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（如果仍在进行中），如果投掷手势又宽又快，则进一步滚动？
如果没有从哪里开始这样的事情？
PS提供赏金。请不要参考 Gallery 或 HorizontalScrollView 的答案
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这里的技术是扩展ViewPager和模仿寻呼机将在内部执行的大部分操作，并结合Gallery小部件的滚动逻辑。总体思路是监控投掷（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。您可以模拟扩展中的计算ViewPager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.
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 ViewPager 替换ScreenSlideActivity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（你会想抓住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ViewPager在适当的情况下将相关事件提供给底层。
于 2013-02-19T23:54:33.730 回答
4
另一种选择是从支持库复制整个ViewPager实现源代码并自定义determineTargetPage(...)方法。它负责确定在滑动手势上滚动到哪个页面。这种方法不是超级方便，但效果很好。见下面的实现代码：
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;
}
于 2014-03-12T15:10:03.990 回答
2
我发现比检查答案更好的实现，当我想停止滚动时，这个 ViewPager 的触摸表现更好 https://github.com/Benjamin-Dobell/VelocityViewPager
于 2016-08-19T10:02:52.670 回答
1
ViewPager 是来自支持库的类。下载支持库源代码，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后的支持库大约一年，因为有时我需要修改几行代码来做一些小改动或添加新方法，我不想复制组件源代码。我使用修改版的 Fragments 和 viewpager。
但是您会遇到一个问题：大约每 6 个月一次，如果您需要新功能，您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，您不想破坏支持库类的兼容性。
于 2013-02-26T10:30:45.063 回答
-1
您可以覆盖 ScrollView 或 HorizontalScrollView 类，并添加该行为。Gallery 中有很多错误，据我所知，它自 api 级别 14 以来已被弃用。
于 2012-10-16T08:39:52.517 回答