[移动开发]Android平滑移动——Scroller类研究，太厉害了

2、computeScrollOffset（）方法：

public boolean computeScrollOffset() {

if (mFinished) {

return false;

}

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

if (timePassed < mDuration) {

switch (mMode) {

case SCROLL_MODE:

float x = timePassed * mDurationReciprocal;

if (mInterpolator == null)

x = viscousFluid(x);

else

x = mInterpolator.getInterpolation(x);

mCurrX = mStartX + Math.round(x * mDeltaX);

mCurrY = mStartY + Math.round(x * mDeltaY);

break;

case FLING_MODE:

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final float t = (float) timePassed / mDuration;

final int index = (int) (NB_SAMPLES * t);

final float t_inf = (float) index / NB_SAMPLES;

final float t_sup = (float) (index + 1) / NB_SAMPLES;

final float d_inf = SPLINE[index];

final float d_sup = SPLINE[index + 1];

final float distanceCoef = d_inf + (t - t_inf) / (t_sup - t_inf) * (d_sup - d_inf);

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;

}

看代码主要当平滑没有完成的情况下，计算出当前滑动的坐标位置，如果平滑完成了，就不需要计算了；这里有一个SCROLL_MODE和FLING_MODEL两种滑动模式，这里是SCROLL_MODE手动拖动平滑模式，FLING_MODEL是由于手指滑动速率来判断惯性滑动。一般这个方法执行完成之后，根据返回值判断是否需要invalidate/postinvalidate，再根据Scroller计算好的坐标值，View将scrollTo/scrollBy到新的坐标位置；

3、fling（）方法：

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 = FloatMath.sqrt(dx * dx + dy * 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 = FloatMath.sqrt(velocityX * velocityX + velocityY * velocityY);

mVelocity = velocity;

final double l = Math.log(START_TENSION * velocity / ALPHA);

mDuration = (int) (1000.0 * Math.exp(l / (DECELERATION_RATE - 1.0)));

mStartTime = AnimationUtils.currentAnimationTimeMillis();

mStartX = startX;

mStartY = startY;

float coeffX = velocity == 0 ? 1.0f : velocityX / velocity;

float coeffY = velocity == 0 ? 1.0f : velocityY / velocity;

int totalDistance =

(int) (ALPHA * Math.exp(DECELERATION_RATE / (DECELERATION_RATE - 1.0) * l));

mMinX = minX;

mMaxX = maxX;

mMinY = minY;

mMaxY = maxY;

mFinalX = startX + Math.round(totalDistance * coeffX);

// Pin to mMinX <= mFinalX <= mMaxX

mFinalX = Math.min(mFinalX, mMaxX);

mFinalX = Math.max(mFinalX, mMinX);

mFinalY = startY + Math.round(totalDistance * coeffY);

// Pin to mMinY <= mFinalY <= mMaxY

mFinalY = Math.min(mFinalY, mMaxY);

mFinalY = Math.max(mFinalY, mMinY);

}

滑动，这个滑就跟GestureDetector.OnGestureListener这个接口中的onFling事件一样，根据滑动速率来判断一些事件。主要处理一些一些惯性坐标变化（惯性行为）。用得比较少；

4、abortAnimation（）方法：

public void abortAnimation() {

mCurrX = mFinalX;

mCurrY = mFinalY;

mFinished = true;

}

看源码，停止动画（就是设置一个标记而已，不再计算坐标的变化值）

在代码的开始外看到这样一段静态代码块：

static {

float x_min = 0.0f;

for (int i = 0; i <= NB_SAMPLES; i++) {

final float t = (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) * START_TENSION + x * END_TENSION) + x * x * x;