Android:基于速度的 ViewPager 滚动

Question

ViewPager 现在滚动的方式是每个手势一个项目。无论是全屏快闪还是慢拖，对挥动手势的处理方式都是一样的；在最后一页只前进了一步。

是否有任何项目或示例可能会添加基于速度的 throw ，根据现有 throw 的速度滚动多个项目(如果仍在进行中)并在 throw 手势又宽又快时进一步滚动？

如果没有从哪里开始这样的事情？

附:提供赏金。请不要引用 Gallery 或 Horizo​​ntalScrollView 的答案

Answer 1

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。总体思路是监控弹跳(以及速度和伴随的滚动)，然后将它们作为虚假的拖动事件提供给底层的 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。您可以在扩展的 ViewPager 中模拟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捕捉到页面或处理 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 下载通过用这个 VelocityViewPager 替换 ScreenSlideActivity 中的 ViewPager(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，比如如果你滚动(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。