想在系统锁屏界面弹出Toast给用户提示，结果死活弹不出来，后来发现Toast其实已经弹出来了，只是被锁屏给挡住了。

原因：
Android的窗口的分层管理，即我们看到的屏幕显示只是z轴（z-ordered）最外面的，后面的其实是被挡住了（虽然看不见但它依然存在），故要想让Toast显示出来，其层级一定要在锁屏之上才行。
应用Window层级范围[1-99], 子Window的层级范围[1000-1999], 系统Window的层级范围[2000-2999]

Android窗口z-order示意图.png

实现代码：

                    Toast mToast = Toast.makeText(mContext, mContext.getString(com.android.internal.R.string.keyguardon_not_switch), Toast.LENGTH_SHORT);
                    mToast.getWindowParams().type = WindowManager.LayoutParams.TYPE_STATUS_BAR_PANEL;
                    mToast.getWindowParams().privateFlags |= WindowManager.LayoutParams.PRIVATE_FLAG_SHOW_FOR_ALL_USERS;
                    mToast.getWindowParams().flags |= WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED;
                    mToast.show();

主要参数：

/frameworks/base/core/java/android/view/WindowManager.java
/** Window flag: special flag to let windows be shown when the screen
         * is locked. This will let application windows take precedence over
         * key guard or any other lock screens. Can be used with
         * {@link #FLAG_KEEP_SCREEN_ON} to turn screen on and display windows
         * directly before showing the key guard window.  Can be used with
         * {@link #FLAG_DISMISS_KEYGUARD} to automatically fully dismisss
         * non-secure keyguards.  This flag only applies to the top-most
         * full-screen window.
         */
        public static final int FLAG_SHOW_WHEN_LOCKED = 0x00080000;

        /** In a multiuser system if this flag is set and the owner is a system process then this
         * window will appear on all user screens. This overrides the default behavior of window
         * types that normally only appear on the owning user's screen. Refer to each window type
         * to determine its default behavior.
         *         * {@hide} */
        public static final int PRIVATE_FLAG_SHOW_FOR_ALL_USERS = 0x00000010;

参考资料
1.锁屏界面无法无法显示Toast（介绍了非系统应用的实现）
https://blog.csdn.net/u013398960/article/details/73194812
2.延伸阅读
2.1 Android窗口管理分析（3）：窗口分组及Z-order的确定
https://blog.csdn.net/happylishang/article/details/77893723
2.2 源码
frameworks/base/core/java/android/view/WindowManager.java
3.疑问
3.1 https://github.com/weidongshan/APP_0010_SurfaceTest/blob/master/SurfaceTest.cpp#L46
window的z-ordered显然要和surface的layer对应，这中间怎么处理的？
surfaceControl->setLayer(100000);