VelocityTracker 速度跟踪器
在写关于Android滑动的控件，如果用户手指在屏幕上（当前位置 - 起始位置 > 某个数值）就做一个界面切换，但是总感觉太生硬，只有满足上面的条件才会触发切换界面，不管用户滑动的速度有多么的快，都要去算当前位置和起始位置的距离；但是ViewPager这个控件如果你滑动速度很快的话，就触发切换页面的效果了，怎么实现的呢；于是我去看了一下Google的ViewPager的源码，发现了这么个神器的东西：VelocityTracker；
以下是Viewpager的源码：
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (mFakeDragging) {
// A fake drag is in progress already, ignore this real one
// but still eat the touch events.
// (It is likely that the user is multi-touching the screen.)
return true;
}
if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
// Don't handle edge touches immediately -- they may actually belong to one of our
// descendants.
return false;
}
if (mAdapter == null || mAdapter.getCount() == 0) {
// Nothing to present or scroll; nothing to touch.
return false;
}
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
final int action = ev.getAction();
boolean needsInvalidate = false;
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
mScroller.abortAnimation();
mPopulatePending = false;
populate();
// Remember where the motion event started
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = mInitialMotionY = ev.getY();
mActivePointerId = ev.getPointerId(0);
break;
}
case MotionEvent.ACTION_MOVE:
if (!mIsBeingDragged) {
final int pointerIndex = ev.findPointerIndex(mActivePointerId);
if (pointerIndex == -1) {
// A child has consumed some touch events and put us into an inconsistent
// state.
needsInvalidate = resetTouch();
break;
}
final float x = ev.getX(pointerIndex);
final float xDiff = Math.abs(x - mLastMotionX);
final float y = ev.getY(pointerIndex);
final float yDiff = Math.abs(y - mLastMotionY);
if (DEBUG) {
Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);
}
if (xDiff > mTouchSlop && xDiff > yDiff) {
if (DEBUG) Log.v(TAG, "Starting drag!");
mIsBeingDragged = true;
requestParentDisallowInterceptTouchEvent(true);
mLastMotionX = x - mInitialMotionX > 0 ? mInitialMotionX + mTouchSlop :
mInitialMotionX - mTouchSlop;
mLastMotionY = y;
setScrollState(SCROLL_STATE_DRAGGING);
setScrollingCacheEnabled(true);
// Disallow Parent Intercept, just in case
ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
}
}
// Not else! Note that mIsBeingDragged can be set above.
if (mIsBeingDragged) {
// Scroll to follow the motion event
final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
final float x = ev.getX(activePointerIndex);
needsInvalidate |= performDrag(x);
}
break;
case MotionEvent.ACTION_UP:
if (mIsBeingDragged) {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) velocityTracker.getXVelocity(mActivePointerId);
mPopulatePending = true;
final int width = getClientWidth();
final int scrollX = getScrollX();
final ItemInfo ii = infoForCurrentScrollPosition();
final float marginOffset = (float) mPageMargin / width;
final int currentPage = ii.position;
final float pageOffset = (((float) scrollX / width) - ii.offset)
/ (ii.widthFactor + marginOffset);
final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
final float x = ev.getX(activePointerIndex);
final int totalDelta = (int) (x - mInitialMotionX);
int nextPage = determineTargetPage(currentPage, pageOffset, initialVelocity,
totalDelta);
setCurrentItemInternal(nextPage, true, true, initialVelocity);
needsInvalidate = resetTouch();
}
break;
case MotionEvent.ACTION_CANCEL:
if (mIsBeingDragged) {
scrollToItem(mCurItem, true, 0, false);
needsInvalidate = resetTouch();
}
break;
case MotionEvent.ACTION_POINTER_DOWN: {
final int index = ev.getActionIndex();
final float x = ev.getX(index);
mLastMotionX = x;
mActivePointerId = ev.getPointerId(index);
break;
}
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));
break;
}
if (needsInvalidate) {
ViewCompat.postInvalidateOnAnimation(this);
}
return true;
}有这么几行代码：
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) velocityTracker.getXVelocity(mActivePointerId);
它在Touch中实例化VelocityTracker ，在UP触摸事件中获取到当前的速度；
可以仿着写个自定义的View：
public class ContinueSlideScrollView extends NestedScrollView {
private final String TAG = "CCB";
public ContinueSlideScrollView(Context context) {
super(context);
this.context = context;
}
public ContinueSlideScrollView(Context context, AttributeSet attrs) {
super(context, attrs);
this.context = context;
}
public ContinueSlideScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
this.context = context;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
switch (ev.getAction()){
case MotionEvent.ACTION_DOWN:
break;
case MotionEvent.ACTION_MOVE:
break;
case MotionEvent.ACTION_UP:
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, ViewConfiguration.get(context).getScaledMaximumFlingVelocity());
int initialVelocity = (int) velocityTracker.getYVelocity();
Log.i(TAG, initialVelocity>0 ? "向下滑动，速度是："+initialVelocity : "向上滑动，速度是："+initialVelocity);
break;
}
return super.onTouchEvent(ev);
}
@Override
protected void onDetachedFromWindow() {
mVelocityTracker.recycle();
super.onDetachedFromWindow();
}
}获取到当前View的滑动速度后，你就可以对它胡作非为了！！！
