Android 深入探究自定義view之事件的分發(fā)機(jī)制與處理詳解

更新時(shí)間：2021年11月02日 09:37:45 作者：臨木小屋

對(duì)于安卓程序員來說，自定義view簡(jiǎn)直不要太重要，畢竟有很多功能，譬如圓形頭像這些，用單純的原生非常難以實(shí)現(xiàn)，而用自定義view，簡(jiǎn)直分分鐘

題引

事件只有一個(gè)，多個(gè)人想要處理，處理的對(duì)象不是我們想給的對(duì)象就是事件沖突。

在這里插入圖片描述

如上圖，RecyclerView 的父布局是ViewPager，左右滑動(dòng)時(shí)沒問題，上下滑動(dòng)時(shí)RecyclerView好像沒收到滑動(dòng)事件一樣，無法達(dá)到我們預(yù)期的效果。我們的觸摸被封裝成MotionEvent事件傳遞，在多個(gè)層級(jí)中它是如何傳遞的呢？又是根據(jù)什么來確定哪個(gè)view處理這個(gè)事件的呢，咱們抽絲剝繭一步步揭開她的面紗！

Activity對(duì)事件的分發(fā)過程

追溯本源，尋找事件分發(fā)的開始。

當(dāng)一個(gè)點(diǎn)擊操作發(fā)生時(shí)，事件最先傳遞給當(dāng)前的Activity，由Activity的dispatchTouchEvent進(jìn)行分發(fā)

    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

這里的getWindow返回的window類只有一個(gè)實(shí)現(xiàn)，PhoneWindow

	private DecorView mDecor

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

我們繼續(xù)看 DecorView的superDispatchTouchEvent方法實(shí)現(xiàn)

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);
    }

DecorView 繼承于 ViewGroup。此時(shí)應(yīng)該理解了，Activity 的事件分發(fā)交給了 DecorView 處理，而 DecorView 又是什么

在這里插入圖片描述

DecorView是activity窗口的根視圖，是一個(gè)FrameLayout，DecorView內(nèi)部又分為兩部分，一部分是ActionBar，另一部分是ContentParent，即activity在setContentView對(duì)應(yīng)的布局。如此一來，事件分發(fā)從系統(tǒng)層面開始向我們寫的布局分發(fā)事件！

事件分發(fā)是一個(gè)遞歸的過程，主要涉及三個(gè)函數(shù)

dispatchTouchEvent
onInterceptTouchEvent
onTouchEvent

三者關(guān)系

public boolean dispatchTouchEvent(MotionEvent ev){
	boolean result = false;
	if(onInterceptTouchEvent(ev)){	//	如果攔截則交給自己的 onTouchEvent 處理事件
		result = onTouchEvent(ev);
	}else{
		//	如果不攔截，交給子布局分發(fā)，這是個(gè)層層遞歸過程
		result = chlid.dispatchTouchEvent(ev);
	}
	return result;
}

直接擼源碼是一件很痛苦的事情，多種可能的發(fā)生讓源碼可讀性很差。下面我們會(huì)從某一種特定邏輯下分析，這樣會(huì)清晰很多。每次只分析一種情境！

父布局?jǐn)r截的分發(fā)處理過程

父布局?jǐn)r截我們分兩步，ACTION_DOWN、ACTION_MOVE

ACTION_DOWN 事件

進(jìn)入 ViewGroup 的 dispatchTouchEvent 方法內(nèi)

   if (actionMasked == MotionEvent.ACTION_DOWN) {
       // Throw away all previous state when starting a new touch gesture.
       // The framework may have dropped the up or cancel event for the previous gesture
       // due to an app switch, ANR, or some other state change.
       cancelAndClearTouchTargets(ev);
       resetTouchState();
   }

因?yàn)槭?ACTION_DOWN 事件，先清空狀態(tài)，一個(gè)是TouchTarget的狀態(tài)，一個(gè)是 mGroupFlags。這個(gè)用不到繼續(xù)走

// Check for interception.
final boolean intercepted;
//	因?yàn)槭堑谝淮芜^來	 mFirstTouchTarget = null ，且是 ACTION_DOWN 事件，走入 if 內(nèi)
if (actionMasked == MotionEvent.ACTION_DOWN
         || mFirstTouchTarget != null) {
     //	咱們走的是父布局?jǐn)r截事件，子布局用尚方寶劍，disallowIntercept =false
     final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
     if (!disallowIntercept) {
     	//	咱們?cè)谶@攔截， intercepted = true
         intercepted = onInterceptTouchEvent(ev);
         ev.setAction(action); // restore action in case it was changed
     } else {
         intercepted = false;
     }
 } else {
     // There are no touch targets and this action is not an initial down
     // so this view group continues to intercept touches.
     intercepted = true;
 }

代碼注釋的比較全，這邊主要是判斷本view是否攔截，如果攔截 intercepted = true 。所以后面的遍歷子view分發(fā)都進(jìn)不去

//	intercepted = true , 進(jìn)不去
if (!canceled && !intercepted) {
	//	這是一個(gè)遍歷子 view 接盤的故事
	for (int i = childrenCount - 1; i >= 0; i--) {
	}
}

一直往下走

// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS);
}

符合這個(gè)條件，深入 dispatchTransformedTouchEvent 函數(shù)，第三個(gè)參數(shù)是 null

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
         View child, int desiredPointerIdBits) {
     
     final boolean handled;
     if (child == null) {
      	//	===== 執(zhí)行位置 ====
          handled = super.dispatchTouchEvent(transformedEvent);
      } else {
          final float offsetX = mScrollX - child.mLeft;
          final float offsetY = mScrollY - child.mTop;
          transformedEvent.offsetLocation(offsetX, offsetY);
          if (! child.hasIdentityMatrix()) {
              transformedEvent.transform(child.getInverseMatrix());
          }

          handled = child.dispatchTouchEvent(transformedEvent);
      }
}

第三個(gè)參數(shù)傳的是 null ，即 child = null 。調(diào)用 super 的 dispatchTouchEvent 。ViewGroup 的 super 即是 View。

handled = View.dispatchTouchEvent(event);

深入 View 的 dispatchTouchEvent 方法，主要處理邏輯是下面兩段代碼

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;
 }

我們可以得出結(jié)論：onTouch 比 onTouchEvent 優(yōu)先級(jí)高，如果onTouch 攔截事件則 onTouchEvent 無法接到事件。這也是為什么我們?cè)趏nTouch方法返回true后onClick事件失效的原因。onTouchEvent 的邏輯比較簡(jiǎn)單，此處不做分析

這里要說明一點(diǎn)，事件分發(fā)機(jī)制的分發(fā)其實(shí)有兩種含義。一是事件在不同view之間的分發(fā)，父布局到子布局的分發(fā)；二是事件在view中對(duì)不同監(jiān)聽的分發(fā)，onTouch、onClick、onLongClick 在分發(fā)時(shí)也是有順序的。

到這里父布局?jǐn)r截的DOWN事件算結(jié)束了，下面是MOVE事件，繼DOWN后的滑動(dòng)，這是個(gè)連續(xù)的過程

ACTION_MOVE 事件

手指點(diǎn)擊后開始滑動(dòng)，繼續(xù)分發(fā)move事件

final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
        || mFirstTouchTarget != null) {
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (!disallowIntercept) {
        intercepted = onInterceptTouchEvent(ev);
        ev.setAction(action); // restore action in case it was changed
    } else {
        intercepted = false;
    }
} else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

actionMasked = ACTION_MOVE ，mFirstTouchTarget = null ，直接走 else 模塊，即 intercepted = true。

//	intercepted = true , 進(jìn)不去
if (!canceled && !intercepted) {
	for (int i = childrenCount - 1; i >= 0; i--) {
	}
}

同樣的分發(fā)子view的模塊我們依舊進(jìn)不去

// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS);
}

到這里就跟之前的邏輯完全一樣了，至此父布局的攔截過程結(jié)束
總結(jié)：

ViewGroup 的 onTouchEvent 方法直接調(diào)用父類（View）的實(shí)現(xiàn)
父布局一旦攔截Down事件，后續(xù)的move事件都直接由父布局執(zhí)行

這么分析的好處是咱們的狀態(tài)是確定的，分析代碼不會(huì)有太多可能性搞亂邏輯，下面是父布局不攔截的情況下事件分發(fā)

父布局不攔截時(shí)的分發(fā)處理過程

父布局不攔截，咱們按照正常流程走一遍，還是按上面那個(gè)思路，先 down 后 move

ACTION_DOWN

進(jìn)入 GroupView 的 dispatchTouchEvent 方法后依舊西先是清空狀態(tài)，然后判斷當(dāng)前布局是否攔截

final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
        || mFirstTouchTarget != null) {
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (!disallowIntercept) {
        intercepted = onInterceptTouchEvent(ev);
        ev.setAction(action); // restore action in case it was changed
    } else {
        intercepted = false;
    }
} else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

咱們的設(shè)定是不攔截，所以 intercepted = false。下面是遍歷子view的代碼

final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
    final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
    //	逆序拿到一個(gè) child ，即從最上層的子view開始往內(nèi)層遍歷
    final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
    
    //	判斷觸點(diǎn)的位置是否在view的范圍之內(nèi)或者view是否在播放動(dòng)畫，如果都不滿足則直接遍歷下一個(gè)
    if (!child.canReceivePointerEvents()|| !isTransformedTouchPointInView(x, y, child, null)) {
        continue;
    }

    newTouchTarget = getTouchTarget(child);
    if (newTouchTarget != null) {
        // Child is already receiving touch within its bounds.
        // Give it the new pointer in addition to the ones it is handling.
        newTouchTarget.pointerIdBits |= idBitsToAssign;
        break;
    }
    resetCancelNextUpFlag(child);
    //	dispatchTransformedTouchEvent 函數(shù)是處理分發(fā)的函數(shù)，父布局處理用的也是這個(gè)
    //	如果子view消費(fèi)了事件則給標(biāo)志位賦值，并 break 結(jié)束循環(huán)，如果沒有消費(fèi)則繼續(xù)循環(huán)尋找分發(fā)
    if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {					注釋1
        // Child wants to receive touch within its bounds.
        mLastTouchDownTime = ev.getDownTime();
        if (preorderedList != null) {
            // childIndex points into presorted list, find original index
            for (int j = 0; j < childrenCount; j++) {
                if (children[childIndex] == mChildren[j]) {
                    mLastTouchDownIndex = j;
                    break;
                }
            }
        } else {
            mLastTouchDownIndex = childIndex;
        }
        mLastTouchDownX = ev.getX();
        mLastTouchDownY = ev.getY();
        //	如果子view消費(fèi)了事件則給 alreadyDispatchedToNewTouchTarget  和 mFirstTouchTarget 賦值
        //	保存 child
        newTouchTarget = addTouchTarget(child, idBitsToAssign);							注釋2
        alreadyDispatchedToNewTouchTarget = true;
        break;
    }

    // The accessibility focus didn't handle the event, so clear
    // the flag and do a normal dispatch to all children.
    ev.setTargetAccessibilityFocus(false);
}

分析上面干了啥

從最上層的子view開始往內(nèi)層遍歷
判斷當(dāng)前的view在位置上是否滿足觸點(diǎn)位置
調(diào)用 dispatchTransformedTouchEvent 判斷是否子view消費(fèi)了事件

如果消費(fèi)了事件則記錄 mFirstTouchTarget 和標(biāo)志位，并跳出循環(huán)

如果沒有沒有消費(fèi)事件則繼續(xù)循環(huán)

注釋1的邏輯 dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
         View child, int desiredPointerIdBits) {
     
     final boolean handled;
     if (child == null) {
          handled = super.dispatchTouchEvent(transformedEvent);
      } else {
          final float offsetX = mScrollX - child.mLeft;
          final float offsetY = mScrollY - child.mTop;
          transformedEvent.offsetLocation(offsetX, offsetY);
          if (! child.hasIdentityMatrix()) {
              transformedEvent.transform(child.getInverseMatrix());
          }
          //	===== 執(zhí)行位置 ====
          handled = child.dispatchTouchEvent(transformedEvent);
      }
}

這次過來 child != null ，調(diào)用的是 child.dispatchTouchEvent(event) 。child 可能是View，也可能是 ViewGroup。如果是 ViewGroup 又是一個(gè)遞歸的過程。層層的遞歸返回 handled 告訴父布局是否消費(fèi)了事件！

再看注釋2的邏輯

 private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
     final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
     //	此時(shí) mFirstTouchTarget = null
     target.next = mFirstTouchTarget;
     mFirstTouchTarget = target;
     return target;
 }

給 mFirstTouchTarget 賦值，下次 move 事件過來時(shí) mFirstTouchTarget 就是有值的了！！即

target.next = null
mFirstTouchTarget = newTouchTarget
保存 child 在 target 中

至此 ACTION_DOWN 事件結(jié)束

ACTION_MOVE

繼上面點(diǎn)擊后開始滑動(dòng)

if (actionMasked == MotionEvent.ACTION_DOWN) {
      cancelAndClearTouchTargets(ev);
      resetTouchState();
  }

move事件不會(huì)重置，繼續(xù)走

 if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null)

記得down事件中給mFirstTouchTarget 賦過值嘛，雖然不是down事件依舊可以進(jìn)入此方法。也就是說這里依舊會(huì)判斷父布局是否要攔截子view，這里也是以后咱們處理事件沖突的重點(diǎn)。當(dāng)前的邏輯是不攔截，所以 intercepted = false

if (actionMasked == MotionEvent.ACTION_DOWN
       || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
        || actionMasked == MotionEvent.ACTION_HOVER_MOVE)

只有ACTION_DOWN事件才會(huì)進(jìn)行分發(fā)，所以不會(huì)進(jìn)入遍歷子view的邏輯代碼！MOVE事件不會(huì)分發(fā)事件！

//	mFirstTouchTarget  有值，走else模塊
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);
} else {
    // Dispatch to touch targets, excluding the new touch target if we already
    // dispatched to it.  Cancel touch targets if necessary.
    TouchTarget predecessor = null;
    TouchTarget target = mFirstTouchTarget;
    while (target != null) {
        final TouchTarget next = target.next;
        //	alreadyDispatchedToNewTouchTarget  是 false
        if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
            handled = true;
        } else {
        	//	此處的結(jié)果是 false
            final boolean cancelChild = resetCancelNextUpFlag(target.child)|| intercepted;
            //	在這里被分發(fā)處理 child就是我們要分發(fā)的對(duì)象
            if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {
                handled = true;
            }
            if (cancelChild) {
                if (predecessor == null) {
                    mFirstTouchTarget = next;
                } else {
                    predecessor.next = next;
                }
                target.recycle();
                target = next;
                continue;
            }
        }
        predecessor = target;
        target = next;
    }
}

alreadyDispatchedToNewTouchTarget 在每次進(jìn)來時(shí)都會(huì)重置為 false ，最后又會(huì)調(diào)用 dispatchTransformedTouchEvent 處理分發(fā)

if (child == null) {
    handled = super.dispatchTouchEvent(transformedEvent);
} else {
    final float offsetX = mScrollX - child.mLeft;
    final float offsetY = mScrollY - child.mTop;
    transformedEvent.offsetLocation(offsetX, offsetY);
    if (! child.hasIdentityMatrix()) {
        transformedEvent.transform(child.getInverseMatrix());
    }
	//	遞歸調(diào)用它來分發(fā)
    handled = child.dispatchTouchEvent(transformedEvent);
}

至此move事件也結(jié)束，做個(gè)總結(jié)

DOWN 事件是事件分發(fā)，尋找接盤的 child 并保存在 mFirstTouchTarget 中
MOVE 事件雖然不需要遍歷尋找接盤的view，但還可以被ViewGroup攔截的（比如ViewPager包裹著RecyclerView，DOWN事件時(shí)被RecyclerView攔截，橫向滑動(dòng)時(shí)被拋棄，這時(shí)候ViewPager是可以攔截橫向滑動(dòng)接盤的）

解決沖突方案

滑動(dòng)沖突解決方案有兩種：內(nèi)部攔截、外部攔截。顧名思義，內(nèi)部攔截是在子View中寫邏輯攔截，外部攔截則是從父布局下手解決問題

都以ViewPager包裹RecyclerView滑動(dòng)沖突為例

外部攔截

public class BadViewPager extends ViewPager {

    private int mLastX, mLastY;

    public BadViewPager(@NonNull Context context) {
        super(context);
    }

    public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    // 外部攔截法：父容器處理沖突
    // 我想要把事件分發(fā)給誰就分發(fā)給誰
    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {

        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                mLastX = (int) event.getX();
                mLastY = (int) event.getY();
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastX;
                int deltaY = y - mLastY;
                if (Math.abs(deltaX) > Math.abs(deltaY)) {	//	橫向滑動(dòng)時(shí)攔截
                    return true;
                }
                break;
            }
            case MotionEvent.ACTION_UP: {
                break;
            }
            default:
                break;
        }

        return super.onInterceptTouchEvent(event);

    }
}

內(nèi)部攔截

ViewPager 代碼

public class BadViewPager extends ViewPager {

    private int mLastX, mLastY;

    public BadViewPager(@NonNull Context context) {
        super(context);
    }

    public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_DOWN){
            super.onInterceptTouchEvent(event);
            //	此處是重點(diǎn)
            return false;
        }
        return true;
    }
}

RecyclerView 代碼

public class MyListView extends ListView {

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

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

    // 內(nèi)部攔截法：子view處理事件沖突
    private int mLastX, mLastY;

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                getParent().requestDisallowInterceptTouchEvent(true);
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastX;
                int deltaY = y - mLastY;
                if (Math.abs(deltaX) > Math.abs(deltaY)) {
                    getParent().requestDisallowInterceptTouchEvent(false);
                }
                break;
            }
            case MotionEvent.ACTION_UP: {
                break;

            }
            default:
                break;
        }

        mLastX = x;
        mLastY = y;
        return super.dispatchTouchEvent(event);
    }
}

此處一定要注意，父布局在 ACTION_DOWN 時(shí)一定要返回false。原因如下：

當(dāng)分發(fā)DOWN事件時(shí)，執(zhí)行了 resetTouchState(); 函數(shù)

    private void resetTouchState() {
        clearTouchTargets();
        resetCancelNextUpFlag(this);
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        mNestedScrollAxes = SCROLL_AXIS_NONE;
    }

mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT

在判斷父布局?jǐn)r截時(shí)

final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
   intercepted = onInterceptTouchEvent(ev);
   ev.setAction(action); // restore action in case it was changed
} else {
    intercepted = false;
}

即 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT & FLAG_DISALLOW_INTERCEPT ！= 0 ==》false
使用 if 語句永遠(yuǎn)是true，在這里ViewPager會(huì)攔截事件，所以RecyclerView無法上下滑動(dòng)。所以內(nèi)部攔截時(shí)要修改父布局的 onInterceptTouchEvent 函數(shù)！

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