如何在不同的类中引用相同的类变量？

Question

I have a variable "wl" of type "PowerManager.WakeLock". I need to use the same "wl" variable and not a different object in different classes. Can this be done?

 public PowerManager.WakeLock wl; 
 // This variable needs to be used like a switch in other classes. 
 // In one class its turned on, in another class I need to call 
 // a method .Release(), to turn it off, 
 // but it has to refer to that same "wl" variable.

NotificationReceiver CLass - "holds the power manager

     public class NotificationReceiver extends Service {
public PowerManager powerManager = (PowerManager) 
      getSystemService(POWER_SERVICE);
public static PowerManager.WakeLock wl = 
       powerManager.newWakeLock(PowerManager.SCREEN_BRIGHT_WAKE_LOCK, 
       "My:WakelockTag"); //Non-static field 'powerManager' cannot be referenced from a static context


@Override
public void onCreate() {
    super.onCreate();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {


    wl.acquire();


    return START_NOT_STICKY;

}

  //--------------------------------------------------------------------

MainActivity class that I need to use the that variable in

public class MainActivity extends AppCompatActivity {
       @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final Button button = (Button) findViewById(R.id.start_button);
    button.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {


            Intent serviceIntent = new Intent (MainActivity.this, ExampleService.class);

            startService(serviceIntent);





            NotificationReceiver nn = new NotificationReceiver();// Here I make a new object to access the .aquire() method 

            nn.wl.acquire();

        }
    });

Answer 1

您可以在构造类时或使用它的各个函数中传递它。

Answer 2

It can be done by making your variable static. You can access it by ClassName.variable. (It will be field of only one class, but it can be accessed from all your classes)

Answer 3

Consider Singleton pattern:

class LockHolder {

  public static final LockHolder INSTANCE = new LockHolder();

  private final PowerManager.WakeLock wakeLock;

  // constructor is private so that noone can create new instances
  private LockHolder() { 
    this.wakeLock = new WakeLock();
  }

  // accessor method is safer than exposing the variable itself;
  // you may eventually end up doing more things here
  public PowerManager.WakeLock wakeLock() {
    return wakeLock;
  }
}

LockHolder is a Singleton since only one instance of this class exists.

You use it as follows:

PowerManager.WakeLock lock = LockHolder.INSTANCE.wakeLock();