Android为什么添加View到Window中会报错？

Question

当我按照如下方法添加View到Window上时：

//改代码参考网上的
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
        if (Build.VERSION.SDK_INT >= 24) {
            if (Settings.canDrawOverlays(this)) {
                addView2Window();
            } else {
                startActivity(new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION));
            }
        } else {
            addView2Window();
        }
    }
    private void addView2Window() {
        Button button = new Button(this);
        button.setText("按钮");
        WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams(WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.TYPE_PHONE, 0, PixelFormat.TRANSPARENT);
        layoutParams.flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE | WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED;
        layoutParams.gravity = Gravity.CENTER;
        layoutParams.x = 100;
        layoutParams.y = 300;
        mWindowManager.addView(button, layoutParams);

    }

报如下错误：

Process: io.github.grooters.eight, PID: 725
    java.lang.RuntimeException: Unable to start activity ComponentInfo{io.github.grooters.eight/io.github.grooters.eight.MainActivity}: android.view.WindowManager$BadTokenException: Unable to add window android.view.ViewRootImpl$W@c6dfe60 -- permission denied for window type 2002
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302)
        at android.app.ActivityThread.-wrap12(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891)
        at android.os.Handler.dispatchMessage(Handler.java:108)
        at android.os.Looper.loop(Looper.java:166)
        at android.app.ActivityThread.main(ActivityThread.java:7425)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
     Caused by: android.view.WindowManager$BadTokenException: Unable to add window android.view.ViewRootImpl$W@c6dfe60 -- permission denied for window type 2002
        at android.view.ViewRootImpl.setView(ViewRootImpl.java:908)
        at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:372)
        at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:128)
        at io.github.grooters.eight.MainActivity.addView2Window(MainActivity.java:42)
        at io.github.grooters.eight.MainActivity.onCreate(MainActivity.java:25)
        at android.app.Activity.performCreate(Activity.java:7372)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3147)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302) 
        at android.app.ActivityThread.-wrap12(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891) 
        at android.os.Handler.dispatchMessage(Handler.java:108) 
        at android.os.Looper.loop(Looper.java:166) 
        at android.app.ActivityThread.main(ActivityThread.java:7425) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921) 

我查了网上很多资料尝试了以下方法：

1. 添加权限

    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>
    <uses-permission android:name="android.permission.SYSTEM_OVERLAY_WINDOW" />

1. 允许在其他应用上层显示内容

但都报错