Android事件傳遞機制

最近在做一個圖片，文字拖拽縮放的控件，所以研究了下android的事件傳遞機制，記錄在此。

一、Activity——dispatchTouchEvent和onTouchEvent

    先來看下Activity裏的dispatchTouchEvent源碼

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()返回的是一個PhoneWindow對象，執行了superDispatchTouchEvent(ev)方法，如果返回true表示事件被消費掉。

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

mDecor是一個DecorView對象，繼承自FrameLayout，它是一個Activity的root view，通過super.dispatchTouchEvent會把事件分發給各個子view及Activity onCreate方法中setContentView裏賦值的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代碼比較多，不全貼了，上邊的代碼是檢查是否攔截事件的，onInterceptTouchEvent(ev)裏代碼很簡單就返回了個false，disallowIntercept用來控制是否禁用攔截事件，可以通過requestDisallowInterceptTouchEvent方法改變值。如果事件沒被攔截會執行下面的代碼：

                    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;
                            }
                        }
                    }

遍歷了所有子view，找到點擊的view執行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依次調用了子view的dispatchTouchEvent方法，當子view的dispatchTouchEvent方法返回true時事件到此結束，返回false時會去執行super.dispatchTouchEvent方法，也就是父類View的dispatchTouchEvent方法，下面介紹View的dispatchTouchEvent方法；

三、View——dispatchTouchEvent、onTouchEvent

先來看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;
    }

判斷了mOnTouchListener是否爲空和它的返回值，如果不爲空並且返回true，dispatchTouchEvent直接返回true，否則執行onTouchEvent(event)，onTouchEvent的默認返回值與CLICKABLE和LONG_CLICKABLE屬性有關，例如button默認返回true，而textview默認返回false。如果onTouchEvent返回true及dispatchTouchEvent返回true時間傳遞結束，否則會執行父佈局的onTouchEvent。

第一次寫的有點亂