Android：java.lang.SecurityException: Not allowed to change Do Not Disturb state

在开发过程中，需要在某个时间段将手机设为静音状态，于是乎，直接写出如下代码：

if（在某个时间范围内）{    setSystemSilent（）；}    private void setSystemSilent() {        mAudioManager.setRingerMode(AudioManager.RINGER_MODE_SILENT);    }

运行程序，就会出现标题的错误，经过网上查询，得到如下结论，缺少权限，需要加上如下权限：

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

ok，在AndroidManife中直接加上这个权限。
再次运行程序，还是出现同样的错误。郁闷了。再去网上寻找答案。
https://stackoverflow.com/questions/39151453/in-android-7-api-level-24-my-app-is-not-allowed-to-mute-phone-set-ringer-mode/39929910#39929910

这里给出了方案。添加如下代码

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N                && !mNotificationManager.isNotificationPolicyAccessGranted()) {            Intent intent = new Intent(Settings.ACTION_NOTIFICATION_POLICY_ACCESS_SETTINGS);            mContext.startActivity(intent);        }

如果应用没有获取权限，会跳转到Setting下Dnd的权限申请界面。对需要该权限的应用，将开关打开。那么应用就可以正常运行，并将设备进行静音了。
下图是控制应用权限界面：

后来经过跟代码，发现在Android的源码中有相关的说明。

frameworks/base/media/java/android/media/AudioManager.java    /**     * Sets the ringer mode.     * <p>     * Silent mode will mute the volume and will not vibrate. Vibrate mode will     * mute the volume and vibrate. Normal mode will be audible and may vibrate     * according to user settings.     * <p>This method has no effect if the device implements a fixed volume policy     * as indicated by {@link #isVolumeFixed()}.     * * <p>From N onward, ringer mode adjustments that would toggle Do Not Disturb are not allowed     * unless the app has been granted Do Not Disturb Access.     * See {@link NotificationManager#isNotificationPolicyAccessGranted()}.     * @param ringerMode The ringer mode, one of {@link #RINGER_MODE_NORMAL},     *            {@link #RINGER_MODE_SILENT}, or {@link #RINGER_MODE_VIBRATE}.     * @see #getRingerMode()     * @see #isVolumeFixed()     */    public void setRingerMode(int ringerMode) {        if (!isValidRingerMode(ringerMode)) {            return;        }        IAudioService service = getService();        try {            service.setRingerModeExternal(ringerMode, getContext().getOpPackageName());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }