Scroller的方法startScroll、fling、computeScrollOffset
最近在使用开源库TableFixHeaders（https://github.com/InQBarna/TableFixHeaders），对于其中Scroller的用法不清楚，故看看这几个方法干了什么。
点与点之间的计算 + 触摸事件的分发。
1. startScroll
当startScroll执行过程中即在duration时间内，computeScrollOffset 方法会一直返回false，但当动画执行完成后会返回返加true.
如果不设置duration，默认DEFAULT_DURATION = 250.
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;
}
看代码，这个方法就是把需要滚动的参数传进来。
2.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);
}
3. computeScrollOffset
功效：返回值为boolean，true说明滚动尚未完成，false说明滚动已经完成。
参考网址：http://www.cnblogs.com/tt_mc/p/3585390.html
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;
}
timePassed表示滚动进行到的时间，若滚动未结束，分SCROLL_MODE和FLING_MODE两种情况计算当前位置。
SCROLL_MODE下，
时间最小单位mDurationReciprocal = 1.0f / (float) mDuration;
通过差值器mInterpolator确定距离百分比，这里x和y方向使用同一差值器。
mInterpolator = new ViscousFluidInterpolator(); 是Scroller的静态内部类，取值算法见getInterpolation()所调用的viscousFluid()。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; }}
FLING_MODE下，
x和y还是同比例变化，这里设定了采样率NB_SAMPLES = 100；（？？）
index和index+1的含义？
速度因子，velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
距离因子，distanceCoef = d_inf + (t - t_inf) * velocityCoef;
4. invalidate() 与 postInvalidate()
5.smoothScrollBy() 与 fling()
6. 其他相关类
参考网址：http://mobile.51cto.com/design-375889.htm
http://www.cnblogs.com/wanqieddy/archive/2012/05/05/2484534.html
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