Scroller的使用用法

Scroller是Android里面用于实现View的弹性滑动的一个Helper类，我们知道android里面ScrollTo，ScrollBy方法实现滑动的效果都是瞬时实现的，没有过渡的动画效果，这样体验式非常不好的，Scroller本身是无法实现滑动的，必须配合computeScroll才能共同完成这个功能。

private Scroller mScroller;
mScroller = new Scroller(getContext());
//缓慢滑动到指定位置
private void smoothScrollTo(int destX, int destY) {
int scrollX = getScrollX();
int deltaX = destX - scrollX;
mScroller.startScroll(scrollX, 0, deltaX, 0, 1000);
invalidate();//这个方法会导致View进行重绘，在View里面的Ondraw方法又会去调用computeScroll方法，
//如此反复，直到滑动过程结束
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());//通过mScroller对象获取到当前的X，Y坐标值
postInvalidate();           //具体的获取方法看下边的computeScrollOffset方法
}
}
//Scroller里面的 computeScrollOffset方法源码
public boolean computeScrollOffset() {
if (mFinished) {
return false;
}

int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);

if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE:
<span style="white-space:pre">	</span>final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
break;
case FLING_MODE:
<span style="white-space:pre">	</span>final float t = (float) timePassed / mDuration;
<span style="white-space:pre">	</span>final int index = (int) (NB_SAMPLES * t);
float distanceCoef = 1.f;
float velocityCoef = 0.f;
if (index < NB_SAMPLES) {
<span style="white-space:pre">	</span>final float t_inf = (float) index / NB_SAMPLES;
<span style="white-space:pre">	</span>final float t_sup = (float) (index + 1) / NB_SAMPLES;
<span style="white-space:pre">	</span>final float d_inf = SPLINE_POSITION[index];
<span style="white-space:pre">	</span>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));//这里的mCurrX和下边的mCurrY就是X，Y坐标了
// 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;
}