[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 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;

}