android 获得屏幕状态

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <!-- 定义获得屏幕状态的按钮 -->
    <Button
        android:id="@+id/reenableKeyguard"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="屏幕的状态" />

</RelativeLayout>

package com.example.yanlei.yl;

import android.app.KeyguardManager;
import android.content.Context;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    // 定义锁屏的按钮
    private Button btnKeyguard;
    //声明KeyguardManager对象
    private KeyguardManager keyguardManager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 获得KeyguardManager服务
        keyguardManager=(KeyguardManager)getSystemService(Context.KEYGUARD_SERVICE);
        //得到布局中的所有对象
        findView();
        //设置对象的监听器
        setListener();
    }

    private void findView() {
        // 得到布局中的所有对象
        btnKeyguard = (Button) findViewById(R.id.reenableKeyguard);
    }

    private void setListener() {
        // 设置对象的监听器
        btnKeyguard.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                //判断当前屏幕的状态
                if(keyguardManager.isKeyguardLocked())
                {
                    Toast.makeText(MainActivity.this, "锁屏", Toast.LENGTH_SHORT).show();
                }
                else
                {
                    Toast.makeText(MainActivity.this, "没有锁屏", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}