本文转载于：http://www.cnblogs.com/avenwu/archive/2014/04/16/3669367.html
侧拉菜单作为常见的导航交互控件，最开始在没有没有android官方控件时，很多时候都是使用开源的SlidingMenu，一直没机会分析侧拉菜单的实现机理，本文将分析android.support.v4.widget.DrawerLayout的使用及实现。
官方介绍
DrawerLayout acts as a top-level container for window content that allows for interactive "drawer" views to be pulled out from the edge of the window.
Drawer positioning and layout is controlled using the
android:layout_gravityattribute on child views corresponding to which side of the view you want the drawer to emerge from: left or right. (Or start/end on platform versions that support layout direction.)To use a DrawerLayout, position your primary content view as the first child with a width and height of
match_parent. Add drawers as child views after the main content view and set thelayout_gravityappropriately. Drawers commonly usematch_parentfor height with a fixed width.
DrawerLayout.DrawerListenercan be used to monitor the state and motion of drawer views. Avoid performing expensive operations such as layout during animation as it can cause stuttering; try to perform expensive operations during theSTATE_IDLEstate.DrawerLayout.SimpleDrawerListeneroffers default/no-op implementations of each callback method.As per the Android Design guide, any drawers positioned to the left/start should always contain content for navigating around the application, whereas any drawers positioned to the right/end should always contain actions to take on the current content. This preserves the same navigation left, actions right structure present in the Action Bar and elsewhere
DrawerLayout直译的事抽屉布局的意思，作为视窗内的顶层容器，它允许用户通过抽屉式的推拉操作，从而把视图视窗外边缘拉到屏幕内，如右图：
抽屉菜单的摆放和布局通过android:layout_gravity属性来控制，可选值为left、right或start、end。通过xml来布局的话，需要把DrawerLayout作为父容器，组界面布局作为其第一个子节点，抽屉布局则紧随其后作为第二个子节点，这样就做就已经把内容展示区和抽屉菜单区独立开来，只需要分别非两个区域设置内容即可。android提供了一些实用的监听器，重载相关的回调方法可以在菜单的交互过程中书写逻辑业务。下面是一个demo布局:
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.aven.myapplication2.app.MainActivity">
<FrameLayout
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
<fragmentandroid:id="@+id/navigation_drawer"
android:layout_width="@dimen/navigation_drawer_width"
android:layout_height="match_parent"
android:layout_gravity="start"
android:name="com.aven.myapplication2.app.NavigationDrawerFragment"
tools:layout="@layout/fragment_navigation_drawer"/>
</android.support.v4.widget.DrawerLayout>
源码分析
DrawerLayout实例化相关辅助类
既然DrawerLayout使用是作为顶层布局layout，那先看看他的构造函数：
public DrawerLayout(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
//根据屏幕分辨率密度计算最小的边距
final float density = getResources().getDisplayMetrics().density;
mMinDrawerMargin = (int) (MIN_DRAWER_MARGIN * density + 0.5f);
final float minVel = MIN_FLING_VELOCITY * density;
//实例化视图滑动的回调接口，包括左右两边
mLeftCallback = new ViewDragCallback(Gravity.LEFT);
mRightCallback = new ViewDragCallback(Gravity.RIGHT);
//创建滑动手势的的辅助类，负责具体的滑动监听实现
mLeftDragger = ViewDragHelper.create(this, TOUCH_SLOP_SENSITIVITY, mLeftCallback);
mLeftDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);
mLeftDragger.setMinVelocity(minVel);
mLeftCallback.setDragger(mLeftDragger);
mRightDragger = ViewDragHelper.create(this, TOUCH_SLOP_SENSITIVITY, mRightCallback);
mRightDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_RIGHT);
mRightDragger.setMinVelocity(minVel);
mRightCallback.setDragger(mRightDragger);
// So that we can catch the back button
setFocusableInTouchMode(true);
ViewCompat.setAccessibilityDelegate(this, new AccessibilityDelegate());
ViewGroupCompat.setMotionEventSplittingEnabled(this,false);
}
从构造函数中，我们发现有两个关键的类ViewDragCallback, ViewDragHelper，命名上来看前者和滑动的回调相关，后者和view的滑动操作实现有关，所以先看ViewDragHelper。
ViewDragHelper负责实现drag操作
从它的类注释信息中可以看到，这个helper是个辅助类，里面封装了一些便于用户拖动ViewGroup内子view的操作及状态记录方法。
/**
* ViewDragHelper is a utility class for writing custom ViewGroups. It offers a number
* of useful operations and state tracking for allowing a user to drag and reposition
* views within their parent ViewGroup.
*/
/**
* Factory method to create a new ViewDragHelper.
*
* @param forParent Parent view to monitor
* @param cb Callback to provide information and receive events
* @return a new ViewDragHelper instance
*/
public static ViewDragHelper create(ViewGroup forParent, Callback cb) {
return new ViewDragHelper(forParent.getContext(), forParent, cb);
}
/**
* Factory method to create a new ViewDragHelper.
*
* @param forParent Parent view to monitor
* @param sensitivity Multiplier for how sensitive the helper should be about detecting
* the start of a drag. Larger values are more sensitive. 1.0f is normal.
* @param cb Callback to provide information and receive events
* @return a new ViewDragHelper instance
*/
public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Callback cb) {
final ViewDragHelper helper = create(forParent, cb);
helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));
return helper;
}
这第二个工厂方法create就是刚才看到的上层调用来创建helper实例的，我们传入了一个viewgroup，也就是说helper将持有我们的DrawerLayout实例引用,第二是一个浮点数，和drag操作的敏感性相关，数值越大表示drag操作更易被监听，最后是一个Callback，即ViewDragCallback实例，它本身继承自ViewDragHelper.Callback，现在来看helper的构造方法：
/**
* Apps should use ViewDragHelper.create() to get a new instance.
* This will allow VDH to use internal compatibility implementations for different
* platform versions.
*
* @param context Context to initialize config-dependent params from
* @param forParent Parent view to monitor
*/
private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {
if (forParent == null) {
throw new IllegalArgumentException("Parent view may not be null");
}
if (cb == null) {
throw new IllegalArgumentException("Callback may not be null");
}
mParentView = forParent;
mCallback = cb;
final ViewConfiguration vc = ViewConfiguration.get(context);
finalfloat density = context.getResources().getDisplayMetrics().density;
mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);
mTouchSlop = vc.getScaledTouchSlop();
mMaxVelocity = vc.getScaledMaximumFlingVelocity();
mMinVelocity = vc.getScaledMinimumFlingVelocity();
mScroller = ScrollerCompat.create(context, sInterpolator);
}
首先需要检测我们传入的DrawerLayout和回调Callback，不允许为空。接下来从ViewConfiguration中获取一些view的默认配置，
vc.getScaledTouchSlop是获取一个pix为单位的距离，代表view在滑动的值；
vc.getScaledMaximumFlingVelocity获取触发view fling的最大每秒滚动的距离，也是pix为单位；
获取view fling的最小每秒滚动距离，同样pix为单位；
这里有scroll和fling，我的理解是scroll表示手没有离开屏幕产生的滑动效果，二fling则是用力一划，然后view自己开始滚动的效果。
最后实例化一个Scroller，这是专门用来处理滚动的一个类，这里用的是扩展包里的campact类做版本兼容。
到此DrawerLayout已经准备好所有资源，接下来就是手势分发时候的各种调用，这一部分留到下一篇文章在做分析
Source：
git clone https://github.com/avenwu/DrawerDemo.git
继续分析DrawerLayout的手势分发部分
谈到手势分发，这本身就是个好话题，DrawerLayout作为继承自ViewGroup得布局他可以拦截手势也可以分发给子view，也就是在onInterceptTouchEvent中做的操作，但是他的下面还有一个onTouchEvent方法，先看哪个呢？追溯代码我们可以知道ViewGroup继承自View，而onTouchEvent是View的方法
我们还是先花点时间把两者的关系先确认再继续。
onInterceptTouchEvent和onTouchEvent---鸡和蛋？
定位到ViewGroup，可以发现onInterceptTouchEvent分定义如下，从它前面一段非常长的注释就可以看出其重要性和复杂，默认的返回是false
/**
* Implement this method to intercept all touch screen motion events. This
* allows you to watch events as they are dispatched to your children, and
* take ownership of the current gesture at any point.
*
* <p>Using this function takes some care, as it has a fairly complicated
* interaction with {@link View#onTouchEvent(MotionEvent)
* View.onTouchEvent(MotionEvent)}, and using it requires implementing
* that method as well as this one in the correct way. Events will be
* received in the following order:
*
* <ol>
* <li> You will receive the down event here.
* <li> The down event will be handled either by a child of this view
* group, or given to your own onTouchEvent() method to handle; this means
* you should implement onTouchEvent() to return true, so you will
* continue to see the rest of the gesture (instead of looking for
* a parent view to handle it). Also, by returning true from
* onTouchEvent(), you will not receive any following
* events in onInterceptTouchEvent() and all touch processing must
* happen in onTouchEvent() like normal.
* <li> For as long as you return false from this function, each following
* event (up to and including the final up) will be delivered first here
* and then to the target's onTouchEvent().
* <li> If you return true from here, you will not receive any
* following events: the target view will receive the same event but
* with the action {@link MotionEvent#ACTION_CANCEL}, and all further
* events will be delivered to your onTouchEvent() method and no longer
* appear here.
* </ol>
*
* @param ev The motion event being dispatched down the hierarchy.
* @return Return true to steal motion events from the children and have
* them dispatched to this ViewGroup through onTouchEvent().
* The current target will receive an ACTION_CANCEL event, and no further
* messages will be delivered here.
*/
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
前两段告诉我们，复写onInterceptTouchEvent方法，可以实现监听所有的动作事件MotionEvent，在向子view传递事件前做我们需要的操作，当然这指的是和这个viewgroup相关的事件；同时我们需要慎重处理该函数，因为他和onTouchEvent关系非常紧密，下面是事件接收的顺序：
首先接收的的事按下事件，down事件，他可以被view处理也可以在自身的onTouchEvent里处理，所以实现onTouchEvent并且返回true，这样onTouchEvent继续才能收到down之后的其他事件，同时onInterceptTouchEvent不会在收到后续事件，因为已经转移到onTouchEvent处理了。
那么什么时候onInterceptTouchEvent会把后续事件转移到他的onTouchEvent呢？这取决于onInterceptTouchEvent的返回值，如果返回false，所有事件都会先分发到这里，然后再到目标view的onTouchEvent；相反如果返回true，那么onInterceptTouchEvent将不再收到后续事件，并且目标view会收到cancel事件，接着自身的onTouchEvent几首后续的事件。
这其实从名字来看是比较好理解的onInterceptTouchEvent表示在截取触摸事件的被调用的方法，既然是截取就可以直接吧事件截下来后不再往后传递，这是就是上面的第二种情况，返回true，即我们自己消耗了触摸事件，子view将没有机会得到唤醒。
大致意思就是如果希望自身消耗掉改事件就可以直接返回true，这一点和onTouchEvent的返回类似目的。
博客园有篇文章对这些事件分发做了很好的分析：http://www.cnblogs.com/sunzn/archive/2013/05/10/3064129.html
详细的阐述了了dispatchTouchEvent，onInterceptTouchEvent以及onTouchEvent之间的关系
现在我们回过头来看DrawerLayout里的分发是如何写的：
重写了后面两个方法，先看onInterceptTouchEvent:
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
final int action = MotionEventCompat.getActionMasked(ev);
// "|" used deliberately here; both methods should be invoked.
final boolean interceptForDrag = mLeftDragger.shouldInterceptTouchEvent(ev) |
mRightDragger.shouldInterceptTouchEvent(ev);
boolean interceptForTap = false;
switch (action) {
case MotionEvent.ACTION_DOWN: {
final float x = ev.getX();
final float y = ev.getY();
mInitialMotionX = x;
mInitialMotionY = y;
if (mScrimOpacity > 0 &&
isContentView(mLeftDragger.findTopChildUnder((int) x, (int) y))) {
interceptForTap = true;
}
mDisallowInterceptRequested = false;
mChildrenCanceledTouch = false;
break;
}
case MotionEvent.ACTION_MOVE: {
// If we cross the touch slop, don't perform the delayed peek for an edge touch.
if (mLeftDragger.checkTouchSlop(ViewDragHelper.DIRECTION_ALL)) {
mLeftCallback.removeCallbacks();
mRightCallback.removeCallbacks();
}
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP: {
closeDrawers(true);
mDisallowInterceptRequested = false;
mChildrenCanceledTouch = false;
}
}
return interceptForDrag || interceptForTap || hasPeekingDrawer() || mChildrenCanceledTouch;
}
1.首先从touch event里面获取当前具体的action动作，MotionEventCompat.getActionMasked(ev)，内部实际上做了一次按位于操作event.getAction() & ACTION_MASK;
2.检查当前是否满足截取drag状态，用于决定onInterceptTouchEvent返回值，这里有个注解说是故意用了|或，而不是||或，两者区别在于||只要第一个条件满足就不在执行第二个检查，二|不同，无论如何都会将两个条件检查一遍；
3.接下来是几个case，根据当前的action做处理；
ACTION_DOWN，当按下时记录按下点的x，y坐标值，根据条件设置当前是否满足tap状态，具体条件有两个，一是mScrimOpacity，表示子view中在屏幕上占据的最大宽度（0-1），二时根据坐标点的位置取得改点对应的最上层view对象，如果是预定义的content view即DrawerLayout里的主内容展示view，也就是同时满足view在屏幕上且点击的位置直接落在了content view上。
ACTION_MOVE,当手按下后开始在屏幕上移动时，如果垂直和水平上的位移差量达到了drag helper的阀值则一处左右两边的回调接口
ACTION_CANCLE和ACTION_UP，手势结束后，关闭菜单
最后结合几个状态来那个来决定onInterceptTouchEvent返回true还是false，
未完待续
在[UI]抽屉菜单DrawerLayout分析（一）和[UI]抽屉菜单DrawerLayout分析（二）中分别介绍了DrawerLayout得基本框架结构和ViewDragerHelper的作用以及手势分发，本文一起来分析其中的Scroller的使用情况。
在ViewDragerHelper中可以发现private ScrollerCompat mScroller;说明抽屉菜单的具体滑动也是依赖于Scroller的使用，检索一下mScroller的引用，定位到forceSettleCapturedViewAt，这个方法回调用Scroller的startScroll来计算位移，它本身适用于计算和保存位移在特定时间的变化情况，最终的在绘制view时我可以获取其保存的x，y坐标值。
/**
* Settle the captured view at the given (left, top) position.
*
* @param finalLeft Target left position for the captured view
* @param finalTop Target top position for the captured view
* @param xvel Horizontal velocity
* @param yvel Vertical velocity
* @return true if animation should continue through {@link #continueSettling(boolean)} calls
*/
private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {
final int startLeft = mCapturedView.getLeft();
final int startTop = mCapturedView.getTop();
final int dx = finalLeft - startLeft;
final int dy = finalTop - startTop;
if (dx == 0 && dy == 0) {
// Nothing to do. Send callbacks, be done.
mScroller.abortAnimation();
setDragState(STATE_IDLE);
returnfalse;
}
final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);
mScroller.startScroll(startLeft, startTop, dx, dy, duration);
setDragState(STATE_SETTLING);
returntrue;
}
这里用的是v4扩展包里的ScrollerCompat用于低版本兼容，它继承自ScrollerCompatImpl，可以看到里面主要的方法声明：
interface ScrollerCompatImpl{
Object createScroller(Context context, Interpolator interpolator);
boolean isFinished(Object scroller);
int getCurrX(Object scroller);
int getCurrY(Object scroller);
float getCurrVelocity(Object scroller);
boolean computeScrollOffset(Object scroller);
void startScroll(Object scroller, int startX, int startY, int dx, int dy);
void startScroll(Object scroller, int startX, int startY, int dx, int dy, int duration);
void fling(Object scroller, int startX, int startY, int velX, int velY,
int minX, int maxX, int minY, int maxY);
void fling(Object scroller, int startX, int startY, int velX, int velY,
int minX, int maxX, int minY, int maxY, int overX, int overY);
void abortAnimation(Object scroller);
void notifyHorizontalEdgeReached(Object scroller, int startX, int finalX, int overX);
void notifyVerticalEdgeReached(Object scroller, int startY, int finalY, int overY);
boolean isOverScrolled(Object scroller);
int getFinalX(Object scroller);
int getFinalY(Object scroller);
}
从Scroller一直往上追溯，可以得到如图的调用流程。
当滑动屏幕时，DrawerLayout中的手势分发被触发，先执行onInterceptTouchEvent根据返回结果确定是否执行onTouchEvent,之后就是一些和ViewDragHelper之间的回调接口处理。
接下来追踪一下什么时候从Scroller中取出x，y来使用：
在View里面有一个实现为空的computeScroll，DrawerLayout对它进行重写，这个方法应该是在view自动重绘是会被调用，回到continueSettling：
/**
* Move the captured settling view by the appropriate amount for the current time.
* If <code>continueSettling</code> returns true, the caller should call it again
* on the next frame to continue.
*
* @param deferCallbacks true if state callbacks should be deferred via posted message.
* Set this to true if you are calling this method from
* {@link android.view.View#computeScroll()} or similar methods
* invoked as part of layout or drawing.
* @return true if settle is still in progress
*/
public boolean continueSettling(boolean deferCallbacks) {
if (mDragState == STATE_SETTLING) {
boolean keepGoing = mScroller.computeScrollOffset();
final int x = mScroller.getCurrX();
final int y = mScroller.getCurrY();
final int dx = x - mCapturedView.getLeft();
final int dy = y - mCapturedView.getTop();
if (dx != 0) {
mCapturedView.offsetLeftAndRight(dx);
}
if (dy != 0) {
mCapturedView.offsetTopAndBottom(dy);
}
if (dx != 0 || dy != 0) {
mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);
}
if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {
// Close enough. The interpolator/scroller might think we're still moving
// but the user sure doesn't.
mScroller.abortAnimation();
keepGoing = mScroller.isFinished();
}
if (!keepGoing) {
if (deferCallbacks) {
mParentView.post(mSetIdleRunnable);
} else {
setDragState(STATE_IDLE);
}
}
}
return mDragState == STATE_SETTLING;
}
当状态处于STATE_SETTLING时开始获取Scroller中的x，y值，结合当前运动view的left，top位置，计算出偏移量，通过offsetLeftAndRight设置，里面是一些具体的位置改变，挺复杂的。
/**
* Offset this view's horizontal location by the specified amount of pixels.
*
* @param offset the number of pixels to offset the view by
*/
public void offsetLeftAndRight(int offset) {
if (offset != 0) {
updateMatrix();
final boolean matrixIsIdentity = mTransformationInfo == null
|| mTransformationInfo.mMatrixIsIdentity;
if (matrixIsIdentity) {
if (mDisplayList != null) {
invalidateViewProperty(false, false);
} else {
final ViewParent p = mParent;
if (p != null && mAttachInfo != null) {
final Rect r = mAttachInfo.mTmpInvalRect;
int minLeft;
int maxRight;
if (offset < 0) {
minLeft = mLeft + offset;
maxRight = mRight;
} else {
minLeft = mLeft;
maxRight = mRight + offset;
}
r.set(0, 0, maxRight - minLeft, mBottom - mTop);
p.invalidateChild(this, r);
}
}
} else {
invalidateViewProperty(false, false);
}
mLeft += offset;
mRight += offset;
if (mDisplayList != null) {
mDisplayList.offsetLeftAndRight(offset);
invalidateViewProperty(false, false);
} else {
if (!matrixIsIdentity) {
invalidateViewProperty(false, true);
}
invalidateParentIfNeeded();
}
}
}
小结
至此DrawerLayout的基本工作流程分析完毕，简单做一个总结，v4包提供了ViewDragHelper类，里面封装了对scroller合view的位移操作，和Callback接口，通过DrawerLayout内的onInterceptTouchEvent和onTouchEvent的重载，触发ViewDragHelper内的相关方法，同时在DrawerLayout内实现ViewDragHelp.Callback.