这里的技术是扩展ViewPager 并模仿寻呼机将在内部执行的大部分操作，以及来自Gallery 小部件的滚动逻辑。一般的想法是监控投掷（以及速度和伴随的滚动），然后将它们作为假拖动事件提供给底层ViewPager。如果您单独执行此操作，则将无法正常工作（您仍然只能获得一页滚动）。发生这种情况是因为假拖动在滚动有效的边界上实现了上限。您可以模拟扩展 ViewPager 中的计算并检测何时会发生这种情况，然后只需翻页并照常继续。使用假拖动的好处意味着您不必处理对齐页面或处理ViewPager 的边缘。
我在动画演示示例中测试了以下代码，可从 http://developer.android.com/training/animation/screen-slide.html 下载，方法是将 ScreenSlideActivity 中的 ViewPager 替换为此 VelocityViewPager（在布局 activity_screen_slide 和 Activity 中的字段中）。
/*
* Copyright 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
* Author: Dororo @ StackOverflow
* An extended ViewPager which implements multiple page flinging.
*
*/
package com.example.android.animationsdemo;
import android.content.Context;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.GestureDetector;
import android.widget.Scroller;
public class VelocityViewPager extends ViewPager implements GestureDetector.OnGestureListener {
private GestureDetector mGestureDetector;
private FlingRunnable mFlingRunnable = new FlingRunnable();
private boolean mScrolling = false;
public VelocityViewPager(Context context) {
super(context);
}
public VelocityViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
mGestureDetector = new GestureDetector(context, this);
}
// We have to intercept this touch event else fakeDrag functions won't work as it will
// be in a real drag when we want to initialise the fake drag.
@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
return true;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
// give all the events to the gesture detector. I'm returning true here so the viewpager doesn't
// get any events at all, I'm sure you could adjust this to make that not true.
mGestureDetector.onTouchEvent(event);
return true;
}
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velX, float velY) {
mFlingRunnable.startUsingVelocity((int)velX);
return false;
}
private void trackMotion(float distX) {
// The following mimics the underlying calculations in ViewPager
float scrollX = getScrollX() - distX;
final int width = getWidth();
final int widthWithMargin = width + this.getPageMargin();
final float leftBound = Math.max(0, (this.getCurrentItem() - 1) * widthWithMargin);
final float rightBound = Math.min(this.getCurrentItem() + 1, this.getAdapter().getCount() - 1) * widthWithMargin;
if (scrollX < leftBound) {
scrollX = leftBound;
// Now we know that we've hit the bound, flip the page
if (this.getCurrentItem() > 0) {
this.setCurrentItem(this.getCurrentItem() - 1, false);
}
}
else if (scrollX > rightBound) {
scrollX = rightBound;
// Now we know that we've hit the bound, flip the page
if (this.getCurrentItem() < (this.getAdapter().getCount() - 1) ) {
this.setCurrentItem(this.getCurrentItem() + 1, false);
}
}
// Do the fake dragging
if (mScrolling) {
this.fakeDragBy(distX);
}
else {
this.beginFakeDrag();
this.fakeDragBy(distX);
mScrolling = true;
}
}
private void endFlingMotion() {
mScrolling = false;
this.endFakeDrag();
}
// The fling runnable which moves the view pager and tracks decay
private class FlingRunnable implements Runnable {
private Scroller mScroller; // use this to store the points which will be used to create the scroll
private int mLastFlingX;
private FlingRunnable() {
mScroller = new Scroller(getContext());
}
public void startUsingVelocity(int initialVel) {
if (initialVel == 0) {
// there is no velocity to fling!
return;
}
removeCallbacks(this); // stop pending flings
int initialX = initialVel < 0 ? Integer.MAX_VALUE : 0;
mLastFlingX = initialX;
// setup the scroller to calulate the new x positions based on the initial velocity. Impose no cap on the min/max x values.
mScroller.fling(initialX, 0, initialVel, 0, 0, Integer.MAX_VALUE, 0, Integer.MAX_VALUE);
post(this);
}
private void endFling() {
mScroller.forceFinished(true);
endFlingMotion();
}
@Override
public void run() {
final Scroller scroller = mScroller;
boolean animationNotFinished = scroller.computeScrollOffset();
final int x = scroller.getCurrX();
int delta = x - mLastFlingX;
trackMotion(delta);
if (animationNotFinished) {
mLastFlingX = x;
post(this);
}
else {
endFling();
}
}
}
@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2, float distX, float distY) {
trackMotion(-distX);
return false;
}
// Unused Gesture Detector functions below
@Override
public boolean onDown(MotionEvent event) {
return false;
}
@Override
public void onLongPress(MotionEvent event) {
// we don't want to do anything on a long press, though you should probably feed this to the page being long-pressed.
}
@Override
public void onShowPress(MotionEvent event) {
// we don't want to show any visual feedback
}
@Override
public boolean onSingleTapUp(MotionEvent event) {
// we don't want to snap to the next page on a tap so ignore this
return false;
}
}
这有一些小问题，可以很容易地解决，但我会留给你，即如果你滚动（拖动，而不是扔）你可能会在页面之间结束（你会想要捕捉 ACTION_UP 事件）。此外，为了做到这一点，触摸事件被完全覆盖，因此您需要在适当的情况下将相关事件提供给底层ViewPager。