在Android 11 输入系统之InputDispatcher和应用窗口建立联系一文中介绍到，当InputDispatcher写入数据后，客户端这边就会调用handleEvent方法接收数据

//frameworks\base\core\jni\android_view_InputEventReceiver.cpp
int NativeInputEventReceiver::handleEvent(int receiveFd, int events, void* data) {
   //省略

    if (events & ALOOPER_EVENT_INPUT) {//之前构造数据的时候，events为ALOOPER_EVENT_INPUT
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        status_t status = consumeEvents(env, false /*consumeBatches*/, -1, nullptr);
        mMessageQueue->raiseAndClearException(env, "handleReceiveCallback");
        return status == OK || status == NO_MEMORY ? 1 : 0;
    }
    
  //省略

继续调用consumeEvents处理

//frameworks\base\core\jni\android_view_InputEventReceiver.cpp
status_t NativeInputEventReceiver::consumeEvents(JNIEnv* env,
        bool consumeBatches, nsecs_t frameTime, bool* outConsumedBatch) {
	//省略
	for (;;) {
        uint32_t seq;
        InputEvent* inputEvent;

        status_t status = mInputConsumer.consume(&mInputEventFactory,
                consumeBatches, frameTime, &seq, &inputEvent);//1
		//省略
			case AINPUT_EVENT_TYPE_MOTION: {
        
                MotionEvent* motionEvent = static_cast<MotionEvent*>(inputEvent);//使用inputEvent构造MotionEvent对象
                if ((motionEvent->getAction() & AMOTION_EVENT_ACTION_MOVE) && outConsumedBatch) {
                    *outConsumedBatch = true;
                }
                inputEventObj = android_view_MotionEvent_obtainAsCopy(env, motionEvent);//创建java层的MotionEvent对象，并将该对象的mNativePtr指向c++的MotionEvent对象
                break;
            }

	//省略
                env->CallVoidMethod(receiverObj.get(),
                        gInputEventReceiverClassInfo.dispatchInputEvent, seq, inputEventObj);//2
       
	//省略
}

注释1处接收InputDispatcher发过来的数据，并将数据封装成InputEvent对象，注释2处通过JNI调用InputEventReceiver的dispatchInputEvent方法
先来看一下如何接收数据的

//frameworks\native\libs\input\InputTransport.cpp
status_t InputConsumer::consume(InputEventFactoryInterface* factory, bool consumeBatches,
                                nsecs_t frameTime, uint32_t* outSeq, InputEvent** outEvent) {
	//省略
	while (!*outEvent) {
        if (mMsgDeferred) {
            // mMsg contains a valid input message from the previous call to consume
            // that has not yet been processed.
            mMsgDeferred = false;
        } else {
            // Receive a fresh message.
            status_t result = mChannel->receiveMessage(&mMsg);//1

		//省略
	switch (mMsg.header.type) {
		case InputMessage::Type::MOTION: {
			//省略
			 	MotionEvent* motionEvent = factory->createMotionEvent();
                if (!motionEvent) return NO_MEMORY;

                updateTouchState(mMsg);
                initializeMotionEvent(motionEvent, &mMsg);//2
                *outSeq = mMsg.body.motion.seq;
                *outEvent = motionEvent;
				//省略
                break;
		}

}

注释1处接收数据，接收到的数据是InputMessage对象。注释2处根据读取到的InputMessage，创建motionEvent对象

//frameworks\native\libs\input\InputTransport.cpp
status_t InputChannel::receiveMessage(InputMessage* msg) {
    ssize_t nRead;
    do {
        nRead = ::recv(mFd.get(), msg, sizeof(InputMessage), MSG_DONTWAIT);//读fd
    } while (nRead == -1 && errno == EINTR);

consume方法得到数据并将数据封装成motionEvent对象后，回到consumeEvents方法，继续调用InputEventReceiver的dispatchInputEvent方法

//frameworks\base\core\java\android\view\InputEventReceiver.java
private void dispatchInputEvent(int seq, InputEvent event) {
        mSeqMap.put(event.getSequenceNumber(), seq);
        onInputEvent(event);
}

调用onInputEvent方法，WindowInputEventReceiver继承InputEventReceiver，调用WindowInputEventReceiver的onInputEvent方法

//frameworks\base\core\java\android\view\ViewRootImpl.java
final class WindowInputEventReceiver extends InputEventReceiver {
        public WindowInputEventReceiver(InputChannel inputChannel, Looper looper) {
            super(inputChannel, looper);
        }

        @Override
        public void onInputEvent(InputEvent event) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "processInputEventForCompatibility");
            //省略
            if (processedEvents != null) {
                //省略
            } else {
                enqueueInputEvent(event, this, 0, true);//注意第二个参数传入的是当前对象
            }
        }

enqueueInputEvent

//frameworks\base\core\java\android\view\ViewRootImpl.java
void enqueueInputEvent(InputEvent event,
            InputEventReceiver receiver, int flags, boolean processImmediately) {
        QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);
       	//省略
        if (processImmediately) {//processImmediately传进来的是true
            doProcessInputEvents();
        } else {
            scheduleProcessInputEvents();
        }
    }

doProcessInputEvents

//frameworks\base\core\java\android\view\ViewRootImpl.java
void doProcessInputEvents() {
        // Deliver all pending input events in the queue.
        while (mPendingInputEventHead != null) {
           
			//省略
            deliverInputEvent(q);
        }

       //省略
    }

deliverInputEvent

//frameworks\base\core\java\android\view\ViewRootImpl.java
private void deliverInputEvent(QueuedInputEvent q) {
        //省略
        try {
            //省略
            InputStage stage;
            if (q.shouldSendToSynthesizer()) {
                stage = mSyntheticInputStage;
            } else {
                stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;//1
            }

           //省略
            if (stage != null) {
                handleWindowFocusChanged();
                stage.deliver(q);//2
            } else {
                finishInputEvent(q);
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

注释1处设置InputStage，对于触摸事件，默认是忽略输入法的，所以stage 为mFirstPostImeInputStage 对象。注释2处 调用mFirstPostImeInputStage 的deliver方法。
系统中有多个InputStage组成的一个链表，在setView方法中设置的

//frameworks\base\core\java\android\view\ViewRootImpl.java
// Set up the input pipeline.
CharSequence counterSuffix = attrs.getTitle();
mSyntheticInputStage = new SyntheticInputStage();
InputStage viewPostImeStage = new ViewPostImeInputStage(mSyntheticInputStage);
InputStage nativePostImeStage = new NativePostImeInputStage(viewPostImeStage,
                        "aq:native-post-ime:" + counterSuffix);
InputStage earlyPostImeStage = new EarlyPostImeInputStage(nativePostImeStage);
InputStage imeStage = new ImeInputStage(earlyPostImeStage,
                        "aq:ime:" + counterSuffix);
InputStage viewPreImeStage = new ViewPreImeInputStage(imeStage);
InputStage nativePreImeStage = new NativePreImeInputStage(viewPreImeStage,
                        "aq:native-pre-ime:" + counterSuffix);

deliver方法的原理就是输入事件会经过这些InputStage依次处理（调用onProcess方法），如果事件已经被上一个消费处理了，后面的stage就不会处理了。触摸事件会传递到ViewPostImeInputStage中处理

//frameworks\base\core\java\android\view\ViewRootImpl.java
final class ViewPostImeInputStage extends InputStage {
        public ViewPostImeInputStage(InputStage next) {
            super(next);
        }

        @Override
        protected int onProcess(QueuedInputEvent q) {
            if (q.mEvent instanceof KeyEvent) {
                return processKeyEvent(q);
            } else {
                final int source = q.mEvent.getSource();
                if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
                    return processPointerEvent(q);
                } else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
                    return processTrackballEvent(q);
                } else {
                    return processGenericMotionEvent(q);
                }
            }
        }

对于触摸事件，调用processPointerEvent继续处理

//frameworks\base\core\java\android\view\ViewRootImpl.java
private int processPointerEvent(QueuedInputEvent q) {
            final MotionEvent event = (MotionEvent)q.mEvent;

            mAttachInfo.mUnbufferedDispatchRequested = false;
            mAttachInfo.mHandlingPointerEvent = true;
            boolean handled = mView.dispatchPointerEvent(event);//1
            //省略
            return handled ? FINISH_HANDLED : FORWARD;
}

主要是调用mView的dispatchPointerEvent方法，这里的mView是DecorView,DecorView中没有实现该方法，在其父类View中实现

//frameworks\base\core\java\android\view\View.java
@UnsupportedAppUsage
    public final boolean dispatchPointerEvent(MotionEvent event) {
        if (event.isTouchEvent()) {
            return dispatchTouchEvent(event);
        } else {
            return dispatchGenericMotionEvent(event);
        }
    }

又回到DecorView的dispatchTouchEvent方法

//frameworks\base\core\java\com\android\internal\policy\DecorView.java
 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        final Window.Callback cb = mWindow.getCallback();
        return cb != null && !mWindow.isDestroyed() && mFeatureId < 0
                ? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev);
    }

这里的callback就是Activity对象，调用Activity的dispatchTouchEvent方法。

//frameworks/base/core/java/android/app/Activity.java
  public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {//1
            return true;
        }
        return onTouchEvent(ev);//2
    }

注释1处调用getWindow的superDispatchTouchEvent方法，getWindow返回的是一个PhoneWindow对象。注意返回值，如果返回ture的话，表明消费事件，注释2处Activity的onTouchEvent方法就不会执行。反之返回false的话使用onTouchEvent进行兜底，onTouchEvent如果是返回true，后面的InputStage就不会处理了，返回false则表明继续交给后面的InputStage处理

//frameworks/base/core/java/com/android/internal/policy/PhoneWindow.java
@Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

又继续调用到DecorView的superDispatchTouchEvent方法

//frameworks/base/core/java/com/android/internal/policy/DecorView.java
public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);
    }

调用其父类ViewGroup的dispatchTouchEvent方法。在分析这个方法之前，先总结下事件是如何分发到ViewGroup的

事件是由DecorView分发给Activity，然后分发给window，最后又回到DecorView,再由DecorView分发给ViewGroup的。
ViewGroup接收到事件后，接下来就是将事件分发给具体的view了
ViewGroup事件分发

//frameworks/base/core/java/android/view/ViewGroup.java
@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
      //省略
        boolean handled = false;//表明是否消费事件
        if (onFilterTouchEventForSecurity(ev)) {//是否符合安全策略
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Check for interception.
            final boolean intercepted;//是否拦截事件
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;//调用requestDisallowInterceptTouchEvent这个方法设置不允许拦截，
                if (!disallowIntercept) {
                    intercepted = onInterceptTouchEvent(ev);//根据返回值判断是否允许拦截
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;//默认是不拦截
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                intercepted = true;//如果第一次的事件不是down的话，直接拦截
            }
		//省略
       
            // Check for cancelation.
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed.
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;//比较重要的参数
            if (!canceled && !intercepted) {//不拦截也不是取消事件的话进入

            //省略

                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    //省略
                    final int childrenCount = mChildrenCount;
                    if (newTouchTarget == null && childrenCount != 0) {
                        final float x = ev.getX(actionIndex);
                        final float y = ev.getY(actionIndex);
                        // Find a child that can receive the event.
                        // Scan children from front to back.
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        for (int i = childrenCount - 1; i >= 0; i--) {//遍历子iew
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);
							//省略
							
							//如果子view不能接收事件或者触摸点不在该view上的话，忽略这个view
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);//取出view的TouchTarget
                           
							//忽略
                            resetCancelNextUpFlag(child);
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {//开始分发并处理了
                                //省略
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);//进入这里表示子view消费了事件，就会设置view的TouchTarget链表，mFirstTouchTarget就不为空
                                alreadyDispatchedToNewTouchTarget = true;//设为true
                                break;
                            }
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }
					//省略
                }
            }

            // 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);//注意第三个参数为null
            } 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;
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {//表明子view消费了事件
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;//判断是否是取消事件
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                       //省略
        return handled;
    }

首先就是看看是不是需要拦截事件，判断是否通过requestDisallowInterceptTouchEvent这个方法，设置了ViewGroup不允许拦截，如果没有设置，再判断onInterceptTouchEvent的返回值，返回flase不表示不拦截。如果没有拦截则会遍历子view，依次使用dispatchTransformedTouchEvent处理，而如果拦截了话，也是通过dispatchTransformedTouchEvent处理，只不过传入的参数中，第3个参数为null
来看一下dispatchTransformedTouchEvent这个方法

//frameworks/base/core/java/android/view/ViewGroup.java
 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);//代表取消事件的话，将action设置为ACTION_CANCEL
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);//又设置回来
            return handled;
        }

     //省略

        // Perform any necessary transformations and dispatch.
        if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);//如果第三个参数传入的是空，则调用自己父类的dispatchTouchEvent方法处理
        } 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);//继续分发给子view处理
        }

        // Done.
        transformedEvent.recycle();
        return handled;
    }

dispatchTransformedTouchEvent的含义是如果child是ViewGroup的话，就继续调用ViewGroup的dispatchTouchEvent方法继续向下分发，如果child是view的话，则调用view的dispatchTouchEvent来处理事件。如果ViewGroup拦截了事件或者ViewGroup的孩子没有消费事件的话，也会调用View的dispatchTouchEvent来处理事件。来看一下view的dispatchTouchEvent方法

//frameworks/base/core/java/android/view/View.java
public boolean dispatchTouchEvent(MotionEvent event) {
        //省略

        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)) {//设置过OnTouchListener优先调用
                result = true;
            }

            if (!result && onTouchEvent(event)) {//调用onTouchEvent方法
                result = true;
            }
        }
	//省略

        return result;
    }

对于事件的处理主要是判断view是不是设置过OnTouchListener，如果设置过，则调用其onTouch方法。如果OnTouch返回true的话，表示事件在这里被消费，后面的onTouchEvent就不会被调用。如果没有设置过OnTouchListener或者设置过，但是OnTouch返回false，则onTouchEvent会被调用。

上面的几个方法内容比较多，理解起来也比较费劲，用一张图总结下ViewGroup的事件分发流程

总结

• 可以通过重写ViewGroup的onInterceptTouchEvent方法来实现对事件的拦截
• 可以通过调用requestDisallowInterceptTouchEvent来禁止ViewGroup对事件拦截，这个优先级更高
• 当事件都没有被View或者ViewGroup消费的话，使用Activity的onTouchEvent进行兜底
• UP和MOVE 事件并不会重新寻找子view，而是直接分发给接收DOWN事件的view