这篇文章是转载的，写的不错，自己好好留着看一下……

[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。

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 theandroid: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 ofmatch_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

@H_301_56@ xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:id="@+id/drawer_layout"

@H_301_56@ android:layout_width="match_parent"

android:layout_height="match_parent"

tools:context="com.aven.myapplication2.app.MainActivity"@H_301_56@>

@H_301_56@ <FrameLayout

android:id="@+id/container"

android:layout_width= android:layout_height="match_parent"@H_301_56@/>

@H_301_56@<fragmentandroid:id="@+id/navigation_drawer"

android:layout_width="@dimen/navigation_drawer_width"

android:layout_gravity="start"

android:name="com.aven.myapplication2.app.NavigationDrawerFragment"

tools:layout="@layout/fragment_navigation_drawer"@H_301_56@/>

</android.support.v4.widget.DrawerLayout>

@H_404_149@

@H_404_149@ 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。

@H_404_149@

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,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); }

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

/** * 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; }

/** * 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); }

UI抽屉菜单DrawerLayout（一）（测拉菜单）

[UI]抽屉菜单DrawerLayout分析（一）

官方介绍

源码分析

DrawerLayout实例化相关辅助类

ViewDragHelper负责实现drag操作

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