我们自定义View的时候难免会需要到滚动的状态，而Scroller可以帮助我们做到顺滑的滚动。
使用的基本套路是下面这样的，然后我们需要滚的的时候调用下smoothScrollTo函数就可以了。
Scroller mScroller = new Scroller(context);
private void smoothScrollTo(int destX, int destY) {
int scrollX = getScrollX();
int delta = destX - scrollX;
mScroller.startScroll(scrollX, 0, delta, 0, 1000);
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}
}
我们知道这个Scroller本身是不能移动我们View的，最终还是要靠本身的scrollTo函数。通过我们希望的滑动距离和时间，然后计算出每次滚动的具体，一直就那样一点点的滚动，从而实现弹性滑动的效果。
现在我们就去看下他背后是做了什么。
起航
API:23
我们先看下构造函数的
public Scroller(Context context) {
this(context, null);
}
public Scroller(Context context, Interpolator interpolator) {
this(context, interpolator,
context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
}
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
}
我们直接调用new Scroller（context）的时候，他会用默认插值器ViscousFluidInterpolator()，设置滚动是否结束的标记为真，同时计算下Deceleration的值。对于0.84是什么鬼，我只想说，对于魔术字，我也不懂。
好了，我们继续去看下那个startScroll()里面的内容
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;
}
嗯，基本都是配置一堆的值。没什么特别的，我们得结合别的来说。
接下来我们看下他配套的函数
public boolean computeScrollOffset() {
if (mFinished) {
return false;
}
int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE: //当前我们要关心的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;
}
很简单的，computeScrollOffset()判断标记是否结束和时间是否过了，没有就分模式处理。
我们的当前的模式是Scroll_mode，那个插值器根据我们的设置的值来计算出一个插值，再和我们的增量相乘的到这次要滚动的距离，最后和我们的其实距离相加，最是我们这次要滚到的点啦。
final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
这样我们的computeScroll() 函数里面，就可以获得者连个值，来滑动界面的。知道
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
接着还有的另外一个模式就是Fling
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);
}
当我们想中断动画的时候，可以调用abortAnimation()，来直接停止滑动，直接滚动到结点。
public void abortAnimation() {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
如果想在拉长下时间，可以调用extendDuration()函数，来延长下时间。
public void extendDuration(int extend) {
int passed = timePassed();
mDuration = passed + extend;
mDurationReciprocal = 1.0f / mDuration;
mFinished = false;
}
public int timePassed() {
return (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
}
后记
写得好没精神..不知不觉一点多了。。。呵呵
明天再做补充。有点困了，还没洗澡。明天居然要早到。