permission denied for window type 2003

今天开发android时遇到这样一个问题：

[email protected] -- permission denied for window type 2003

这个原因是我想要用系统悬浮弹框，但是没有权限；

fun showDialog(){
    var dialog:AlertDialog.Builder = AlertDialog.Builder(this)
    var alear:AlertDialog =dialog.setIcon(R.drawable.ic_launcher_background).setTitle(R.string.app_name)
            .setMessage(R.string.msg).setPositiveButton(R.string.ok, object : DialogInterface.OnClickListener {
                override fun onClick(p0: DialogInterface?, p1: Int) {
                    Log.e("kotlin","button_click")
                }
            }).create()
    alear.window.setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT)
    alear.show()
}

这是我代码，代码提示TYPE_SYSTEM_ALERT 已经过时，于是在网上又找到另外一个，要使用TYPE_APPLICATION_OVERLAY

类型。试了还是不行，这是Manifest的代码；

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

怎么办;在程序信息里看到没有此类的权限，原来是另外的设置：

permission denied for window type 2003

设置后，报另外一个错误：permission denied for window type 2038

然后是我的系统7.1，权限改为TYPE_PHONE可以，但是没实现悬浮的感觉，只是需要自定义View,下面是完整代码.

fun showDialog() {
    var dialog: AlertDialog.Builder = AlertDialog.Builder(this)
    var alear: AlertDialog = dialog.setIcon(R.drawable.ic_launcher_background).setTitle(R.string.app_name)
            .setMessage(R.string.msg).setPositiveButton(R.string.ok, object : DialogInterface.OnClickListener {
                override fun onClick(p0: DialogInterface?, p1: Int) {
                    Log.e("kotlin", "button_click")
                }
            }).create()
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.O) {
        alear.window.setType(WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY)
    } else {
        alear.window.setType(WindowManager.LayoutParams.TYPE_PHONE)
    }
    alear.window.setFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL, WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE)
    alear.show()
}

permission denied for window type 2003