虚拟导航栏常见有三个按钮：分别是Back键，Home键，Recent键。Android 4.0以后默认不显示Menu键，在API 22以上系统源码对Menu键的显示也有修改。本篇将开启Menu键并且适配Android 4.0 - Android 7.0
QQ一直沿用Menu键
纵观目前很多应用都未采用Menu键，QQ却一直沿用，并且利用Menu键显示一些快捷菜单，例如：版本更新、意见反馈、退出登录。京东也利用Menu键做一些快捷菜单。
| QQ Menu键 | 京东 Menu键 |
|---|---|
 |
 |
利用反射开启Menu键
分析PhoneWindow源码后，利用反射可以强行显示Memu键，由于API 22以后此部分的系统代码略有改动，在此进行适配。完整代码如下：
/**
* 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 7.0，Android 8.0 未测试
* 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();
}
}效果如图
测试手机为：Nexus 6，系统分别5.0（API 21）、6.0（API 23）、7.1.1（API 25）
源码分析
1.首先有视图的地方就会有Window，常用的Activity也对应这一个Window。我们在Activity的onCreate方法中设置布局。
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}进入setContentView方法中，我们可以看到它和Window关联在一起了
/**
* 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();
}2.在setContentView过程中，Activity会将具体实现交给Window，Window是一个抽象类，它的唯一实现类是PhoneWindow，那么直接分析它的具体逻辑。
在PhoneWindow的setContentView法中会初始化DecorView，如果没有DecorView则创建它。
@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;
}3.创建好DecorView后，继续按照设定的主题样式形成最终的DecorView。
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
}4.在关键的generateLayout方法中，即可找到Menu键的相关调用。
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
}5.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
}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
ScreenUtil.showNavigationMenuKey(getWindow());
initView();
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_MENU) {
Toast.makeText(this, "menu click", Toast.LENGTH_LONG).show();
return true;
}
return super.onKeyDown(keyCode, event);
}