android自定义控件系列----Scroller类详解
说在前面的话：
为什么要来说Scroller这个类呢？这个类到底是拿来干什么的呢？如果你看了ListView这类控件那么你肯定会发现里面有一个Sroller类，其实它的作用就是辅助记录和计算我们滑动的距离和速度这些。从而让我们在自定义控件的时候可以方便的做一些滑动和回弹的动画，为什么呢？因为Sroller类都给你计算好了嘛。
类分析
public class Scroller {
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;
......
/**
* 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:
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:
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;
}
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 = 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;
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);
}
/**
* 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;
}
....* Call this when you want to know the new location. If it returns true,当你想得到一个新的位置的时候call这个方法。其实在这个类里面已经给了我们一个很好的例子我这里也贴出来看看到底是怎么样的？
* 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();
* ...
* } 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();
* }可以看到上面的代码，注释都很nice对不对？需要注意的是最后一行invalidate这个时候需要我们手动的去发起一次刷新请求，不然结果会跟你想得不一样的。接下来就是fling()这个方法，我们从它的注释可以读出来是什么意思 /** * Start scrolling based on a fling gesture. The distance travelled will * depend on the initial velocity of the fling.根据你的手势来滑动一段距离取决于你的初速度，那么这个函数应该什么时候调用呢？那么我来告诉你，listview是不是快速滑动的时候你会看到listview的视图快速网上滑动一些条目过后才会停止下来，就是在我们快速滑动屏幕的时候就调用这个函数。从下一篇开始就要开始讲实战了。