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。无论是全屏快速拖动还是慢速拖动，它都以同样的方式对待投掷手势;在结束页面只前进一步。

是否有任何项目或示例可以添加基于速度的投掷，根据现有投掷的速度滚动多个项目（如果它仍在进行中）并且如果投掷手势宽而快，则进一步滚动？

如果没有从哪里开始这样的事情？

P.S。提供赏金。请不要参考Gallery或Horizo​​ntalScrollView

Answer 1

这里的技术是扩展ViewPager并模仿寻呼机内部工作的大部分内容，以及来自Gallery小部件的滚动逻辑。一般的想法是监视fling（和速度和伴随的滚动），然后将它们作为假拖动事件提供给底层ViewPager。如果你单独这样做，它虽然不会工作（你仍然只能滚动一页）。发生这种情况是因为假拖动在滚动有效的边界上实现了限制。您可以模拟扩展ViewPager中的计算并检测何时会发生这种情况，然后只需翻转页面并继续照常处理。使用假拖动的好处意味着您不必处理对页面的捕捉或处理ViewPager的边缘。

我在动画演示示例上测试了以下代码，可以从http://developer.android.com/training/animation/screen-slide.html下载，将ScreenSlideActivity中的ViewPager替换为VelocityViewPager（两者都在布局activity_screen_slide中活动中的字段。

/*
 * 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，而不是投掷），你可以在页面之间的一半（你＆＃39;我想要捕捉ACTION_UP事件）。此外，触摸事件正在被完全覆盖，因此您需要在适当的时候将相关事件提供给基础ViewPager。

Answer 2

另一个选择是从支持库复制整个ViewPager实现源代码并自定义determineTargetPage(...)方法。它负责确定在fling手势上滚动到哪个页面。这种方法不是很方便，但效果很好。 请参阅以下实施代码：

private int determineTargetPage(int curPage, float pageOffset, int velocity, int dx) {
    int target;
    if (Math.abs(dx) > mFlingDistance && Math.abs(velocity) > mMinimumVelocity) {
        target = calculateFinalPage(curPage, velocity);
    } else {
        final float truncator = curPage >= mCurItem ? 0.4f : 0.6f;
        target = (int) (curPage + pageOffset + truncator);
    }
    if (mItems.size() > 0) {
        final ItemInfo first = mItems.get(0);
        final ItemInfo last = mItems.get(mItems.size() - 1);

        // Only let the user target pages we have items for
        target = Math.max(first.position, Math.min(target, last.position));
    }
    return target;
}

private int calculateFinalPage(int curPage, int velocity) {
    float distance = Math.abs(velocity) * MAX_SETTLE_DURATION / 1000f;
    float normalDistance = (float) Math.sqrt(distance / 2) * 25;
    int step = (int) - Math.signum(velocity);
    int width = getClientWidth();
    int page = curPage;
    for (int i = curPage; i >= 0 && i < mAdapter.getCount(); i += step) {
        float pageWidth = mAdapter.getPageWidth(i);
        float remainingDistance = normalDistance - pageWidth * width;
        if (remainingDistance >= 0) {
            normalDistance = remainingDistance;
        } else {
            page = i;
            break;
        }
    }
    return page;
}

Answer 3

我找到了一个比检查答案更好的实现，当我想要停止滚动时，这个ViewPager在触摸时表现得更好 https://github.com/Benjamin-Dobell/VelocityViewPager

Answer 4

ViewPager是支持库中的类。下载支持库源代码并在onTouchEvent方法中更改大约10行代码以添加所需的功能。

我在我的项目中使用修改的支持库大约一年，因为有时我需要修改几行代码来进行一些更改或添加新方法，我不想复制组件源代码。我使用片段和viewpager的修改版本。

但是你会遇到一个问题：如果需要新功能，一旦进入大约6 mounth，你必须将自定义支持库与新的官方版本合并。并且要小心更改，您不希望破坏支持库类的兼容性。

Answer 5

您可以覆盖ScrollView或Horizo​​ntalScrollView类，并添加该行为。 Gallery中有很多错误，我记得它自api 14级以来已被弃用。