【Android 事件分发】事件分发源码分析 ( ViewGroup 事件传递机制 三 )

for (int i = childrenCount - 1; i >= 0; i--) {

                        	// 获取索引
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            // 获取索引对应组件 
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);

    /**
     * Returns true if a child view can receive pointer events.
     * 判定控件是否可见 / 是否处于动画中 
     * @hide
     */
    private static boolean canViewReceivePointerEvents(@NonNull View child) {
        return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                || child.getAnimation() != null;
    }

    /**
     * Returns true if a child view contains the specified point when transformed
     * into its coordinate space.
     * Child must not be null.
     * 判定手指是否触摸到了组件 , 是否在组件区域范围内 
     * @hide
     */
    protected boolean isTransformedTouchPointInView(float x, float y, View child,
            PointF outLocalPoint) {
        // 获取当前坐标 
        final float[] point = getTempPoint();
        point[0] = x;
        point[1] = y;
        transformPointToViewLocal(point, child);
        final boolean isInView = child.pointInView(point[0], point[1]);
        if (isInView && outLocalPoint != null) {
            outLocalPoint.set(point[0], point[1]);
        }
        return isInView;
    }

							// X 控件范围 A , 如果手指按在 B 范围 , 不会触发 X 控件的事件 
							// 判定当前的组件是否可见 , 是否处于动画过程中 
							// ① canViewReceivePointerEvents 判定组件是否可见 , 会否处于动画 
							// ② isTransformedTouchPointInView 判定手指是否在控件上面 ; 
							// 上述两种情况 , 不触发事件 
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                // 不触发事件 
                                continue;
                            }

@UiThread
public abstract class ViewGroup extends View implements ViewParent, ViewManager {
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
		...
						// 倒序遍历 按照 Z 轴的上下顺序 , 排列好的组件 
						// 先遍历的 Z 轴方向上 , 放在最上面的组件 , 也就是顶层组件 
                        for (int i = childrenCount - 1; i >= 0; i--) {
                        	// 获取索引
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            // 获取索引对应组件 
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);

                            // If there is a view that has accessibility focus we want it
                            // to get the event first and if not handled we will perform a
                            // normal dispatch. We may do a double iteration but this is
                            // safer given the timeframe.
                            // 无障碍 辅助功能 
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

							// X 控件范围 A , 如果手指按在 B 范围 , 不会触发 X 控件的事件 
							// 判定当前的组件是否可见 , 是否处于动画过程中 
							// ① canViewReceivePointerEvents 判定组件是否可见 , 会否处于动画 
							// ② isTransformedTouchPointInView 判定手指是否在控件上面 ; 
							// 上述两种情况 , 不触发事件 
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                // 不触发事件 
                                continue;
                            }
							
							// 截止到此处 , 可以获取子组件进行操作   
		...
	}

    /**
     * Returns true if a child view can receive pointer events.
     * 判定控件是否可见 / 是否处于动画中 
     * @hide
     */
    private static boolean canViewReceivePointerEvents(@NonNull View child) {
        return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                || child.getAnimation() != null;
    }

    /**
     * Returns true if a child view contains the specified point when transformed
     * into its coordinate space.
     * Child must not be null.
     * 判定手指是否触摸到了组件 , 是否在组件区域范围内 
     * @hide
     */
    protected boolean isTransformedTouchPointInView(float x, float y, View child,
            PointF outLocalPoint) {
        // 获取当前坐标 
        final float[] point = getTempPoint();
        point[0] = x;
        point[1] = y;
        transformPointToViewLocal(point, child);
        final boolean isInView = child.pointInView(point[0], point[1]);
        if (isInView && outLocalPoint != null) {
            outLocalPoint.set(point[0], point[1]);
        }
        return isInView;
    }
}

newTouchTarget = getTouchTarget(child);

dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)

        // 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.
        // 处理取消状态 , 暂时不分析 ; 
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }

        // If the number of pointers is the same and we don't need to perform any fancy
        // irreversible transformations, then we can reuse the motion event for this
        // dispatch as long as we are careful to revert any changes we make.
        // Otherwise we need to make a copy.
        final MotionEvent transformedEvent;
        if (newPointerIdBits == oldPointerIdBits) {
            if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                	// 被遍历的 child 子组件为空 
                	// 调用父类的分发方法 
                    handled = super.dispatchTouchEvent(event);
                } else {
                	// 被遍历的 child 子组件不为空 
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;
                    event.offsetLocation(offsetX, offsetY);

					// 子组件分发触摸事件 
					// 此处调用的是 View 组件的 dispatchTouchEvent 方法 ; 
                    handled = child.dispatchTouchEvent(event);

                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }
            transformedEvent = MotionEvent.obtain(event);
        } else {
            transformedEvent = event.split(newPointerIdBits);
        }

@UiThread
public abstract class ViewGroup extends View implements ViewParent, ViewManager {
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
		...
							// 提取当前的子组件 
							// 第一次执行 getTouchTarget 代码时 , 是没有 mFirstTouchTarget 的
							// 此时第一次返回 null 
                            newTouchTarget = getTouchTarget(child);
                            // 该分支操作第一次不执行 
                            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);
                            // 正式开始分发触摸事件
                            // 处理以下两种情况 : 
                            // ① 情况一 : 子控件触摸事件返回 true 
                            // ② 情况二 : 子控件触摸事件返回 false 
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { 
		...
	}

    /**
     * Gets the touch target for specified child view.
     * Returns null if not found.
     * 
     */
    private TouchTarget getTouchTarget(@NonNull View child) {
    	// 判断 mFirstTouchTarget 中的 child 字段 , 是否是当前遍历的 子组件 View 
    	// 如果是 , 则返回该 TouchTarget 
    	// 如果不是 , 则返回空
        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            if (target.child == child) {
                return target;
            }
        }
        return null;
    }

    /**
     * Transforms a motion event into the coordinate space of a particular child view,
     * filters out irrelevant pointer ids, and overrides its action if necessary.
     * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
     * 该方法是正式分发触摸事件的方法 
     * 注意参数中传入了当前正在被遍历的 child 子组件 
     */
    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.
        // 处理取消状态 , 暂时不分析 ; 
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }

        // Calculate the number of pointers to deliver.
        final int oldPointerIdBits = event.getPointerIdBits();
        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

        // If for some reason we ended up in an inconsistent state where it looks like we
        // might produce a motion event with no pointers in it, then drop the event.
        if (newPointerIdBits == 0) {
            return false;
        }

        // If the number of pointers is the same and we don't need to perform any fancy
        // irreversible transformations, then we can reuse the motion event for this
        // dispatch as long as we are careful to revert any changes we make.
        // Otherwise we need to make a copy.
        final MotionEvent transformedEvent;
        if (newPointerIdBits == oldPointerIdBits) {
            if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                	// 被遍历的 child 子组件为空 
                	// 调用父类的分发方法 
                    handled = super.dispatchTouchEvent(event);
                } else {
                	// 被遍历的 child 子组件不为空 
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;
                    event.offsetLocation(offsetX, offsetY);

					// 子组件分发触摸事件 
					// 此处调用的是 View 组件的 dispatchTouchEvent 方法 ; 
                    handled = child.dispatchTouchEvent(event);

                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }
            transformedEvent = MotionEvent.obtain(event);
        } else {
            transformedEvent = event.split(newPointerIdBits);
        }

        // Perform any necessary transformations and dispatch.
        if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);
        } 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);
        }

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

@UiThread
public abstract class ViewGroup extends View implements ViewParent, ViewManager {

    // First touch target in the linked list of touch targets.
    private TouchTarget mFirstTouchTarget;

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
    	// 辅助功能 , 残疾人相关辅助 , 跨进程调用 无障碍 功能
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
        }

        // If the event targets the accessibility focused view and this is it, start
        // normal event dispatch. Maybe a descendant is what will handle the click.
        // 判断产生事件的目标组件是可访问性的 , 那么按照普通的事件分发进行处理 ; 
        // 可能由其子类处理点击事件 ; 
        // 判断当前是否正在使用 无障碍 相关功能产生事件 
        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
            ev.setTargetAccessibilityFocus(false);
        }

		// 是否按下操作 , 最终的对外返回结果 , 该方法的最终返回值 
        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Handle an initial down.
            // 判断是否是第一次按下 , 如果是第一次按下 , 则执行下面的业务逻辑 
            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);
                // 如果是第一次按下 , 那么重置触摸状态 
                resetTouchState();
            }

            // Check for interception.
            // 判定是否拦截 
            // 用于多点触控按下操作的判定 
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                // 判断是否需要拦截 , 可以使用 requestDisallowInterceptTouchEvent 方法进行设置
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                	// 进行事件拦截 
                	// 该 onInterceptTouchEvent 方法只返回是否进行事件拦截 , 返回一个布尔值 , 没有进行具体的事件拦截 
                	// 是否进行拦截 , 赋值给了 intercepted 局部变量 
                	// 该值决定是否进行拦截 
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                	// 不进行事件拦截 
                    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;
            }

            // If intercepted, start normal event dispatch. Also if there is already
            // a view that is handling the gesture, do normal event dispatch.
            if (intercepted || mFirstTouchTarget != null) {
                ev.setTargetAccessibilityFocus(false);
            }

            // Check for cancelation.
            // 检查是否取消操作 , 手指是否移除了组件便捷 ; 
            // 一般情况默认该值是 false ; 
            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;
            // 注意此处 newTouchTarget 为空 
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;

			// 此处判定 , 是否拦截 
			// 假定不取消 , 也不拦截 
			// canceled 和 intercepted 二者都是 false , 才不能拦截 ; 
            if (!canceled && !intercepted) {

                // If the event is targeting accessibility focus we give it to the
                // view that has accessibility focus and if it does not handle it
                // we clear the flag and dispatch the event to all children as usual.
                // We are looking up the accessibility focused host to avoid keeping
                // state since these events are very rare.
                // 无障碍 辅助功能 
                View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                        ? findChildWithAccessibilityFocus() : null;

				// 判断是否是按下操作 
                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    // 获取触摸索引值 
                    final int actionIndex = ev.getActionIndex(); // always 0 for down
                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                            : TouchTarget.ALL_POINTER_IDS;

                    // Clean up earlier touch targets for this pointer id in case they
                    // have become out of sync.
                    removePointersFromTouchTargets(idBitsToAssign);

					// 计算 ViewGroup 父容器下面有多少个子 View 组件 ; 
                    final int childrenCount = mChildrenCount;
                    // TouchTarget newTouchTarget = null; 在上面声明为空 , 此处肯定为 null ; 
                    // childrenCount 子组件个数不为 0 
                    // 如果子组件个数为 0 , 则不走下一段代码 , 如果子组件个数大于 0 , 则执行下一段代码 ; 
                    // 说明下面的代码块中处理的是 ViewGroup 中子组件的事件分发功能 ; 
                    if (newTouchTarget == null && childrenCount != 0) {
                    	// 获取单个手指的 x,y 坐标 
                        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.
                        // 子组件排序 , 按照 Z 轴排列的层级 , 从上到下进行排序 , 
                        // 控件会相互重叠 , Z 轴的排列次序上 , 
                        // 顶层的组件优先获取到触摸事件 
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
	
						// 倒序遍历 按照 Z 轴的上下顺序 , 排列好的组件 
						// 先遍历的 Z 轴方向上 , 放在最上面的组件 , 也就是顶层组件 
                        for (int i = childrenCount - 1; i >= 0; i--) {
                        	// 获取索引
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            // 获取索引对应组件 
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);

                            // If there is a view that has accessibility focus we want it
                            // to get the event first and if not handled we will perform a
                            // normal dispatch. We may do a double iteration but this is
                            // safer given the timeframe.
                            // 无障碍 辅助功能 
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

							// X 控件范围 A , 如果手指按在 B 范围 , 不会触发 X 控件的事件 
							// 判定当前的组件是否可见 , 是否处于动画过程中 
							// ① canViewReceivePointerEvents 判定组件是否可见 , 会否处于动画 
							// ② isTransformedTouchPointInView 判定手指是否在控件上面 ; 
							// 上述两种情况 , 不触发事件 
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                // 不触发事件 
                                continue;
                            }
							
							// 截止到此处 , 可以获取子组件进行操作   

							// 提取当前的子组件 
							// 第一次执行 getTouchTarget 代码时 , 是没有 mFirstTouchTarget 的
							// 此时第一次返回 null 
                            newTouchTarget = getTouchTarget(child);
                            // 该分支操作第一次不执行 
                            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);
                            // 正式开始分发触摸事件
                            // 处理以下两种情况 : 
                            // ① 情况一 : 子控件触摸事件返回 true 
                            // ② 情况二 : 子控件触摸事件返回 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);
                                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;
                    }
                }
            }

            // 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);
            } 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) {
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        if (cancelChild) {
                            if (predecessor == null) {
                                mFirstTouchTarget = next;
                            } else {
                                predecessor.next = next;
                            }
                            target.recycle();
                            target = next;
                            continue;
                        }
                    }
                    predecessor = target;
                    target = next;
                }
            }

            // Update list of touch targets for pointer up or cancel, if needed.
            if (canceled
                    || actionMasked == MotionEvent.ACTION_UP
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                resetTouchState();
            } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
                final int actionIndex = ev.getActionIndex();
                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
                removePointersFromTouchTargets(idBitsToRemove);
            }
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }

    @Override
    public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
		// disallowIntercept 存在一个默认值 , 如果值为默认值 , 直接退出 
        if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
            // We're already in this state, assume our ancestors are too
            return;
        }

		// 如果不是默认值 , 则进行相应更改 
		// 最终的值影响 mGroupFlags 是 true 还是 false 
        if (disallowIntercept) {
            mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
        } else {
            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        }

        // Pass it up to our parent
        if (mParent != null) {
            mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
        }
    }

    public boolean onInterceptTouchEvent(MotionEvent ev) {
    	// 该方法只返回是否进行事件拦截 , 返回一个布尔值 , 没有进行具体的事件拦截 
        if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
                && ev.getAction() == MotionEvent.ACTION_DOWN
                && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
                && isOnScrollbarThumb(ev.getX(), ev.getY())) {
            return true;
        }
        return false;
    }

    /**
     * Provide custom ordering of views in which the touch will be dispatched.
     * 按照事件传递的顺序进行组件排序 
     *
     * This is called within a tight loop, so you are not allowed to allocate objects, including
     * the return array. Instead, you should return a pre-allocated list that will be cleared
     * after the dispatch is finished.
     * @hide
     */
    public ArrayList<View> buildTouchDispatchChildList() {
        return buildOrderedChildList();
    }

    /**
     * Populates (and returns) mPreSortedChildren with a pre-ordered list of the View's children,
     * sorted first by Z, then by child drawing order (if applicable). This list must be cleared
     * after use to avoid leaking child Views.
     *
     * Uses a stable, insertion sort which is commonly O(n) for ViewGroups with very few elevated
     * children.
     */
    ArrayList<View> buildOrderedChildList() {
        final int childrenCount = mChildrenCount;
        if (childrenCount <= 1 || !hasChildWithZ()) return null;

        if (mPreSortedChildren == null) {
            mPreSortedChildren = new ArrayList<>(childrenCount);
        } else {
            // callers should clear, so clear shouldn't be necessary, but for safety...
            mPreSortedChildren.clear();
            mPreSortedChildren.ensureCapacity(childrenCount);
        }

        final boolean customOrder = isChildrenDrawingOrderEnabled();
        // 下面的组件排序的核心逻辑 
        // 获取当前所有组件的子组件的 Z 轴的深度 
        // 按照 Z 轴深度进行排序 
        // Z 轴方向上 , 对于事件传递 , 上面的组件优先级高于被覆盖的下面的组件优先级
        for (int i = 0; i < childrenCount; i++) {
            // add next child (in child order) to end of list
            final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
            final View nextChild = mChildren[childIndex];
            final float currentZ = nextChild.getZ();

            // insert ahead of any Views with greater Z
            // 计算当前遍历的组件应该被放到的索引位置
            int insertIndex = i;
            while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
                insertIndex--;
            }
            // 将当前遍历的组件插入到指定索引位置上 
            mPreSortedChildren.add(insertIndex, nextChild);
        }
        return mPreSortedChildren;
    }

	// 获取排序后的子组件的索引值
    private int getAndVerifyPreorderedIndex(int childrenCount, int i, boolean customOrder) {
        final int childIndex;
        if (customOrder) {
            final int childIndex1 = getChildDrawingOrder(childrenCount, i);
            if (childIndex1 >= childrenCount) {
                throw new IndexOutOfBoundsException("getChildDrawingOrder() "
                        + "returned invalid index " + childIndex1
                        + " (child count is " + childrenCount + ")");
            }
            childIndex = childIndex1;
        } else {
            childIndex = i;
        }
        return childIndex;
    }

	// 获取索引值对应的组件 
    private static View getAndVerifyPreorderedView(ArrayList<View> preorderedList, View[] children,
            int childIndex) {
        final View child;
        if (preorderedList != null) {
            child = preorderedList.get(childIndex);
            if (child == null) {
                throw new RuntimeException("Invalid preorderedList contained null child at index "
                        + childIndex);
            }
        } else {
            child = children[childIndex];
        }
        return child;
    }

    /**
     * Returns true if a child view can receive pointer events.
     * 判定控件是否可见 / 是否处于动画中 
     * @hide
     */
    private static boolean canViewReceivePointerEvents(@NonNull View child) {
        return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                || child.getAnimation() != null;
    }

    /**
     * Returns true if a child view contains the specified point when transformed
     * into its coordinate space.
     * Child must not be null.
     * 判定手指是否触摸到了组件 , 是否在组件区域范围内 
     * @hide
     */
    protected boolean isTransformedTouchPointInView(float x, float y, View child,
            PointF outLocalPoint) {
        // 获取当前坐标 
        final float[] point = getTempPoint();
        point[0] = x;
        point[1] = y;
        transformPointToViewLocal(point, child);
        final boolean isInView = child.pointInView(point[0], point[1]);
        if (isInView && outLocalPoint != null) {
            outLocalPoint.set(point[0], point[1]);
        }
        return isInView;
    }

    /**
     * Gets the touch target for specified child view.
     * Returns null if not found.
     * 
     */
    private TouchTarget getTouchTarget(@NonNull View child) {
    	// 判断 mFirstTouchTarget 中的 child 字段 , 是否是当前遍历的 子组件 View 
    	// 如果是 , 则返回该 TouchTarget 
    	// 如果不是 , 则返回空
        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            if (target.child == child) {
                return target;
            }
        }
        return null;
    }


    /**
     * Transforms a motion event into the coordinate space of a particular child view,
     * filters out irrelevant pointer ids, and overrides its action if necessary.
     * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
     * 该方法是正式分发触摸事件的方法 
     * 注意参数中传入了当前正在被遍历的 child 子组件 
     */
    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.
        // 处理取消状态 , 暂时不分析 ; 
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }

        // Calculate the number of pointers to deliver.
        final int oldPointerIdBits = event.getPointerIdBits();
        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

        // If for some reason we ended up in an inconsistent state where it looks like we
        // might produce a motion event with no pointers in it, then drop the event.
        if (newPointerIdBits == 0) {
            return false;
        }

        // If the number of pointers is the same and we don't need to perform any fancy
        // irreversible transformations, then we can reuse the motion event for this
        // dispatch as long as we are careful to revert any changes we make.
        // Otherwise we need to make a copy.
        final MotionEvent transformedEvent;
        if (newPointerIdBits == oldPointerIdBits) {
            if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                	// 被遍历的 child 子组件为空 
                	// 调用父类的分发方法 
                    handled = super.dispatchTouchEvent(event);
                } else {
                	// 被遍历的 child 子组件不为空 
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;
                    event.offsetLocation(offsetX, offsetY);

					// 子组件分发触摸事件 
					// 此处调用的是 View 组件的 dispatchTouchEvent 方法 ; 
                    handled = child.dispatchTouchEvent(event);

                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }
            transformedEvent = MotionEvent.obtain(event);
        } else {
            transformedEvent = event.split(newPointerIdBits);
        }

        // Perform any necessary transformations and dispatch.
        if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);
        } 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);
        }

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

}