<!-- Base "touch slop" value used by ViewConfiguration as amovement threshold where scrolling should begin. -->
<dimen name="config_viewConfigurationTouchSlop">8dp</dimen>

Log.i("songzheweiwang", "TouchSlop=" + ViewConfiguration.get(this).getScaledTouchSlop());

07-15 17:02:22.382 6789-6789/com.example.demos I/songzheweiwang: TouchSlop=24

 1 //======android.view.ViewConfiguration.java======
 2 private final int mTouchSlop;
 3 /**
 4   * @return Distance in pixels a touch can wander before we think the user is scrolling
 5  */
 6 public int getScaledTouchSlop() {
 7     return mTouchSlop;
 8 }
 9 ......
10 private ViewConfiguration(Context context) {
11     ......
12     mTouchSlop = res.getDimensionPixelSize( com.android.internal.R.dimen.config_viewConfigurationTouchSlop);
13     ......
14 }

VelocityTracker mVelocityTracker = VelocityTracker.obtain();

 1 /**
 2      * Retrieve a new VelocityTracker object to watch the velocity of a
 3      * motion.  Be sure to call {@link #recycle} when done.  You should
 4      * generally only maintain an active object while tracking a movement,
 5      * so that the VelocityTracker can be re-used elsewhere.
 6      *
 7      * @return Returns a new VelocityTracker.
 8      */
 9     static public VelocityTracker obtain() {
10         VelocityTracker instance = sPool.acquire();
11         return (instance != null) ? instance : new VelocityTracker(null);
12     }

mVelocityTracker.addMovement(event);

 1 /**
 2      * Add a user's movement to the tracker.  You should call this for the
 3      * initial {@link MotionEvent#ACTION_DOWN}, the following
 4      * {@link MotionEvent#ACTION_MOVE} events that you receive, and the
 5      * final {@link MotionEvent#ACTION_UP}.  You can, however, call this
 6      * for whichever events you desire.
 7      *
 8      * @param event The MotionEvent you received and would like to track.
 9      */
10     public void addMovement(MotionEvent event) {
11         if (event == null) {
12             throw new IllegalArgumentException("event must not be null");
13         }
14         nativeAddMovement(mPtr, event);
15     }
16 ......
17 private static native void nativeAddMovement(long ptr, MotionEvent event);

mVelocityTracker.computeCurrentVelocity(int units);

 1 /**
 2  * Equivalent to invoking {@link #computeCurrentVelocity(int, float)} with a maximum
 3  * velocity of Float.MAX_VALUE.
 4  *
 5  * @see #computeCurrentVelocity(int, float)
 6  */
 7 public void computeCurrentVelocity(int units) {
 8     nativeComputeCurrentVelocity(mPtr, units, Float.MAX_VALUE);
 9 }
10 ......
11 private static native void nativeComputeCurrentVelocity(long ptr, int units, float maxVelocity);

 float xVelocity = mVelocityTracker.getXVelocity();float yVelocity = mVelocityTracker.getYVelocity();

 1 /**
 2  * Retrieve the last computed X velocity.  You must first call
 3  * {@link #computeCurrentVelocity(int)} before calling this function.
 4  *
 5  * @return The previously computed X velocity.
 6  */
 7 public float getXVelocity() {
 8     return nativeGetXVelocity(mPtr, ACTIVE_POINTER_ID);
 9 }
10 /**
11  * Retrieve the last computed Y velocity.  You must first call
12  * {@link #computeCurrentVelocity(int)} before calling this function.
13  *
14  * @return The previously computed Y velocity.
15  */
16 public float getYVelocity() {
17     return nativeGetYVelocity(mPtr, ACTIVE_POINTER_ID);
18 }
19 ......
20 private static native float nativeGetXVelocity(long ptr, int id);
21 private static native float nativeGetYVelocity(long ptr, int id);

mVelocityTracker.recycle();

 1 /**
 2  * Return a VelocityTracker object back to be re-used by others.  You must
 3  * not touch the object after calling this function.
 4  */
 5 public void recycle() {
 6     if (mStrategy == null) {
 7         clear();
 8         sPool.release(this);
 9     }
10 }
11 ......
12 /**
13   * Reset the velocity tracker back to its initial state.
14   */
15  public void clear() {
16      nativeClear(mPtr);
17  }
18 ......
19 private static native void nativeClear(long ptr);

 1 public class VelocityView extends View {
 2     private static final String TAG = "songzheweiwang";
 3     private VelocityTracker mVelocityTracker;
 4
 5     public VelocityView(Context context, @Nullable AttributeSet attrs) {
 6         super(context, attrs);
 7         mVelocityTracker = VelocityTracker.obtain();
 8     }
 9
10     @Override
11     public boolean onTouchEvent(MotionEvent event) {
12         mVelocityTracker.addMovement(event);
13         switch (event.getActionMasked()) {
14             case MotionEvent.ACTION_UP:
15                 mVelocityTracker.computeCurrentVelocity(1000);
16                 float xVelocity = mVelocityTracker.getXVelocity();
17                 float yVelocity = mVelocityTracker.getYVelocity();
18                 Log.i(TAG, "xVelocity=" + xVelocity + ";yVelocity=" + yVelocity);
19                 break;
20         }
21         return true;
22     }
23
24     @Override
25     protected void onDetachedFromWindow() {
26         Log.i(TAG, "onDetachedFromWindow");
27         mVelocityTracker.recycle();
28         super.onDetachedFromWindow();
29     }
30 }

07-16 10:15:18.951 10338-10338/com.example.demos I/songzheweiwang: xVelocity=643.2775;yVelocity=543.7565
07-16 10:15:26.406 10338-10338/com.example.demos I/songzheweiwang: onDetachedFromWindow