_EARTH // g (m/s^2)
* 39.37f // inch/meter
* mPpi // pixels per inch
* friction;
}
/**
*
* Returns whether the scroller has finished scrolling.
*
* @return True if the scroller has finished scrolling, false otherwise.
*/
public final boolean isFinished() {
return mFinished;
}
/**
* Force the finished field to a particular value.
*
* @param finished The new finished value.
*/
public final void forceFinished(boolean finished) {
mFinished = finished;
}
/**
* Returns how long the scroll event will take, in milliseconds.
*
* @return The duration of the scroll in milliseconds.
*/
public final int getDuration() {
return mDuration;
}
/**
* Returns the current X offset in the scroll.
*
* @return The new X offset as an absolute distance from the origin.
*/
public final int getCurrX() {
return mCurrX;
}
/**
* Returns the current Y offset in the scroll.
*
* @return The new Y offset as an absolute distance from the origin.
*/
public final int getCurrY() {
return mCurrY;
}
/**
* Returns the current velocity.
*
* @return The original velocity less the deceleration. Result may be
* negative.
*/
public float getCurrVelocity() {
return mMode == FLING_MODE ?
mCurrVelocity : mVelocity - mDeceleration * timePassed() / 2000.0f;
}
/**
* Returns the start X offset in the scroll.
*
* @return The start X offset as an absolute distance from the origin.
*/
public final int getStartX() {
return mStartX;
}
/**
* Returns the start Y offset in the scroll.
*
* @return The start Y offset as an absolute distance from the origin.
*/
public final int getStartY() {
return mStartY;
}
/**
* Returns where the scroll will end. Valid only for "fling" scrolls.
*
* @return The final X offset as an absolute distance from the origin.
*/
public final int getFinalX() {
return mFinalX;
}
/**
* Returns where the scroll will end. Valid only for "fling" scrolls.
*
* @return The final Y offset as an absolute distance from the origin.
*/
public final int getFinalY() {
return mFinalY;
}
/**
* Call this when you want to know the new location. If it returns true,
* the animation is not yet finished.
*/
public boolean computeScrollOffset() {
if (mFinished) {
return false;
}
int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE:
final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
break;
case FLING_MODE:
final float t = (float) timePassed / mDuration;
final int index = (int) (NB_SAMPLES * t);
float distanceCoef = 1.f;
float velocityCoef = 0.f;
if (index < NB_SAMPLES) {
final float t_inf = (float) index / NB_SAMPLES;
final float t_sup = (float) (index + 1) / NB_SAMPLES;
final float d_inf = SPLINE_POSITION[index];
final float d_sup = SPLINE_POSITION[index + 1];
velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
distanceCoef = d_inf + (t - t_inf) * velocityCoef;
}
mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
// Pin to mMinX <= mCurrX <= mMaxX
mCurrX = Math.min(mCurrX, mMaxX);
mCurrX = Math.max(mCurrX, mMinX);
mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
// Pin to mMinY <= mCurrY <= mMaxY
mCurrY = Math.min(mCurrY, mMaxY);
mCurrY = Math.max(mCurrY, mMinY);
if (mCurrX == mFinalX && mCurrY == mFinalY) {
mFinished = true;
}
break;
}
}
else {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
return true;
}
/**
* Start scrolling by providing a starting point and the distance to travel.
* The scroll will use the default value of 250 milliseconds for the
* duration.
*
* @param startX Starting horizontal scroll offset in pixels. Positive
* numbers will scroll the content to the left.
* @param startY Starting vertical scroll offset in pixels. Positive numbers
* will scroll the content up.
* @param dx Horizontal distance to travel. Positive numbers will scroll the
* content to the left.
* @param dy Vertical distance to travel. Positive numbers will scroll the
* content up.
*/
public void startScroll(int startX, int startY, int dx, int dy) {
startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
}
/**
* Start scrolling by providing a starting point, the distance to travel,
* and the duration of the scroll.
*
* @param startX Starting horizontal scroll offset in pixels. Positive
* numbers will scroll the content to the left.
* @param startY Starting vertical scroll offset in pixels. Positive numbers
* will scroll the content up.
* @param dx Horizontal distance to travel. Positive numbers will scroll the
* content to the left.
* @param dy Vertical distance to travel. Positive numbers will scroll the
* content up.
* @param duration Duration of the scroll in milliseconds.
*/
public void startScroll(int startX, int startY, int dx, int dy, int duration) {
mMode = SCROLL_MODE;
mFinished = false;
mDuration = duration;
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
mFinalX = startX + dx;
mFinalY = startY + dy;
mDeltaX = dx;
mDeltaY = dy;
mDurationReciprocal = 1.0f / (float) mDuration;
}
/**
* Start scrolling based on a fling gesture. The distance travelled will
* depend on the initial velocity of the fling.
*
* @param startX Starting point of the scroll (X)
* @param startY Starting point of the scroll (Y)
* @param velocityX Initial velocity of the fling (X) measured in pixels per
* second.
* @param velocityY Initial velocity of the fling (Y) measured in pixels per
* second
* @param minX Minimum X value. The scroller will not scroll past this
* point.
* @param maxX Maximum X value. The scroller will not scroll past this
* point.
* @param minY Minimum Y value. The scroller will not scroll past this
* point.
* @param maxY Maximum Y value. The scroller will not scroll past this
* point.
*/
public void fling(int startX, int startY, int velocityX, int velocityY,
int minX, int maxX, int minY, int maxY) {
// Continue a scroll or fling in progress
if (mFlywheel && !mFinished) {
float oldVel = getCurrVelocity();
float dx = (float) (mFinalX - mStartX);
float dy = (float) (mFinalY - mStartY);
float hyp = (float) Math.hypot(dx, dy);
float ndx = dx / hyp;
float ndy = dy / hyp;
float oldVelocityX = ndx * oldVel;
float oldVelocityY = ndy * oldVel;
if (Math.signum(velocityX) == Math.signum(oldVelocityX) &&
Math.signum(velocityY) == Math.signum(oldVelocityY)) {
velocityX += oldVelocityX;
velocityY += oldVelocityY;
}
}
mMode = FLING_MODE;
mFinished = false;
float velocity = (float) Math.hypot(velocityX, velocityY);
mVelocity = velocity;
mDuration = getSplineFlingDuration(velocity);
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
float coeffX = velocity == 0 ? 1.0f : velocityX / velocity;
float coeffY = velocity == 0 ? 1.0f : velocityY / velocity;
double totalDistance = getSplineFlingDistance(velocity);
mDistance = (int) (totalDistance * Math.signum(velocity));
mMinX = minX;
mMaxX = maxX;
mMinY = minY;
mMaxY = maxY;
mFinalX = startX + (int) Math.round(totalDistance * coeffX);
// Pin to mMinX <= mFinalX <= mMaxX
mFinalX = Math.min(mFinalX, mMaxX);
mFinalX = Math.max(mFinalX, mMinX);
mFinalY = startY + (int) Math.round(totalDistance * coeffY);
// Pin to mMinY <= mFinalY <= mMaxY
mFinalY = Math.min(mFinalY, mMaxY);
mFinalY = Math.max(mFinalY, mMinY);
}
private double getSplineDeceleration(float velocity) {
return Math.log(INFLEXION * Math.abs(velocity) / (mFlingFriction * mPhysicalCoeff));
}
private int getSplineFlingDuration(float velocity) {
final double l = getSplineDeceleration(velocity);
final double decelMinusOne = DECELERATION_RATE - 1.0;
return (int) (1000.0 * Math.exp(l / decelMinusOne));
}
private double getSplineFlingDistance(float velocity) {
final double l = getSplineDeceleration(velocity);
final double decelMinusOne = DECELERATION_RATE - 1.0;
return mFlingFriction * mPhysicalCoeff * Math.exp(DECELERATION_RATE / decelMinusOne * l);
}
/**
* Stops the animation. Contrary to {@link #forceFinished(boolean)},
* aborting the animating cause the scroller to move to the final x and y
* position
*
* @see #forceFinished(boolean)
*/
public void abortAnimation() {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
/**
* Extend the scroll animation. This allows a running animation to scroll
* further and longer, when used with {@link #setFinalX(int)} or {@link #setFinalY(int)}.
*
* @param extend Additional time to scroll in milliseconds.
* @see #setFinalX(int)
* @see #setFinalY(int)
*/
public void extendDuration(int extend) {
int passed = timePassed();
mDuration = passed + extend;
mDurationReciprocal = 1.0f / mDuration;
mFinished = false;
}
/**
* Returns the time elapsed since the beginning of the scrolling.
*
* @return The elapsed time in milliseconds.
*/
public int timePassed() {
return (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
}
/**
* Sets the final position (X) for this scroller.
*
* @param newX The new X offset as an absolute distance from the origin.
* @see #extendDuration(int)
* @see #setFinalY(int)
*/
public void setFinalX(int newX) {
mFinalX = newX;
mDeltaX = mFinalX - mStartX;
mFinished = false;
}
/**
* Sets the final position (Y) for this scroller.
*
* @param newY The new Y offset as an absolute distance from the origin.
* @see #extendDuration(int)
* @see #setFinalX(int)
*/
public void setFinalY(int newY) {
mFinalY = newY;
mDeltaY = mFinalY - mStartY;
mFinished = false;
}
/**
* @hide
*/
public boolean isScrollingInDirection(float xvel, float yvel) {
return !mFinished && Math.signum(xvel) == Math.signum(mFinalX - mStartX) &&
Math.signum(yvel) == Math.signum(mFinalY - mStartY);
}
static class ViscousFluidInterpolator implements Interpolator {
/** Controls the viscous fluid effect (how much of it). */
private static final float VISCOUS_FLUID_SCALE = 8.0f;
private static final float VISCOUS_FLUID_NORMALIZE;
private static final float VISCOUS_FLUID_OFFSET;
static {
// must be set to 1.0 (used in viscousFluid())
VISCOUS_FLUID_NORMALIZE = 1.0f / viscousFluid(1.0f);
// account for very small floating-point error
VISCOUS_FLUID_OFFSET = 1.0f - VISCOUS_FLUID_NORMALIZE * viscousFluid(1.0f);
}
private static float viscousFluid(float x) {
x *= VISCOUS_FLUID_SCALE;
if (x < 1.0f) {
x -= (1.0f - (float)Math.exp(-x));
} else {
float start = 0.36787944117f; // 1/e == exp(-1)
x = 1.0f - (float)Math.exp(1.0f - x);
x = start + x * (1.0f - start);
}
return x;
}
@Override
public float getInterpolation(float input) {
final float interpolated = VISCOUS_FLUID_NORMALIZE * viscousFluid(input);
if (interpolated > 0) {
return interpolated + VISCOUS_FLUID_OFFSET;
}
return interpolated;
}
}
}