GestureDetector

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 2019/12/08   Share

尽管应用不应依赖轻触手势来执行基本行为（因为手势可能无法在所有上下文中供所有用户使用，用户可能不知道需要轻触手势来执行某些操作），但向应用添加基于轻触的互动可以极大地提升其实用性和吸引力（可以考虑做一个行为备份，即用户可以同时通过更常见的方式来执行这些基本操作）。

本文中，我们主要介绍以下几个方面的内容

检测常用手势：了解如何使用 GestureDetector 检测基本轻触手势，例如滚动、滑动和点按两次。
跟踪轻触和指针移动：了解如何跟踪指针移动。
以动画方式显示滚动手势：了解如何使用滚动条（Scroller 或 OverScroller）生成滚动动画以响应轻触事件。
处理多点触控手势：了解如何检测多指针（手指）手势。
拖动和缩放：了解如何实现基于轻触的拖动和缩放。
在 ViewGroup 中管理轻触事件：了解如何在 ViewGroup 中管理轻触事件，以确保将轻触事件正确分配给目标视图。

View的dispatchTouchEvent

为什么这里要先讲一下 View 的 dispatchTouchEvent 呢，因为我们平常处理手势事件，也是在 View 层面，由于 View 可以注册很多监听器，所以一个事件到了 View 层级，是一个怎么样的处理流程就很重要了。

下面结合一下源码来分析一下这个 “分发” 过程（这里的分发，意指分发给哪个监听事件来处理）

​``` /**
     * Pass the touch screen motion event down to the target view, or this
     * view if it is the target.
     *
     * @param event The motion event to be dispatched.
     * @return True if the event was handled by the view, false otherwise.
     */
    public boolean dispatchTouchEvent(MotionEvent event) {
        // If the event should be handled by accessibility focus first.
        if (event.isTargetAccessibilityFocus()) {
            // We don't have focus or no virtual descendant has it, do not handle the event.
            if (!isAccessibilityFocusedViewOrHost()) {
                return false;
            }
            // We have focus and got the event, then use normal event dispatch.
            event.setTargetAccessibilityFocus(false);
        }

        boolean result = false;

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }

        final int actionMasked = event.getActionMasked();
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            // Defensive cleanup for new gesture
            stopNestedScroll();
        }

        if (onFilterTouchEventForSecurity(event)) {
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                result = true;
            }

            if (!result && onTouchEvent(event)) {
                result = true;
            }
        }

        if (!result && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }

        // Clean up after nested scrolls if this is the end of a gesture;
        // also cancel it if we tried an ACTION_DOWN but we didn't want the rest
        // of the gesture.
        if (actionMasked == MotionEvent.ACTION_UP ||
                actionMasked == MotionEvent.ACTION_CANCEL ||
                (actionMasked == MotionEvent.ACTION_DOWN && !result)) {
            stopNestedScroll();
        }

        return result;
    }

检测常用手势

“轻触手势”指的是用户将一个或多个手指放在触摸屏上，并且您的应用会将这种轻触模式解读为特定手势（为手势赋归类，并赋予含义）。手势检测可相应地划为两个阶段：

收集轻触事件的相关数据
解读数据以确定其是否复合您的应用支持的任何手势的事件

支持库类：GestureDetectorCompat 和 MotionEventCompat, 这些类均包含在支持库中。以便与搭载在 Android 1.6 及更高版本的设备兼容

收集数据

手势会在用户首次轻触屏幕时开始，在系统跟踪用户手指的位置时继续，并在捕获到用户手指离开屏幕的最终事件时结束。在此互动过程中，传递给 onTouchEvent() 的 MotionEvent 会提供每次互动的详细信息。您的应用可以使用 MotionEvent 提供的数据确定是否发生了它所关注的手势。

常用的事件

public static final int ACTION_DOWN             = 0;
public static final int ACTION_UP               = 1;
public static final int ACTION_MOVE             = 2;
public static final int ACTION_CANCEL           = 3;
public static final int ACTION_OUTSIDE          = 4;
public static final int ACTION_POINTER_DOWN     = 5;
public static final int ACTION_POINTER_UP       = 6;

public class MainActivity extends Activity {
...
// This example shows an Activity, but you would use the same approach if
// you were subclassing a View.
@Override
public boolean onTouchEvent(MotionEvent event){

    int action = MotionEventCompat.getActionMasked(event);

    switch(action) {
        case (MotionEvent.ACTION_DOWN) :
            Log.d(DEBUG_TAG,"Action was DOWN");
            return true;
        case (MotionEvent.ACTION_MOVE) :
            Log.d(DEBUG_TAG,"Action was MOVE");
            return true;
        case (MotionEvent.ACTION_UP) :
            Log.d(DEBUG_TAG,"Action was UP");
            return true;
        case (MotionEvent.ACTION_CANCEL) :
            Log.d(DEBUG_TAG,"Action was CANCEL");
            return true;
        case (MotionEvent.ACTION_OUTSIDE) :
            Log.d(DEBUG_TAG,"Movement occurred outside bounds " +
                    "of current screen element");
            return true;
        default :
            return super.onTouchEvent(event);
    }
}

除了重写 onTouchEvent 外，您还可以设置 View.OnTouchListener . 这样可以避免创建现有 View 的子类

mView.setOnTouchListener(new OnTouchListener(){
     public boolean onTouch(View v,MotionEvent event){
     		//... respond to touch events
     		return true;
     }
})

检测手势

Android 提供了检测常用手势的 GestureDetector 类，该类支持的一些手势包括 onDown() / onLongPress() / onFling() 等等。您可以将 GestureDetector 与上述 onTouchEvent 结合使用（结合使用不是说一起使用，而是视具体情况使用）

Author：Dan

原文链接：https://passedby.github.io/2019/12/08/GestureDetector/

发表日期：December 8th 2019, 12:37:00 am

更新日期：January 6th 2020, 10:30:21 pm

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CATALOG

1. View的dispatchTouchEvent
2. 检测常用手势
1. 2.1. 收集数据
2. 2.2. 检测手势

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1	``` /**
2	* Pass the touch screen motion event down to the target view, or this
3	* view if it is the target.
4	*
5	* @param event The motion event to be dispatched.
6	* @return True if the event was handled by the view, false otherwise.
7	*/
8	public boolean dispatchTouchEvent(MotionEvent event) {
9	// If the event should be handled by accessibility focus first.
10	if (event.isTargetAccessibilityFocus()) {
11	// We don't have focus or no virtual descendant has it, do not handle the event.
12	if (!isAccessibilityFocusedViewOrHost()) {
13	return false;
14	}
15	// We have focus and got the event, then use normal event dispatch.
16	event.setTargetAccessibilityFocus(false);
17	}
18
19	boolean result = false;
20
21	if (mInputEventConsistencyVerifier != null) {
22	mInputEventConsistencyVerifier.onTouchEvent(event, 0);
23	}
24
25	final int actionMasked = event.getActionMasked();
26	if (actionMasked == MotionEvent.ACTION_DOWN) {
27	// Defensive cleanup for new gesture
28	stopNestedScroll();
29	}
30
31	if (onFilterTouchEventForSecurity(event)) {
32	if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
33	result = true;
34	}
35	//noinspection SimplifiableIfStatement
36	ListenerInfo li = mListenerInfo;
37	if (li != null && li.mOnTouchListener != null
38	&& (mViewFlags & ENABLED_MASK) == ENABLED
39	&& li.mOnTouchListener.onTouch(this, event)) {
40	result = true;
41	}
42
43	if (!result && onTouchEvent(event)) {
44	result = true;
45	}
46	}
47
48	if (!result && mInputEventConsistencyVerifier != null) {
49	mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
50	}
51
52	// Clean up after nested scrolls if this is the end of a gesture;
53	// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
54	// of the gesture.
55	if (actionMasked == MotionEvent.ACTION_UP \|\|
56	actionMasked == MotionEvent.ACTION_CANCEL \|\|
57	(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
58	stopNestedScroll();
59	}
60
61	return result;
62	}

1	public static final int ACTION_DOWN = 0;
2	public static final int ACTION_UP = 1;
3	public static final int ACTION_MOVE = 2;
4	public static final int ACTION_CANCEL = 3;
5	public static final int ACTION_OUTSIDE = 4;
6	public static final int ACTION_POINTER_DOWN = 5;
7	public static final int ACTION_POINTER_UP = 6;

1	public class MainActivity extends Activity {
2	...
3	// This example shows an Activity, but you would use the same approach if
4	// you were subclassing a View.
5	@Override
6	public boolean onTouchEvent(MotionEvent event){
7
8	int action = MotionEventCompat.getActionMasked(event);
9
10	switch(action) {
11	case (MotionEvent.ACTION_DOWN) :
12	Log.d(DEBUG_TAG,"Action was DOWN");
13	return true;
14	case (MotionEvent.ACTION_MOVE) :
15	Log.d(DEBUG_TAG,"Action was MOVE");
16	return true;
17	case (MotionEvent.ACTION_UP) :
18	Log.d(DEBUG_TAG,"Action was UP");
19	return true;
20	case (MotionEvent.ACTION_CANCEL) :
21	Log.d(DEBUG_TAG,"Action was CANCEL");
22	return true;
23	case (MotionEvent.ACTION_OUTSIDE) :
24	Log.d(DEBUG_TAG,"Movement occurred outside bounds " +
25	"of current screen element");
26	return true;
27	default :
28	return super.onTouchEvent(event);
29	}
30	}

1	mView.setOnTouchListener(new OnTouchListener(){
2	public boolean onTouch(View v,MotionEvent event){
3	//... respond to touch events
4	return true;
5	}
6	})