Analyze Android event distribution mechanism according to popular interview questions (I)

（ One ） Overview of event distribution mechanism

Interview questions ： You know Android Event distribution mechanism ？ Please give us a general introduction

After the click event , First pass it on to Activity Of dispatchTouchEvent Method , This will call getWindow().superDispatchTouchEvent(ev), because PhoneWindow yes Android Window The only implementation class of , So it will pass PhoneWindow Medium mDecor.superDispatchTouchEvent(event) To call the parent class's dispatchTouchEvent Method ,mDecor yes DecorView, It is inherited from FrameLayout,FrameLayout Inherited from ViewGroup, because FrameLayout No rewriting dispatchTouchEvent Method , So in DecorView in ：

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

Will call ViewGroup Of dispatchTouchEvent Method , In this method onInterecptTouchEvent Method to determine whether to intercept , Without interception , Can traverse ViewGroup Child elements , Intron view Of dispatchTouchEvent Method , If son view Set up onTouchListener, Is executed onTouch Method , And according to onTouch The return value of the method is true still false Decide whether to execute onTouchEvent Method , If it is false, Then continue onTouchEvent Method , stay onTouchEvent Of ACTION_UP Judging from the events , If set onClickListener, Is executed onClick Method .

（ Two ） Source code analysis of high-frequency examination sites in the interview

Interview questions 1: If the father view China doesn't intercept down event , Intercept move,up event , In the child view Set up in requestDisallowInterceptTouchEvent(true);（ Ask the father view Don't intercept events ） After this sign , Son view To receive move,up Incident ？

analysis ： As the case may be ：（1） If son view consumption down event , And set up requestDisallowInterceptTouchEvent(true), Son view You can receive move,up event （2） Son view Don't consume down event , Will only receive down event , Will not receive move,up event . Here's why ：
（1） Son view consumption down event ： In our father view Don't rewrite dispatchTouchEvent Under the circumstances , Will use ViewGroup Of dispatchTouchEvent Method to distribute Events . Let's first understand the relevant parts of this problem through the source code ：

 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
    
    ...// Omitted code 
     // Handle an initial down.
            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();// If it is down When the incident came , Reset Touch The state of 
            }
            
   private void resetTouchState() {
    
        clearTouchTargets();
        resetCancelNextUpFlag(this);
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;// This state can be achieved by 
        //requestDisallowInterceptTouchEvent(true) This method sets ,
        // It will be reset here 
        mNestedScrollAxes = SCROLL_AXIS_NONE;
    }
    

The above code means when down Some status resets will be done when the event comes , because down Events are the beginning of event processing , So every time down What happened , Are equivalent to the beginning of an event processing , Relevant status needs to be reset .

Then it is to determine whether to intercept the source code of the event part ：

   @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
    
    ......// Omitted code 
// Check for interception.
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
    
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                // If the event is down Words of events ,disallowIntercept
                // It will always be false, because down The state of was reset at the beginning 
                //mFirstTouchTarget It's a linked list , It is used to save the information that the user clicks for the first time , If son view Consuming Events , Will assign a value to this variable , Otherwise, it will not 
                if (!disallowIntercept) {
    // therefore down When an event is , This judgment is bound to enter 
                    intercepted = onInterceptTouchEvent(ev);
                    // Finally, whether to intercept down The event depends on this , If it intercepts down event 
                   // onInterceptTouchEvent(ev) by true, that 
                    // Son view You won't receive the event , It's no use setting any flags , If we don't intercept ,down
                    // Events can be passed through , Son view Have the opportunity to deal with Events 
                    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;
            }

After the above code is executed, you will get a intercepted value , This value will directly affect the distribution of events , Also in viewGroup Of dispatchTouchEvent In the method ：

  if (!canceled && !intercepted) {
    // Without interception, you will enter the following code block 
....// Omitted code 

                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();// Ergodic view
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();// Check whether the sub view The order of drawing 
                        final View[] children = mChildren;
                        for (int i = childrenCount - 1; i >= 0; i--) {
    
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                                    // Get child view The index of the value 
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);
                            if (!child.canReceivePointerEvents()
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
    
                                    // If son view Can't receive the event or it is not within the click range , That is, I didn't click view On , End this cycle , Perform the next loop traversal 
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);
                            if (newTouchTarget != null) {
    
                       //newTouchTarget Not empty , Represents the current view The incident has been dealt with , End the cycle directly .
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }

                            resetCancelNextUpFlag(child);
                            //dispatchTransformedTouchEvent This method is to ask view Whether to deal with the incident , If processed, return true, Do not process the return false
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
    
                                // 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();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);// If view If you deal with the incident, you will pass addTouchTarget（） by mFirstTouchTarget assignment , follow-up move and up Events will not be intercepted .
                                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);
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }

                    if (newTouchTarget == null && mFirstTouchTarget != null) {
    
                        // Did not find a child to receive the event.
                        // Assign the pointer to the least recently added target.
                        newTouchTarget = mFirstTouchTarget;
                        while (newTouchTarget.next != null) {
    
                            newTouchTarget = newTouchTarget.next;
                        }
                        newTouchTarget.pointerIdBits |= idBitsToAssign;
                    }
                }
            }

So the situation 1 can be summarized as ：
（1）down After the event , First, the relevant state will be reset , such as mFirstTouchTarget,FLAG_DISALLOW_INTERCEPT etc.
（2） Then it will judge whether to intercept the event , If it is down event , It will directly enter the code block of whether to intercept , At this time, I will judge disallowIntercept This flag bit , But because of down event , This flag bit has been reset , therefore down In the event of , This flag bit has always been false, therefore down The event will enter the code block of judgment , from onInterceptTouchEvent Decide whether to intercept , If it intercepts , Son view You won't receive the event , What is discussed here is the case without interception ：

 if (!disallowIntercept) {
    
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                }

（3） Don't intercept down In the event of ,mFirstTouchTarget Will be assigned , So when move and up When the event comes , You will enter the following code block

 if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
    
                    //mFirstTouchTarget Not empty , Will go into the block of code to determine whether to intercept 
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;//disallowIntercept Set to true, Will not enter the following judgment code block , Therefore, the father will not be executed view Of onInterceptTouchEvent(ev) Method , So in the father view It's useless to set any interception in , It can't be implemented anyway . If it is set to false, Immediate son view Set up requestDisallowInterceptTouchEvent(false), Then it will enter the following code block , Whether to intercept or not depends on the parent view Of onInterceptTouchEvent(ev) The method decides 
                if (!disallowIntercept) {
    
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
    
                    intercepted = false;
                }
            }

Through the above source code analysis, we can know , In the father view Don't intercept down At the time of the event , Son view Set up requestDisallowInterceptTouchEvent(true), Then even father view Intercepted move and up Events are useless , Because it can't be executed by the father view Of onInterceptTouchEvent(ev) Method . If son view Set up requestDisallowInterceptTouchEvent(false) after , Then view Whether you can receive the event depends on your father view Of onInterceptTouchEvent Method processing logic , In this question is interception move and up event , So son view You will only receive one father view There is no interception down event , And one. cancel event . Will receive cancel The event is because move and up Events have been intercepted , It will not enter into the event distribution logic , That is, the father view Of ：

@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
    
    ......
       if (!canceled && !intercepted) {
    
       ......// Omit 
       }

In the child view In the case of consumption events （ Son view Of dispatchTouchEvent return true）

// Son view Consumer events , be mFirstTouchTarget Not empty , If son view Do not consume events , Then father view I will deal with the incident by myself 
 if (mFirstTouchTarget == null) {
    
                // No touch targets so treat this as an ordinary view.
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
    
            
                TouchTarget predecessor = null;
                TouchTarget target = mFirstTouchTarget;
                while (target != null) {
    
                    final TouchTarget next = target.next;
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
    
                        handled = true;
                    } else {
    
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;// Because of intercepting move and up event , So this cancelChild Would be true,
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
    
                            handled = true;
                        }
                   ......
            }
         private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
    
        final boolean handled;

        // Canceling motions is a special case. We don't need to perform any transformations
        // or filtering. The important part is the action, not the contents.
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
    
            event.setAction(MotionEvent.ACTION_CANCEL);
           // cancel It's from above cancelChild, There will be a ACTION_CANCEL event , So son view You will receive this event in the case of consumption event 
            if (child == null) {
    
                handled = super.dispatchTouchEvent(event);
            } else {
    
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }

So in self view In the case of consumption events , Father view Zhongruo move and up The event was intercepted , Then son view I'll get one cancel event .

Interview questions 2: If view Set up onTouchListener,onClickListener,onTouchEvent, Which one will be executed first ？
analysis ： In this place , When answering, many people may answer , Execute first onTouchListener Of onTouch Method . And then it's gone , There is nothing wrong with this answer on the surface , But it's easy to kill chatting . The interviewer doesn't know how to ask , I can only spit out three words , Why? ？ Based on my interview experience , It's hard to get the approval of the interviewer , Even if you answer correctly, which function to execute first , It will only make the interviewer feel that you are on the back of the test question , Although we often recite , But we need to clarify the principle , Try not to let the interviewer ask too many questions . Strive for an answer and answer all the questions he wants to ask .
Now let's see how to answer this question , The interviewer asks you this question , I just want to check your understanding of the event distribution mechanism . Let's look at the source code first ; space in between view Don't rewrite dispatchTouchEvent When , The event was distributed to view after , Will execute view Of dispatchTouchEvent（） Method .

 public boolean dispatchTouchEvent(MotionEvent event) {
    
 ...// Omit irrelevant code 
 // It's not hard to see from the code below , When the user sets the onTouchListener When monitoring , Will execute onTouch Method , according to onTouch() Method to determine whether to execute onTouchEvent.
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
    
                result = true;
            }
// If onTouch() Return to true, The following code will not be executed , because java The condition is short circuited , That is to say if //(！result) On the condition that false, With another condition “&&” If you connect , Then the latter condition will not be implemented , The whole judgment returns false
            if (!result && onTouchEvent(event)) {
    
                result = true;
            }

From the above code, we can know , If the user has set onTouchListener,onTouchevent and onClickListener Method , Then it will be executed first onTouchListener Medium onTouch Method , And then according to onTouch The return result of determines whether to execute onTouchEvent, hypothesis onTouch() return false, Carry on onTouchEvent(), Let's keep looking at the source code ：

 public boolean onTouchEvent(MotionEvent event) {
    
 ... Omitted code 
case MotionEvent.ACTION_UP:
       //...... Omitted code 
       // In the event of  ACTION_UP  Will carry out clickListener Of onClick（） Method . 
       if (mPerformClick == null) {
    
            mPerformClick = new PerformClick();
          }
        if (!post(mPerformClick)) {
    
            performClickInternal();
         }
                            }
                        }
    public boolean performClick() {
    
        // We still need to call this method to handle the cases where performClick() was called
        // externally, instead of through performClickInternal()
        notifyAutofillManagerOnClick();

        final boolean result;
        final ListenerInfo li = mListenerInfo;
        // If the user has set onClickListener Words . Will execute onClick（） Method 
        if (li != null && li.mOnClickListener != null) {
    
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
    
            result = false;
        }

        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

Sum up ： Our answer should be , If the user has set onTouchListener,onTouchEvent,onClickListener when , First of all, it will execute onTouchEvent Of onTouch Method , according to onTouch The return value of determines whether to execute onTouchEvent Method , If onTouch Method returns false, Then continue onTouchEvent, If onTouchEvent There is no direct return true perhaps false( This will cause the event to fail view Of OnTouchEvent Method , As a result, our settings will not be implemented onClickListener Medium onClick Method ), Then it will be implemented to view Of onTouchEvent Method , Then it will be executed ClickListener Of onClick Method

Length is too long , So the sliding conflict problem of event distribution mechanism is summarized in the next blog , It is also the way of problem plus source code analysis , I hope all friends can put forward valuable suggestions , Make progress together ～～～～, Together read the fucking sorce code!