SYSTEM_ALERT_WINDOW

<permission android:name="android.permission.SYSTEM_ALERT_WINDOW"
         android:label="@string/permlab_systemAlertWindow"
         android:description="@string/permdesc_systemAlertWindow"
        android:protectionLevel="signature|setup|appop|installer|pre23|development" />

如何获取该权限:
如果有系统签名,则只需要在AndroidManifest.xml 中注册既可
如果没有系统签名, 可以动态申请:

    public static void getPermission(Activity activity) {
        Intent intent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION,
                Uri.parse("package:" + activity.getPackageName()));
        activity.startActivityForResult(intent, ACTION_MANAGE_OVERLAY_PERMISSION_REQUEST_CODE);
    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == ACTION_MANAGE_OVERLAY_PERMISSION_REQUEST_CODE) {
            if (!Settings.canDrawOverlays(this)) {
                // You don't have permission
                SystemAlertUtil.getPermission(this);
            } else {
                if (!showText) {
                    SystemAlertUtil.showSystemAlert(this);
                } else {
                    SystemAlertUtil.showTextAlert(this);
                }
            }
        }
    }


获得system alert window权限后可以做什么?
1.可以startActivity in Background(可以是自己的activity ,也可以是其他应用的Activity) , 并且Activity处于顶层
2.可以弹出顶层弹框


public static void showSystemDialog(Activity activity) {
        WindowManager.LayoutParams params = new WindowManager.LayoutParams();
        params.type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
        AlertDialog.Builder builder = new AlertDialog.Builder(activity);
        builder.setIcon(R.drawable.app_icon_your_company);
        builder.setTitle("下线通知").setMessage("该账号在另一台Android设备上登录。")
                .setPositiveButton("重新登陆", new DialogInterface.OnClickListener() {

                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        //do somthing
                    }

                }).setNegativeButton("退出", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {

            }
        });
        AlertDialog alertDialog = builder.create();
        alertDialog.setCancelable(false);
        alertDialog.setCanceledOnTouchOutside(false);
        alertDialog.getWindow().setType(params.type);
        alertDialog.show();
    }

    static TextView textView;
    static WindowManager mWindowManager;

    public static void showSystemView(Activity mContext) {
        textView = new TextView(mContext);
        textView.setText("此为悬浮窗口View");
        textView.setTextSize(TypedValue.COMPLEX_UNIT_PX, 20);
        WindowManager.LayoutParams mLayoutParams = new WindowManager.LayoutParams();
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            mLayoutParams.type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
        } else {
            mLayoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ALERT;
        }
        mLayoutParams.format = PixelFormat.RGBA_8888; //窗口透明
        mLayoutParams.gravity = Gravity.CENTER | Gravity.TOP; //窗口位置
        mLayoutParams.flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE;
        DisplayMetrics displayMetrics = mContext.getResources().getDisplayMetrics();
        int layoutY = displayMetrics.heightPixels / 2;
        int layoutX = displayMetrics.widthPixels - textView.getMeasuredWidth();
        mLayoutParams.width = WindowManager.LayoutParams.WRAP_CONTENT;
        mLayoutParams.height = WindowManager.LayoutParams.WRAP_CONTENT;
        mLayoutParams.x = layoutX;
        mLayoutParams.y = layoutY;
        mWindowManager = (WindowManager) mContext.getSystemService(Context.WINDOW_SERVICE);
        mWindowManager.addView(textView, mLayoutParams);
    }

    public static void removeMessage(Context context) {
        if (textView != null) {
            if (mWindowManager != null) {
                mWindowManager.removeView(textView);
            }
            textView = null;
            mWindowManager = null;
        }
    }