在公司的项目中主界面用了侧滑菜单栏(DrawerLayout)，类似滴滴打车，当然还有很多其他的软件都使用这样的布局。用本篇对DrawerLayout一些学习进行记录。
Drawable出现了事件穿透
1.界面布局
类似滴滴打车的侧滑界面，其中有一部分是空白区域，当点击空白区域，会将点击事件传递到被遮挡的主布局上，现象：如果是类似滴滴，主界面是地图的话，手指在侧滑栏空白区域移动，主界面地图可以拖动。
2.解决方法
在侧滑栏的根布局添加android:clickable=”true” 属性。代码示例如下：
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical"><!-- 主界面布局--><FrameLayout android:layout_width="match_parent"android:layout_height="match_parent"></FrameLayout><!-- 侧滑栏布局,需要将android:clickable属性设置为true --><LinearLayout android:layout_width="240dp"android:layout_height="match_parent"android:gravity="start|left"android:clickable="true"></LinearLayout>
</android.support.v4.widget.DrawerLayout>3.从Android事件分发机制来分析
3.1 点击事件传递规则
public boolean dispatchTouchEvent(MotionEvent ev) {boolean consume = false;if (onInterceptTouchEvent(ev)) {consume = onTouchEvent(ev);} else {consume = child.dispatchTouchEvent(ev);}return consume;
}ViewGroup会调用onInterceptTouchEvent方法进行拦截，如果被拦截则调用自身的onTouchEvent方法进行处理，如果不拦截则交由子元素进行后续分发/处理。
一个点击事件产生后，传递规则:Activity->Window->View，如果View的onTouchEvent方法返回false，则它的父容器的onTouchEvent方法会被调用，以此类推直到返回到Activity的onTouchEvent方法。
3.2 源码分析
Activity的dispatchTouchEvent方法如下：
/*** Called to process touch screen events. You can override this to* intercept all touch screen events before they are dispatched to the* window. Be sure to call this implementation for touch screen events* that should be handled normally.** @param ev The touch screen event.** @return boolean Return true if this event was consumed.*/public boolean dispatchTouchEvent(MotionEvent ev) {if (ev.getAction() == MotionEvent.ACTION_DOWN) {onUserInteraction();}if (getWindow().superDispatchTouchEvent(ev)) {return true;}return onTouchEvent(ev);}点击事件由Activity传递给Window，如果点击事件被消费，则返回true并结束，如果Window没有消费事件，则调用Activity自身的onTouchEvent方法。
Window源码
/*** Used by custom windows, such as Dialog, to pass the touch screen event* further down the view hierarchy. Application developers should* not need to implement or call this.**/public abstract boolean superDispatchTouchEvent(MotionEvent event);Window是一个抽象类，superDispatchTouchEvent也是抽象方法，在Window的类注释中，可以看到它的唯一实现类是PhoneWindow，如下：
/*** Abstract base class for a top-level window look and behavior policy. An* instance of this class should be used as the top-level view added to the* window manager. It provides standard UI policies such as a background, title* area, default key processing, etc.** <p>The only existing implementation of this abstract class is* android.view.PhoneWindow, which you should instantiate when needing a* Window.*/
public abstract class Window {.....
}PhoneWindow源码
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {return mDecor.superDispatchTouchEvent(event);
}在Window实现类中，PhoneWindow将点击事件传递给DecorView(继承FrameLayout),至此，事件已经从Activity传递到了View。
ViewGroup源码：
// 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;
}首先事件到达ViewGroup的dispatchTouchEvent方法，先进行拦截检查：
1.事件为按下事件；
2.mFirstTouchTarget不为空;
则表示进行拦截检查，mFirstTouchTarget后续是会指向ViewGroup处理事件成功的子元素,如果事件来Move或者Up，一旦之前ViewGroup已经拦截，则不会再调用onInterceptTouchEvent方法，因为mFirstTouchTarget为空。如果事件不被拦截或者不被取消，那么接下会遍历ViewGroup的子元素
// 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;
//从后往前遍历
for (int i = childrenCount - 1; i >= 0; i--) {final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
....遍历子元素是从后往前遍历，后续分发给可以处理事件的子元素
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
....
}这个dispatchTransformedTouchEvent方法会对child参数进行判断，如果child为空，则调用父容器的onTouch方法，如果不为空则调用child的onTouch方法
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;}如果没有子元素或者子元素没有处理点击事件，则事件转给ViewGroup的父类View进行处理。
View源码：
/*** Pass the touch screen motion event down to the target view, or this* view if it is the target.** @param event The motion event to be dispatched.* @return True if the event was handled by the view, false otherwise.*/public boolean dispatchTouchEvent(MotionEvent event) {// If the event should be handled by accessibility focus first.if (event.isTargetAccessibilityFocus()) {// We don't have focus or no virtual descendant has it, do not handle the event.if (!isAccessibilityFocusedViewOrHost()) {return false;}// We have focus and got the event, then use normal event dispatch.event.setTargetAccessibilityFocus(false);}boolean result = false;if (mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onTouchEvent(event, 0);}final int actionMasked = event.getActionMasked();if (actionMasked == MotionEvent.ACTION_DOWN) {// Defensive cleanup for new gesturestopNestedScroll();}if (onFilterTouchEventForSecurity(event)) {if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {result = true;}//noinspection SimplifiableIfStatementListenerInfo li = mListenerInfo;if (li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)) {result = true;}if (!result && onTouchEvent(event)) {result = true;}}if (!result && mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);}// Clean up after nested scrolls if this is the end of a gesture;// also cancel it if we tried an ACTION_DOWN but we didn't want the rest// of the gesture.if (actionMasked == MotionEvent.ACTION_UP ||actionMasked == MotionEvent.ACTION_CANCEL ||(actionMasked == MotionEvent.ACTION_DOWN && !result)) {stopNestedScroll();}return result;}如果View设置了OnTouchListener，它的优先级最大，如果它的onTouch方法返回true，则不会执行onTouchEvent方法。接下来看onTouchEvent方法：
public boolean onTouchEvent(MotionEvent event) {final float x = event.getX();final float y = event.getY();final int viewFlags = mViewFlags;final int action = event.getAction();if ((viewFlags & ENABLED_MASK) == DISABLED) {if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {setPressed(false);}// A disabled view that is clickable still consumes the touch// events, it just doesn't respond to them.return (((viewFlags & CLICKABLE) == CLICKABLE|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);}if (mTouchDelegate != null) {if (mTouchDelegate.onTouchEvent(event)) {return true;}}if (((viewFlags & CLICKABLE) == CLICKABLE ||(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {switch (action) {case MotionEvent.ACTION_UP:boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
...如果View是不可用状态下，依旧会消耗点击事件，如果View的CLICKABLE，LONG_CLICKABLE， CONTEXT_CLICKABLE，其中有一个为true则会消耗点击事件。至此，事件传递已经完成分析。那开头说事件穿透的解决方法是android:clickable=”true”，根据源码分析，这样也有了根据了。