Android事件传递机制

最近在做一个图片，文字拖拽缩放的控件，所以研究了下android的事件传递机制，记录在此。

一、Activity——dispatchTouchEvent和onTouchEvent

    先来看下Activity里的dispatchTouchEvent源码

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

getWindow().superDispatchTouchEvent(ev)，getWindow()返回的是一个PhoneWindow对象，执行了superDispatchTouchEvent(ev)方法，如果返回true表示事件被消费掉。

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

mDecor是一个DecorView对象，继承自FrameLayout，它是一个Activity的root view，通过super.dispatchTouchEvent会把事件分发给各个子view及Activity onCreate方法中setContentView里赋值的view。

二、ViewGroup——dispatchTouchEvent、onInterceptTouchEvent和onTouchEvent

            // Check for interception.
            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;
            }

dispatchTouchEvent代码比较多，不全贴了，上边的代码是检查是否拦截事件的，onInterceptTouchEvent(ev)里代码很简单就返回了个false，disallowIntercept用来控制是否禁用拦截事件，可以通过requestDisallowInterceptTouchEvent方法改变值。如果事件没被拦截会执行下面的代码：

                    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 View[] children = mChildren;

                        final boolean customOrder = isChildrenDrawingOrderEnabled();
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder ?
                                    getChildDrawingOrder(childrenCount, i) : i;
                            final View child = children[childIndex];
                            if (!canViewReceivePointerEvents(child)
                                    || !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);
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                mLastTouchDownTime = ev.getDownTime();
                                mLastTouchDownIndex = childIndex;
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                    }

遍历了所有子view，找到点击的view执行dispatchTransformedTouchEvent方法：

if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                    handled = super.dispatchTouchEvent(event);
                } else {
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;
                    event.offsetLocation(offsetX, offsetY);

                    handled = child.dispatchTouchEvent(event);

                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }

可以看到child.dispatchTouchEvent(event)，所以ViewGroup的dispatchTouchEvent依次调用了子view的dispatchTouchEvent方法，当子view的dispatchTouchEvent方法返回true时事件到此结束，返回false时会去执行super.dispatchTouchEvent方法，也就是父类View的dispatchTouchEvent方法，下面介绍View的dispatchTouchEvent方法；

三、View——dispatchTouchEvent、onTouchEvent

先来看dispatchTouchEvent：

public boolean dispatchTouchEvent(MotionEvent event) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }

        if (onFilterTouchEventForSecurity(event)) {
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                return true;
            }

            if (onTouchEvent(event)) {
                return true;
            }
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        return false;
    }

判断了mOnTouchListener是否为空和它的返回值，如果不为空并且返回true，dispatchTouchEvent直接返回true，否则执行onTouchEvent(event)，onTouchEvent的默认返回值与CLICKABLE和LONG_CLICKABLE属性有关，例如button默认返回true，而textview默认返回false。如果onTouchEvent返回true及dispatchTouchEvent返回true时间传递结束，否则会执行父布局的onTouchEvent。

第一次写的有点乱

版权声明：本文为博主原创文章，未经博主允许不得转载。