[Android进阶笔记]Android触摸事件的拦截机制

第一句话总是最重要的：
Android的拦截机制是一个自顶向下的事件分发与自底向上的事件响应机制
自顶向下的分发，就是我从View树的顶部开始向下分发事件
自底向上的响应，就是当事件传递到View树的底层，那么他就开始往上层层响应

View

View有2个方法 dispatchTouchEvent和onTouchEvent，源码如下：

3363        public boolean dispatchTouchEvent(MotionEvent event) {
3364            if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
3365                    mOnTouchListener.onTouch(this, event)) {
3366                return true;
3367            }
3368            return onTouchEvent(event);
3369        }

根据我上边的话，可以这么理解
dispatchTouchEvent 是用来事件分发的
onTouchEvent 是用来事件响应的

View的onTouchEvent源码

3792        public boolean onTouchEvent(MotionEvent event) {
3793            final int viewFlags = mViewFlags;
3794    
3795            if ((viewFlags & ENABLED_MASK) == DISABLED) {
3796                // A disabled view that is clickable still consumes the touch
3797                // events, it just doesn't respond to them.
3798                return (((viewFlags & CLICKABLE) == CLICKABLE ||
3799                        (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
3800            }
3801    
3802            if (mTouchDelegate != null) {
3803                if (mTouchDelegate.onTouchEvent(event)) {
3804                    return true;
3805                }
3806            }
3807    
3808            if (((viewFlags & CLICKABLE) == CLICKABLE ||
3809                    (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
3810                switch (event.getAction()) {
3811                    case MotionEvent.ACTION_UP:
3812                        if ((mPrivateFlags & PRESSED) != 0) {
3813                            // take focus if we don't have it already and we should in
3814                            // touch mode.
3815                            boolean focusTaken = false;
3816                            if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
3817                                focusTaken = requestFocus();
3818                            }
3819    
3820                            if (!mHasPerformedLongPress) {
3821                                // This is a tap, so remove the longpress check
3822                                if (mPendingCheckForLongPress != null) {
3823                                    removeCallbacks(mPendingCheckForLongPress);
3824                                }
3825    
3826                                // Only perform take click actions if we were in the pressed state
3827                                if (!focusTaken) {
3828                                    performClick();
3829                                }
3830                            }
3831    
3832                            if (mUnsetPressedState == null) {
3833                                mUnsetPressedState = new UnsetPressedState();
3834                            }
3835    
3836                            if (!post(mUnsetPressedState)) {
3837                                // If the post failed, unpress right now
3838                                mUnsetPressedState.run();
3839                            }
3840                        }
3841                        break;
3842    
3843                    case MotionEvent.ACTION_DOWN:
3844                        mPrivateFlags |= PRESSED;
3845                        refreshDrawableState();
3846                        if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
3847                            postCheckForLongClick();
3848                        }
3849                        break;
3850    
3851                    case MotionEvent.ACTION_CANCEL:
3852                        mPrivateFlags &= ~PRESSED;
3853                        refreshDrawableState();
3854                        break;
3855    
3856                    case MotionEvent.ACTION_MOVE:
3857                        final int x = (int) event.getX();
3858                        final int y = (int) event.getY();
3859    
3860                        // Be lenient about moving outside of buttons
3861                        int slop = ViewConfiguration.get(mContext).getScaledTouchSlop();
3862                        if ((x < 0 - slop) || (x >= getWidth() + slop) ||
3863                                (y < 0 - slop) || (y >= getHeight() + slop)) {
3864                            // Outside button
3865                            if ((mPrivateFlags & PRESSED) != 0) {
3866                                // Remove any future long press checks
3867                                if (mPendingCheckForLongPress != null) {
3868                                    removeCallbacks(mPendingCheckForLongPress);
3869                                }
3870    
3871                                // Need to switch from pressed to not pressed
3872                                mPrivateFlags &= ~PRESSED;
3873                                refreshDrawableState();
3874                            }
3875                        } else {
3876                            // Inside button
3877                            if ((mPrivateFlags & PRESSED) == 0) {
3878                                // Need to switch from not pressed to pressed
3879                                mPrivateFlags |= PRESSED;
3880                                refreshDrawableState();
3881                            }
3882                        }
3883                        break;
3884                }
3885                return true;
3886            }
3887    
3888            return false;

如果消化了点击事件的Action_Down，则返回true。若不消耗则返回false，返回false则对应的dispatchTouchEvent也返回fasle。则表示不拦截，继续向下分发事件。至于怎么继续向下分发呢，这个放在ViewGroup的事件分发机制来说。返回True 则表示这个View拦截这个事件，这样这个完整的点击事件（Action_Down到Up）都交给这个View来处理。

ViewGroup:
ViewGroup有三个方法dispatchTouchEvent、onInterceptTouchEvent和onTouchEvent
dispatchTouchEvent 先看看源码里的一小段，就能马上理解ViewGroup的分发机制了

if (child.dispatchTouchEvent(ev))  {
// Event handled, we have a target now.
    mMotionTarget = child;
return true;
}
// The event didn't get handled, try the next view.
// Don't reset the event's location, it's not
// necessary here.

依次调用ViewGroup里所包含的childView的dispatchTouchEvent，倘若该ViewGroup里的childView，有一个选择了拦截，那么这个ViewGroup的dispatchTouchEvent就返回True，而事件的传递也就会在拦截的childView的onTouchEvent 里停止。如果ViewGroup里所有的childView的dispatchTouchEvent都返回false，即子View都不拦截，那么就相当于这个点击事件已经传递到底了，只能逐层向上响应，即调用ViewGroup的onTouchEvent，直到某个ViewGroup的onTouchEvent返回True，否则一直向上响应，直至rootView。

废话一堆不如代码来的实在。请看例子！
先写2个自定义的ViewGroup，ViewGroupB代码同ViewGroupA，只是在三个事件拦截的方法里加了打印信息

package com.dongua.toucheventtest;


import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.RelativeLayout;

public class ViewGroupA extends RelativeLayout
{

public ViewGroupA(Context context, AttributeSet attrs, int defStyle)
    {
super(context, attrs, defStyle);
    }

public ViewGroupA(Context context)
    {
super(context);
    }

public ViewGroupA(Context context, AttributeSet attrs)
    {
super(context, attrs);
    }

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.i("click", "dispatchTouchEvent: ViewGroupA");
return super.dispatchTouchEvent(ev);
    }

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.i("click", "onInterceptTouchEvent: ViewGroupA");
return super.onInterceptTouchEvent(ev);
    }

@Override
public boolean onTouchEvent(MotionEvent event) {
        Log.i("click", "onTouchEvent: ViewGroupA");
return super.onTouchEvent(event);
    }


}

和一个自定义的View，这里先继承自TextView，因为TextView对点击事件是默认不响应，当然如果熟悉的话你继承一个Button，然后手动的在dispatchTouchEvent里返回fasle，使button不拦截也是ok 的~

package com.dongua.toucheventtest;

import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.TextView;

/**
 * Created by dongua on 2016/11/21.
 */
public class ViewC extends TextView {

public ViewC(Context context) {
super(context);
    }

public ViewC(Context context, AttributeSet attrs) {
super(context, attrs);
    }

public ViewC(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
    }


@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.i("click", "dispatchTouchEvent: ViewC");
return super.dispatchTouchEvent(ev);
    }


@Override
public boolean onTouchEvent(MotionEvent event) {
        Log.i("click", "onTouchEvent: ViewC");
return super.onTouchEvent(event);
    }
}

然后写一下布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.dongua.toucheventtest.MainActivity">

    <com.dongua.toucheventtest.ViewGroupA
        android:id="@+id/A"
        android:background="#0000ff"
        android:layout_width="200dp"
        android:layout_height="200dp">
        <com.dongua.toucheventtest.ViewGroupB
            android:id="@+id/B"
            android:background="#00ff00"
            android:layout_width="100dp"
            android:layout_height="100dp">
            <com.dongua.toucheventtest.ViewC
                android:id="@+id/C"
                android:background="#ff0000"
                android:layout_width="60dp"
                android:layout_height="60dp"
                android:text="View C 文本"/>

        </com.dongua.toucheventtest.ViewGroupB>
    </com.dongua.toucheventtest.ViewGroupA>
</LinearLayout>

布局如图：
[Android进阶笔记]Android触摸事件的拦截机制

然后试着点击一下TextView，看看打印消息

11-22 18:11:36.770 7412-7412/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewGroupA
11-22 18:11:36.770 7412-7412/com.dongua.toucheventtest I/click: onInterceptTouchEvent: ViewGroupA
11-22 18:11:36.770 7412-7412/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewGroupB
11-22 18:11:36.770 7412-7412/com.dongua.toucheventtest I/click: onInterceptTouchEvent: ViewGroupB
11-22 18:11:36.770 7412-7412/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewC
11-22 18:11:36.771 7412-7412/com.dongua.toucheventtest I/click: onTouchEvent: ViewC
11-22 18:11:36.771 7412-7412/com.dongua.toucheventtest I/click: onTouchEvent: ViewGroupB
11-22 18:11:36.771 7412-7412/com.dongua.toucheventtest I/click: onTouchEvent: ViewGroupA

这就很好理解了吧这次我们让TextView强行拦截，即ViewC的dispatchTouchEvent返回一个true，我们在看看打印的消息

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.i("click", "dispatchTouchEvent: ViewC");
return true;
    }

11-22 18:17:55.295 15435-15435/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewGroupA
11-22 18:17:55.295 15435-15435/com.dongua.toucheventtest I/click: onInterceptTouchEvent: ViewGroupA
11-22 18:17:55.296 15435-15435/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewGroupB
11-22 18:17:55.296 15435-15435/com.dongua.toucheventtest I/click: onInterceptTouchEvent: ViewGroupB
11-22 18:17:55.296 15435-15435/com.dongua.toucheventtest I/click: dispatchTouchEvent: ViewC

可以看到它只到ViewC的dispatchTouchEvent就结束了整个事件的拦截。因为ViewC我们手动设置了拦截，但ViewC继承自TextView，如果我们对onTouchEvent也强行返回true，那么就会执行到ViewC的onTouchEvent方法。
同理的大家可以试试在ViewGroupB的dispatchTouchEvent直接返回true，这就使得事件向下的传递到此为止。对onTouchEvent直接返回true，这就使得事件向上的响应到此为止。

例子就这些，大家想要熟悉的话肯定要自己去手动实现一遍，我觉得一定要在反复的看几遍文字的描述之后，再去写代码才能理解其中深意，盲目的堆代码只会让你的了解停留在表面。之前看完了老是不自己写一遍，结果一阵子之后又忘了，如果多做笔记，在做笔记的过程中无疑又是一次学习的过程。
以上为个人学习笔记，如有错误，请不吝赐教~

秒客网

[Android进阶笔记]Android触摸事件的拦截机制

View

View的onTouchEvent源码

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