Android：如何启用设备管理？

我想通过我的应用程序关闭屏幕，但我无法获取设备管理员

这是我的代码：

DevicePolicyManager mDPM;
ComponentName mDeviceAdminSample;
private static final int REQUEST_CODE_ENABLE_ADMIN = 1;


/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);


    mContext=this;


    mDeviceAdminSample = new ComponentName(this, Act_01_LockScreen.class);
    mDPM = (DevicePolicyManager) getSystemService(Context.DEVICE_POLICY_SERVICE);




}

@Override
public void onResume() {
    super.onResume();


    Boolean mAdminActive = isActiveAdmin();



    if (!mAdminActive) {

            // Launch the activity to have the user enable our admin.
            Intent intent = new Intent(DevicePolicyManager.ACTION_ADD_DEVICE_ADMIN);
            intent.putExtra(DevicePolicyManager.EXTRA_DEVICE_ADMIN, mDeviceAdminSample);
            intent.putExtra(DevicePolicyManager.EXTRA_ADD_EXPLANATION,
                    mContext.getResources().getString(R.string.app_name));
            startActivityForResult(intent, REQUEST_CODE_ENABLE_ADMIN);
            // return false - don't update checkbox until we're really active

    }else{
        mDPM.lockNow();
    }





}

private boolean isActiveAdmin() {
    return mDPM.isAdminActive(mDeviceAdminSample);
}

}

这个代码没有显示任何内容。我该怎么办

非常感谢

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请不要向我显示任何没有soltuion的链接…

作为一种解决方法，您可以根据亮度级别的变化尝试此技巧

Window window = getWindow();
WindowManager.LayoutParams windowParams = window.getAttributes();
windowParams.screenBrightness = 0;
window.setAttributes(windowParams);

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非常感谢您的回答，但这并不能回答我的问题，如果我使用此代码，我将无法再退出待机状态。