Android InputEventReceiver事件接收流程分析

本文基于Android 12。

InputEvent经过inputflinger读取后，通过Inputchannel发送到Java层的InputEventReceiver对象，输入事件和View的状态强相关，事件发送需要确定当前的焦点App，焦点Window（View），事件接收者是谁，所以InputEventReceiver对象也在View的创建流程中被初始化，ViewRootImpl中通过InputChannel参数实例化WindowInputEventReceiver，其覆盖了父类InputEventReceiver的onInputEvent()方法。

一、WindowInputEventReceiver

    // ViewRootImpl.java    
    	final class WindowInputEventReceiver extends InputEventReceiver {
            public WindowInputEventReceiver(InputChannel inputChannel, Looper looper) {
                super(inputChannel, looper);
            }
          
            @Override
            public void onInputEvent(InputEvent event) {
                    enqueueInputEvent(event, this, 0, true);
            }
        }

调用enqueueInputEvent方法。

    // ViewRootImpl.java 
        QueuedInputEvent mPendingInputEventHead;
    
        void enqueueInputEvent(InputEvent event,
                InputEventReceiver receiver, int flags, boolean processImmediately) {
            QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);
            QueuedInputEvent last = mPendingInputEventTail;
            if (last == null) {
                mPendingInputEventHead = q;
                mPendingInputEventTail = q;
            } else {
                last.mNext = q;
                mPendingInputEventTail = q;
            }
            mPendingInputEventCount += 1;
            doProcessInputEvents();
        }

enqueueInputEvent对事件通过mPendingInputEventHead变量维护一个事件队列，doProcessInputEvents()进行下一步处理。

    // ViewRootImpl.java 
    	void doProcessInputEvents() {
            while (mPendingInputEventHead != null) {
                QueuedInputEvent q = mPendingInputEventHead;
                mPendingInputEventHead = q.mNext;
                if (mPendingInputEventHead == null) {
                    mPendingInputEventTail = null;
                }
                q.mNext = null;
              	deliverInputEvent(q);
            }
        }

doProcessInputEvents对mPendingInputEventHead队列中所有事件进行分发。

    // ViewRootImpl.java     
    	private void deliverInputEvent(QueuedInputEvent q) {
            try {
              	InputStage stage;
                if (q.shouldSendToSynthesizer()) {
                    stage = mSyntheticInputStage;
                } else {
                    stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;
                }
                if (stage != null) {
                    handleWindowFocusChanged();
                    stage.deliver(q);
                } else {
                    finishInputEvent(q);
                }
            }
        }

将事件交给InputStage处理，InputStage是Android实现事件责任链的基类，deliver()方法中处理是否传递给下一个阶段：forward()，或者结束事件的传递：finish()。

二、InputState

    // ViewRootImpl.java 
    abstract class InputStage {        
        public final void deliver(QueuedInputEvent q) {
            if ((q.mFlags & QueuedInputEvent.FLAG_FINISHED) != 0) {
                forward(q);
            } else if (shouldDropInputEvent(q)) {
                finish(q, false);
            } else {
                traceEvent(q, Trace.TRACE_TAG_VIEW);
                final int result;
                try {
                    result = onProcess(q);
                } finally {
                    Trace.traceEnd(Trace.TRACE_TAG_VIEW);
                }
                apply(q, result);
            }
        }
    }

InputStage是一个抽象类，deliver方法中调用了onProcess(q)，子类覆盖onProcess方法实现自己的操作逻辑。

在ViewRootImpl的setView方法中定义了InputState的责任链：

    // ViewRootImpl.java 
    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
            int userId) {
            // 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);
            mFirstInputStage = nativePreImeStage;
            mFirstPostImeInputStage = earlyPostImeStage;
    }

从实例化的顺序可知事件传递阶段的流程：

nativePreImeStage->viewPreImeStage->imeStage->earlyPostImeStage->nativePostImeStage->viewPostImeStage->mSyntheticInputStage

关于各个InputStage的官方说明：

可参考文章：https://blog.csdn.net/vviccc/article/details/93377708

一般用户接触到最多的Activity和View属于ViewPostImeInputStage阶段。

三、ViewPostImeInputStage

    // ViewRootImpl.java 
    	final class ViewPostImeInputStage extends InputStage {
            @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);
                    }
                }
            }
        }

processKeyEvent处理KeyEvent，MotionEvent有两种情况，触屏操作的MotionEvent调用processPointerEvent处理，轨迹球操作的MotionEvent调用processTrackballEvent处理。

接着查看processPointerEvent方法：

    // ViewRootImpl.java 
    private int processPointerEvent(QueuedInputEvent q) {
        final MotionEvent event = (MotionEvent)q.mEvent;
        boolean handled = mView.dispatchPointerEvent(event);
        maybeUpdatePointerIcon(event);
        maybeUpdateTooltip(event);
        return handled ? FINISH_HANDLED : FORWARD;
    }

调用mView.dispatchPointerEvent(event)方法。

    // View.java    
    	@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
        public final boolean dispatchPointerEvent(MotionEvent event) {
            if (event.isTouchEvent()) {
                return dispatchTouchEvent(event);
            } else {
                return dispatchGenericMotionEvent(event);
            }
        }

终于到了熟悉的dispatchTouchEvent(event)方法。接下来就是熟悉onInterceptTouchEvent()，onTouchEvent()。