何时应该使用 unbindService()，以及如何正确使用它来解除与使用 AIDL 接口的远程服务的绑定？

Question

I'm writing a simple music player, and I've created a playback service which implements an AIDL interface to bind with the clients, one a simple track browser and the other an even simpler player activity. The service controls the MediaPlayer object while the two activities use ServiceConnections to obtain connections to the service.

This is included in the onStart() methods of both activities:

@Override
public void onStart()
{
  super.onStart();
  Intent i = new Intent(this, PureService.class);
  startService(i);
  bindService(i, mConnection, 0);
}

I did this so that the service would not immediately stop upon unbinding. Of course, that hasn't actually been a problem because my activity refuses to unbind from the service at all. Anytime my application gets to unbindService in either of these activities, unbindService throws IllegalArgumentException every time, without exception (hehe).

In the onStop methods:

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

  if (mBound) {
    try {
      unbindService(mConnection);
    } catch (java.lang.IllegalArgumentException e)
    {
      //Print to log or make toast that it failed
    }
  }
  mBound = false;
}

What I'm wondering is this:

• Should I be calling unbindService() in the onStop() method? Or at all?
• Am I calling it correctly?
• Is there anything peculiar about the way I'm starting/binding the service that I should know about?
• Am I doing something utterly, completely wrong? I'm new to android programming so that's certainly not out of the question.

Thanks in advance.

EDIT: Here are the ServiceConnection overrides

public void onServiceConnected(ComponentName className, IBinder service) {
  mBound = true;
  mService = IPureService.Stub.asInterface(service);
}
public void onServiceDisconnected(ComponentName arg0) {
  mBound = false;
}

There's some additional code in the player activity, but it's unrelated to the binding itself.

Answer 1

First, unless you actually need to make calls to this service across processes (that is, from other.apks, or you are using android:process to split up your own.apk into multiple processes for some reason), then I really recommend just dropping the use of aidl. It is more complexity for no gain. The "Local Service Sample" in the Service documentation shows how to do this: http://developer.android.com/reference/android/app/Service.html

Second, doing a bind at the same time as a start is a strong indication of some basic flaw in the design. Starting a service and binding to a service are semantically very different, so will be done at different places based on those different semantics. That is, if both are even done at all... in fact it is an unusual situation where you are using both start and bind with the same service.

In the class implementation of a service for doing music playback, it would use start when it is actively performing playback (so its process doesn't get killed by the system when the user is no longer actively interacting with the application's UI). Starting the service when the user has enters the UI is likely to cause pain because now the start/stopped state of the service is not clearly defined -- it could be started either because it is doing playback or because the user happens to have gone into the app's UI, and now when is the right time to stop it? This is going to be troublesome.

Now as far as when to unbind -- you just need to make sure you always match an unbindService() with a previous bindService(). From your snippets of code it looks like you are doing this, but there are strange things in it like mBound never being set. In fact if you are consistently binding in onStart() and unbinding in onStop(), you should never need to have an mBound to decide whether to unbind, because onStop() is always called after onStart().

So with the code you give here, it doesn't look like there is a problem. If you are getting exceptions, though, there clearly is so it may be elsewhere in your app. To help narrow the problem down, you can use this flag when you call bindService() to get additional information in the log when the failure happens: http://developer.android.com/reference/android/content/Context.html#BIND_DEBUG_UNBIND

Answer 2

A couple of points:

1. Return START_STICKY in onStartCommand if you want your Service to live longer than the bound Activities.
2. unbindService() in onStop is fine: this is where I call it in multiple apps and I've never seen that particular error. I expect you have some other issue with your ServiceConnection: show the code for your mConnection object and we can probably figure out what's wrong with it.

Answer 3

I had this weird error as well. Then I tried doing the bind to the service in onResume() instead of in onStart() in the Activity, and voila, no more exceptions. I am still a bit clueless as to why this works, If someone could explain. I'd be one happy coder: :)