Android 4.0以上设备虚拟按键中显示Menu键

在Android4.0以后，谷歌添加了虚拟导航键来替换实体键，虚拟导航栏有三个按钮分别是Back键，Home键，Recent键。一般的，Android默认不显示Menu键，本篇将讲述如何开启Menu键以及它背后的实现原理。

在一些产品中可以发现在虚拟导航栏上有菜单键功能，而一般的应用是没有这个功能的，效果如图右下角所示：

那是如何实现的呢。先看一下代码：

/**
     * 显示虚拟导航栏菜单按钮.
     * 虚拟导航栏菜单按钮在4.0以后默认不显示，可以利用反射强行设置，调用位置须在setContentView之后
     * 具体可以参考5.0以及6.0中的PhoneWindow类源码
     *
     * @param window {@link Window}
     */
    public static void showNavigationMenuKey(Window window) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP_MR1) {
            showNavigationLollipopMR1(window);
        }
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            showNavigationIceCreamSandwich(window);
        }
    }

    /**
     * 显示虚拟导航栏菜单按钮.
     * Android 4.0 - Android 5.0
     * API 14 - 21
     *
     * @param window {@link Window}
     */
    private static void showNavigationIceCreamSandwich(Window window) {
        try {
            int flags = WindowManager.LayoutParams.class.getField("FLAG_NEEDS_MENU_KEY").getInt(null);
            window.addFlags(flags);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }

    /**
     * 显示虚拟导航栏菜单按钮.
     * Android 5.1.1 - Android 8.0
     * API 22 - 25
     *
     * @param window {@link Window}
     */
    private static void showNavigationLollipopMR1(Window window) {
        try {
            Method setNeedsMenuKey = Window.class.getDeclaredMethod("setNeedsMenuKey", int.class);
            setNeedsMenuKey.setAccessible(true);
            int value = WindowManager.LayoutParams.class.getField("NEEDS_MENU_SET_TRUE").getInt(null);
            setNeedsMenuKey.invoke(window, value);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }

源码分析

通过上面代码可以知道是使用反射来完成设置导航栏菜单键的，下面来分析一下为什么要使用这个方案。

Android中所有视图都对应着Window来负责管理，在Activity的setContentView方法中会与Window关联在一起。

/**
     * Set the activity content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the activity.
     *
     * @param layoutResID Resource ID to be inflated.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

在setContentView过程中，Activity将具体实现交给Window来处理。Window是一个抽象类，它的唯一实现类是PhoneWindow，所以直接分析它的具体逻辑。

@Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor();//从调用此方法继续分析
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            view.setLayoutParams(params);
            final Scene newScene = new Scene(mContentParent, view);
            transitionTo(newScene);
        } else {
            mContentParent.addView(view, params);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

可以看到在Window的setContentView法中会初始化DecorView，如果没有DecorView则创建它。创建好DecorView后，继续按照设定的主题样式形成最终的DecorView。如下：

private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            mDecor = generateDecor(-1);
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);//设置主题样式
            ......//省略后续代码
        }

generateLayout方法非常关键，这里就是系统设置导航栏菜单键的地方。

protected ViewGroup generateLayout(DecorView decor) {
    //......省略代码段
    final Context context = getContext();
    final int targetSdk = context.getApplicationInfo().targetSdkVersion;
    final boolean targetPreHoneycomb = targetSdk<android.os.Build.VERSION_CODES.HONEYCOMB;
    final boolean targetPreIcs = targetSdk<android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH;
    final boolean targetPreL = targetSdk < android.os.Build.VERSION_CODES.LOLLIPOP;
    final boolean targetHcNeedsOptions = context.getResources().getBoolean(
                R.bool.target_honeycomb_needs_options_menu);
    final boolean noActionBar = !hasFeature(FEATURE_ACTION_BAR) || hasFeature(FEATURE_NO_TITLE);
    //Menu的显示或者隐藏
    if (targetPreHoneycomb || (targetPreIcs && targetHcNeedsOptions && noActionBar)) {
        setNeedsMenuKey(WindowManager.LayoutParams.NEEDS_MENU_SET_TRUE);
    } else {
        setNeedsMenuKey(WindowManager.LayoutParams.NEEDS_MENU_SET_FALSE);
    }
    //...省略代码段
}

上述分析的系统源码基于API 26，在API 21中，此部分略有不同，如下：

protected ViewGroup generateLayout(DecorView decor) {
    //......省略代码段
    final Context context = getContext();
    final int targetSdk = context.getApplicationInfo().targetSdkVersion;
    final boolean targetPreHoneycomb = targetSdk < android.os.Build.VERSION_CODES.HONEYCOMB;
    final boolean targetPreIcs = targetSdk<android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH;
    final boolean targetPreL = targetSdk < android.os.Build.VERSION_CODES.LOLLIPOP;
    final boolean targetHcNeedsOptions = context.getResources().getBoolean(
                R.bool.target_honeycomb_needs_options_menu);
    final boolean noActionBar = !hasFeature(FEATURE_ACTION_BAR) || hasFeature(FEATURE_NO_TITLE);

    if (targetPreHoneycomb || (targetPreIcs && targetHcNeedsOptions && noActionBar)) {
        addFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
    } else {
        clearFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
    }
    //...省略代码段
}

在Android API 26的Window类中多了一个方法，setNeedMenuKey 函数，该函数的作用就是设置是否显示虚拟菜单键。在Android 5.1.1之前是否显示菜单键是WindowManager.LayoutParams 中的一个flags，而在Android 5.1.1及以后，谷歌把这个标记为改到了WindowManager.LayoutParams类中的needsMenuKey 字段去了，可以通过setNeedMenuKey 方法来修改。

因此利用反射即可完成显示Menu键，注意调用方法时需要在setContentView之后，也就是在系统完成DecorView之后调用，强行显示。Menu的点击事件只需要监听onKeyDown，判断keyCode即可。

@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);//注意调用顺序
        showNavigationMenuKey(getWindow());
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_MENU) {
            // 业务逻辑
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

• https://blog.csdn.net/scau_zhangpeng/article/details/78955886
• https://blog.csdn.net/u014651216/article/details/53188698