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  • [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
    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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  • 原文地址:https://www.cnblogs.com/Free-Thinker/p/4204914.html
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