[UI]抽屉菜单DrawerLayout分析（一）

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。

Device 2014 04 16 191818

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_gravity attribute 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 the layout_gravity appropriately. Drawers commonly use match_parent for height with a fixed width.

DrawerLayout.DrawerListener can 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 the STATE_IDLE state. DrawerLayout.SimpleDrawerListener offers 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>

所以DrawerLayout的使用非常简单，和很多容器类布局一样，它本身也继承自ViewGroup，只是在内部实现中会默认将第一个子节点作为内容区，第二个作为抽屉菜单，所以写布局的事后必须牢记，好在现在的IDE已经非常智能，通过引导来创建Drawerlayout时,会自动生成Activity和xml layout布局，比如使用AndroidStudio就非常方便。

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.
*/

现在来看看这个helper到底是怎么封装的滑动操作，从上面的实例化我们知道这个helper通过工厂方法来构造实例，工厂方法有两个如下：

/** * 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

作者：小文字
出处：http://www.cnblogs.com/avenwu/
本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利.

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