Question

I am using an android service as explained here. I am calling doBindService() in onCreate() and trying to call one of mBoundservice‘s methods in onResume(). This is where I run into an issue. At this point, mBoundService is still null, presumably because mConnection.onServiceConnected() hasn’t yet been called.

Is there some way to be able to call methods of mBoundService in onResume(), or is there no way around it’s being null at that point?

Answer 1

It hasn’t been clear stated in the official dev guide that bindService() is actually an asynchronous call:

A client can bind to the service by calling bindService(). When it does, it must provide an implementation of ServiceConnection, which monitors the connection with the service. The bindService() method returns immediately without a value, but when the Android system creates the connection between the client and service, it calls onServiceConnected() on the ServiceConnection, to deliver the IBinder that the client can use to communicate with the service.

There is a lag (although instantaneous but still a lag) after calling bindService() and before system prepare/instantiate a usable service instance (not NULL) and hand it back in ServiceConnection.onServiceConnected() callback. the time interval between onCreate() and onResume() is too short to overcome the lag (in case if activity is opened first time).

Suppose you want to call mBoundservice.foo() in onResume(), a common workaround is call it in onServiceConnected() callback when activity is first created, and set a boolean state, and in onResume() method, only call it iff the state is set, to conditional control the code execution i.e. calling mBoundservice.foo() based on different Activity lifecycle:

LocalService mBoundservice = null;
boolean mBound = false;

... ...

private ServiceConnection mConnection = new ServiceConnection() {
  @Override
  public void onServiceConnected(ComponentName className, IBinder service) {
    LocalBinder binder = (LocalBinder) service;
    mBoundservice = binder.getService();
    mBound = true;
    // when activity is first created:
    mBoundservice.foo();
  }

  ... ...
};

... ...

@Override
public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  // call bindService here:
  doBindService();
}

@Override
protected void onResume() {
  super.onResume();
  // when activity is resumed:
  // mBound will not be ready if Activity is first created, in this case use onServiceConnected() callback perform service call.
  if (mBound) // <- or simply check if (mBoundservice != null)
    mBoundservice.foo();
}

... ...

Hope this helps.