1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
|
page.title=Bound Services
parent.title=Services
parent.link=services.html
@jd:body
<div id="qv-wrapper">
<ol id="qv">
<h2>Quickview</h2>
<ul>
<li>A bound service allows other components to bind to it, in order to interact with it and
perform interprocess communication</li>
<li>A bound service is destroyed once all clients unbind, unless the service was also started</li>
</ul>
<h2>In this document</h2>
<ol>
<li><a href="#Basics">The Basics</a></li>
<li><a href="#Creating">Creating a Bound Service</a>
<ol>
<li><a href="#Binder">Extending the Binder class</a></li>
<li><a href="#Messenger">Using a Messenger</a></li>
</ol>
</li>
<li><a href="#Binding">Binding to a Service</a></li>
<li><a href="#Lifecycle">Managing the Lifecycle of a Bound Service</a></li>
</ol>
<h2>Key classes</h2>
<ol>
<li>{@link android.app.Service}</li>
<li>{@link android.content.ServiceConnection}</li>
<li>{@link android.os.IBinder}</li>
</ol>
<h2>Samples</h2>
<ol>
<li><a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/RemoteService.html">{@code
RemoteService}</a></li>
<li><a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalService.html">{@code
LocalService}</a></li>
</ol>
<h2>See also</h2>
<ol>
<li><a href="{@docRoot}guide/components/services.html">Services</a></li>
</ol>
</div>
<p>A bound service is the server in a client-server interface. A bound service allows components
(such as activities) to bind to the service, send requests, receive responses, and even perform
interprocess communication (IPC). A bound service typically lives only while it serves another
application component and does not run in the background indefinitely.</p>
<p>This document shows you how to create a bound service, including how to bind
to the service from other application components. However, you should also refer to the <a
href="{@docRoot}guide/components/services.html">Services</a> document for additional
information about services in general, such as how to deliver notifications from a service, set
the service to run in the foreground, and more.</p>
<h2 id="Basics">The Basics</h2>
<p>A bound service is an implementation of the {@link android.app.Service} class that allows
other applications to bind to it and interact with it. To provide binding for a
service, you must implement the {@link android.app.Service#onBind onBind()} callback method. This
method returns an {@link android.os.IBinder} object that defines the programming interface that
clients can use to interact with the service.</p>
<div class="sidebox-wrapper">
<div class="sidebox">
<h3>Binding to a Started Service</h3>
<p>As discussed in the <a href="{@docRoot}guide/components/services.html">Services</a>
document, you can create a service that is both started and bound. That is, the service can be
started by calling {@link android.content.Context#startService startService()}, which allows the
service to run indefinitely, and also allow a client to bind to the service by calling {@link
android.content.Context#bindService bindService()}.
<p>If you do allow your service to be started and bound, then when the service has been
started, the system does <em>not</em> destroy the service when all clients unbind. Instead, you must
explicitly stop the service, by calling {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}.</p>
<p>Although you should usually implement either {@link android.app.Service#onBind onBind()}
<em>or</em> {@link android.app.Service#onStartCommand onStartCommand()}, it's sometimes necessary to
implement both. For example, a music player might find it useful to allow its service to run
indefinitely and also provide binding. This way, an activity can start the service to play some
music and the music continues to play even if the user leaves the application. Then, when the user
returns to the application, the activity can bind to the service to regain control of playback.</p>
<p>Be sure to read the section about <a href="#Lifecycle">Managing the Lifecycle of a Bound
Service</a>, for more information about the service lifecycle when adding binding to a
started service.</p>
</div>
</div>
<p>A client can bind to the service by calling {@link android.content.Context#bindService
bindService()}. When it does, it must provide an implementation of {@link
android.content.ServiceConnection}, which monitors the connection with the service. The {@link
android.content.Context#bindService bindService()} method returns immediately without a value, but
when the Android system creates the connection between the
client and service, it calls {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} on the {@link
android.content.ServiceConnection}, to deliver the {@link android.os.IBinder} that
the client can use to communicate with the service.</p>
<p>Multiple clients can connect to the service at once. However, the system calls your service's
{@link android.app.Service#onBind onBind()} method to retrieve the {@link android.os.IBinder} only
when the first client binds. The system then delivers the same {@link android.os.IBinder} to any
additional clients that bind, without calling {@link android.app.Service#onBind onBind()} again.</p>
<p>When the last client unbinds from the service, the system destroys the service (unless the
service was also started by {@link android.content.Context#startService startService()}).</p>
<p>When you implement your bound service, the most important part is defining the interface
that your {@link android.app.Service#onBind onBind()} callback method returns. There are a few
different ways you can define your service's {@link android.os.IBinder} interface and the following
section discusses each technique.</p>
<h2 id="Creating">Creating a Bound Service</h2>
<p>When creating a service that provides binding, you must provide an {@link android.os.IBinder}
that provides the programming interface that clients can use to interact with the service. There
are three ways you can define the interface:</p>
<dl>
<dt><a href="#Binder">Extending the Binder class</a></dt>
<dd>If your service is private to your own application and runs in the same process as the client
(which is common), you should create your interface by extending the {@link android.os.Binder} class
and returning an instance of it from
{@link android.app.Service#onBind onBind()}. The client receives the {@link android.os.Binder} and
can use it to directly access public methods available in either the {@link android.os.Binder}
implementation or even the {@link android.app.Service}.
<p>This is the preferred technique when your service is merely a background worker for your own
application. The only reason you would not create your interface this way is because
your service is used by other applications or across separate processes.</dd>
<dt><a href="#Messenger">Using a Messenger</a></dt>
<dd>If you need your interface to work across different processes, you can create
an interface for the service with a {@link android.os.Messenger}. In this manner, the service
defines a {@link android.os.Handler} that responds to different types of {@link
android.os.Message} objects. This {@link android.os.Handler}
is the basis for a {@link android.os.Messenger} that can then share an {@link android.os.IBinder}
with the client, allowing the client to send commands to the service using {@link
android.os.Message} objects. Additionally, the client can define a {@link android.os.Messenger} of
its own so the service can send messages back.
<p>This is the simplest way to perform interprocess communication (IPC), because the {@link
android.os.Messenger} queues all requests into a single thread so that you don't have to design
your service to be thread-safe.</p>
</dd>
<dt>Using AIDL</dt>
<dd>AIDL (Android Interface Definition Language) performs all the work to decompose objects into
primitives that the operating system can understand and marshall them across processes to perform
IPC. The previous technique, using a {@link android.os.Messenger}, is actually based on AIDL as
its underlying structure. As mentioned above, the {@link android.os.Messenger} creates a queue of
all the client requests in a single thread, so the service receives requests one at a time. If,
however, you want your service to handle multiple requests simultaneously, then you can use AIDL
directly. In this case, your service must be capable of multi-threading and be built thread-safe.
<p>To use AIDL directly, you must
create an {@code .aidl} file that defines the programming interface. The Android SDK tools use
this file to generate an abstract class that implements the interface and handles IPC, which you
can then extend within your service.</p>
</dd>
</dl>
<p class="note"><strong>Note:</strong> Most applications <strong>should not</strong> use AIDL to
create a bound service, because it may require multithreading capabilities and
can result in a more complicated implementation. As such, AIDL is not suitable for most applications
and this document does not discuss how to use it for your service. If you're certain that you need
to use AIDL directly, see the <a href="{@docRoot}guide/components/aidl.html">AIDL</a>
document.</p>
<h3 id="Binder">Extending the Binder class</h3>
<p>If your service is used only by the local application and does not need to work across processes,
then you can implement your own {@link android.os.Binder} class that provides your client direct
access to public methods in the service.</p>
<p class="note"><strong>Note:</strong> This works only if the client and service are in the same
application and process, which is most common. For example, this would work well for a music
application that needs to bind an activity to its own service that's playing music in the
background.</p>
<p>Here's how to set it up:</p>
<ol>
<li>In your service, create an instance of {@link android.os.Binder} that either:
<ul>
<li>contains public methods that the client can call</li>
<li>returns the current {@link android.app.Service} instance, which has public methods the
client can call</li>
<li>or, returns an instance of another class hosted by the service with public methods the
client can call</li>
</ul>
<li>Return this instance of {@link android.os.Binder} from the {@link
android.app.Service#onBind onBind()} callback method.</li>
<li>In the client, receive the {@link android.os.Binder} from the {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback method and
make calls to the bound service using the methods provided.</li>
</ol>
<p class="note"><strong>Note:</strong> The reason the service and client must be in the same
application is so the client can cast the returned object and properly call its APIs. The service
and client must also be in the same process, because this technique does not perform any
marshalling across processes.</p>
<p>For example, here's a service that provides clients access to methods in the service through
a {@link android.os.Binder} implementation:</p>
<pre>
public class LocalService extends Service {
// Binder given to clients
private final IBinder mBinder = new LocalBinder();
// Random number generator
private final Random mGenerator = new Random();
/**
* Class used for the client Binder. Because we know this service always
* runs in the same process as its clients, we don't need to deal with IPC.
*/
public class LocalBinder extends Binder {
LocalService getService() {
// Return this instance of LocalService so clients can call public methods
return LocalService.this;
}
}
@Override
public IBinder onBind(Intent intent) {
return mBinder;
}
/** method for clients */
public int getRandomNumber() {
return mGenerator.nextInt(100);
}
}
</pre>
<p>The {@code LocalBinder} provides the {@code getService()} method for clients to retrieve the
current instance of {@code LocalService}. This allows clients to call public methods in the
service. For example, clients can call {@code getRandomNumber()} from the service.</p>
<p>Here's an activity that binds to {@code LocalService} and calls {@code getRandomNumber()}
when a button is clicked:</p>
<pre>
public class BindingActivity extends Activity {
LocalService mService;
boolean mBound = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
protected void onStart() {
super.onStart();
// Bind to LocalService
Intent intent = new Intent(this, LocalService.class);
bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
}
@Override
protected void onStop() {
super.onStop();
// Unbind from the service
if (mBound) {
unbindService(mConnection);
mBound = false;
}
}
/** Called when a button is clicked (the button in the layout file attaches to
* this method with the android:onClick attribute) */
public void onButtonClick(View v) {
if (mBound) {
// Call a method from the LocalService.
// However, if this call were something that might hang, then this request should
// occur in a separate thread to avoid slowing down the activity performance.
int num = mService.getRandomNumber();
Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
}
}
/** Defines callbacks for service binding, passed to bindService() */
private ServiceConnection mConnection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName className,
IBinder service) {
// We've bound to LocalService, cast the IBinder and get LocalService instance
LocalBinder binder = (LocalBinder) service;
mService = binder.getService();
mBound = true;
}
@Override
public void onServiceDisconnected(ComponentName arg0) {
mBound = false;
}
};
}
</pre>
<p>The above sample shows how the client binds to the service using an implementation of
{@link android.content.ServiceConnection} and the {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback. The next
section provides more information about this process of binding to the service.</p>
<p class="note"><strong>Note:</strong> The example above doesn't explicitly unbind from the service,
but all clients should unbind at an appropriate time (such as when the activity pauses).</p>
<p>For more sample code, see the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalService.html">{@code
LocalService.java}</a> class and the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalServiceActivities.html">{@code
LocalServiceActivities.java}</a> class in <a
href="{@docRoot}resources/samples/ApiDemos/index.html">ApiDemos</a>.</p>
<h3 id="Messenger">Using a Messenger</h3>
<div class="sidebox-wrapper">
<div class="sidebox">
<h4>Compared to AIDL</h4>
<p>When you need to perform IPC, using a {@link android.os.Messenger} for your interface is
simpler than implementing it with AIDL, because {@link android.os.Messenger} queues
all calls to the service, whereas, a pure AIDL interface sends simultaneous requests to the
service, which must then handle multi-threading.</p>
<p>For most applications, the service doesn't need to perform multi-threading, so using a {@link
android.os.Messenger} allows the service to handle one call at a time. If it's important
that your service be multi-threaded, then you should use <a
href="{@docRoot}guide/components/aidl.html">AIDL</a> to define your interface.</p>
</div>
</div>
<p>If you need your service to communicate with remote processes, then you can use a
{@link android.os.Messenger} to provide the interface for your service. This technique allows
you to perform interprocess communication (IPC) without the need to use AIDL.</p>
<p>Here's a summary of how to use a {@link android.os.Messenger}:</p>
<ul>
<li>The service implements a {@link android.os.Handler} that receives a callback for each
call from a client.</li>
<li>The {@link android.os.Handler} is used to create a {@link android.os.Messenger} object
(which is a reference to the {@link android.os.Handler}).</li>
<li>The {@link android.os.Messenger} creates an {@link android.os.IBinder} that the service
returns to clients from {@link android.app.Service#onBind onBind()}.</li>
<li>Clients use the {@link android.os.IBinder} to instantiate the {@link android.os.Messenger}
(that references the service's {@link android.os.Handler}), which the client uses to send
{@link android.os.Message} objects to the service.</li>
<li>The service receives each {@link android.os.Message} in its {@link
android.os.Handler}—specifically, in the {@link android.os.Handler#handleMessage
handleMessage()} method.</li>
</ul>
<p>In this way, there are no "methods" for the client to call on the service. Instead, the
client delivers "messages" ({@link android.os.Message} objects) that the service receives in
its {@link android.os.Handler}.</p>
<p>Here's a simple example service that uses a {@link android.os.Messenger} interface:</p>
<pre>
public class MessengerService extends Service {
/** Command to the service to display a message */
static final int MSG_SAY_HELLO = 1;
/**
* Handler of incoming messages from clients.
*/
class IncomingHandler extends Handler {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_SAY_HELLO:
Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
break;
default:
super.handleMessage(msg);
}
}
}
/**
* Target we publish for clients to send messages to IncomingHandler.
*/
final Messenger mMessenger = new Messenger(new IncomingHandler());
/**
* When binding to the service, we return an interface to our messenger
* for sending messages to the service.
*/
@Override
public IBinder onBind(Intent intent) {
Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
return mMessenger.getBinder();
}
}
</pre>
<p>Notice that the {@link android.os.Handler#handleMessage handleMessage()} method in the
{@link android.os.Handler} is where the service receives the incoming {@link android.os.Message}
and decides what to do, based on the {@link android.os.Message#what} member.</p>
<p>All that a client needs to do is create a {@link android.os.Messenger} based on the {@link
android.os.IBinder} returned by the service and send a message using {@link
android.os.Messenger#send send()}. For example, here's a simple activity that binds to the
service and delivers the {@code MSG_SAY_HELLO} message to the service:</p>
<pre>
public class ActivityMessenger extends Activity {
/** Messenger for communicating with the service. */
Messenger mService = null;
/** Flag indicating whether we have called bind on the service. */
boolean mBound;
/**
* Class for interacting with the main interface of the service.
*/
private ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
// This is called when the connection with the service has been
// established, giving us the object we can use to
// interact with the service. We are communicating with the
// service using a Messenger, so here we get a client-side
// representation of that from the raw IBinder object.
mService = new Messenger(service);
mBound = true;
}
public void onServiceDisconnected(ComponentName className) {
// This is called when the connection with the service has been
// unexpectedly disconnected -- that is, its process crashed.
mService = null;
mBound = false;
}
};
public void sayHello(View v) {
if (!mBound) return;
// Create and send a message to the service, using a supported 'what' value
Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
try {
mService.send(msg);
} catch (RemoteException e) {
e.printStackTrace();
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
protected void onStart() {
super.onStart();
// Bind to the service
bindService(new Intent(this, MessengerService.class), mConnection,
Context.BIND_AUTO_CREATE);
}
@Override
protected void onStop() {
super.onStop();
// Unbind from the service
if (mBound) {
unbindService(mConnection);
mBound = false;
}
}
}
</pre>
<p>Notice that this example does not show how the service can respond to the client. If you want the
service to respond, then you need to also create a {@link android.os.Messenger} in the client. Then
when the client receives the {@link android.content.ServiceConnection#onServiceConnected
onServiceConnected()} callback, it sends a {@link android.os.Message} to the service that includes
the client's {@link android.os.Messenger} in the {@link android.os.Message#replyTo} parameter
of the {@link android.os.Messenger#send send()} method.</p>
<p>You can see an example of how to provide two-way messaging in the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/MessengerService.html">{@code
MessengerService.java}</a> (service) and <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/MessengerServiceActivities.html">{@code
MessengerServiceActivities.java}</a> (client) samples.</p>
<h2 id="Binding">Binding to a Service</h2>
<p>Application components (clients) can bind to a service by calling
{@link android.content.Context#bindService bindService()}. The Android
system then calls the service's {@link android.app.Service#onBind
onBind()} method, which returns an {@link android.os.IBinder} for interacting with the service.</p>
<p>The binding is asynchronous. {@link android.content.Context#bindService
bindService()} returns immediately and does <em>not</em> return the {@link android.os.IBinder} to
the client. To receive the {@link android.os.IBinder}, the client must create an instance of {@link
android.content.ServiceConnection} and pass it to {@link android.content.Context#bindService
bindService()}. The {@link android.content.ServiceConnection} includes a callback method that the
system calls to deliver the {@link android.os.IBinder}.</p>
<p class="note"><strong>Note:</strong> Only activities, services, and content providers can bind
to a service—you <strong>cannot</strong> bind to a service from a broadcast receiver.</p>
<p>So, to bind to a service from your client, you must: </p>
<ol>
<li>Implement {@link android.content.ServiceConnection}.
<p>Your implementation must override two callback methods:</p>
<dl>
<dt>{@link android.content.ServiceConnection#onServiceConnected onServiceConnected()}</dt>
<dd>The system calls this to deliver the {@link android.os.IBinder} returned by
the service's {@link android.app.Service#onBind onBind()} method.</dd>
<dt>{@link android.content.ServiceConnection#onServiceDisconnected
onServiceDisconnected()}</dt>
<dd>The Android system calls this when the connection to the service is unexpectedly
lost, such as when the service has crashed or has been killed. This is <em>not</em> called when the
client unbinds.</dd>
</dl>
</li>
<li>Call {@link
android.content.Context#bindService bindService()}, passing the {@link
android.content.ServiceConnection} implementation. </li>
<li>When the system calls your {@link android.content.ServiceConnection#onServiceConnected
onServiceConnected()} callback method, you can begin making calls to the service, using
the methods defined by the interface.</li>
<li>To disconnect from the service, call {@link
android.content.Context#unbindService unbindService()}.
<p>When your client is destroyed, it will unbind from the service, but you should always unbind
when you're done interacting with the service or when your activity pauses so that the service can
shutdown while its not being used. (Appropriate times to bind and unbind is discussed
more below.)</p>
</li>
</ol>
<p>For example, the following snippet connects the client to the service created above by
<a href="#Binder">extending the Binder class</a>, so all it must do is cast the returned
{@link android.os.IBinder} to the {@code LocalService} class and request the {@code
LocalService} instance:</p>
<pre>
LocalService mService;
private ServiceConnection mConnection = new ServiceConnection() {
// Called when the connection with the service is established
public void onServiceConnected(ComponentName className, IBinder service) {
// Because we have bound to an explicit
// service that is running in our own process, we can
// cast its IBinder to a concrete class and directly access it.
LocalBinder binder = (LocalBinder) service;
mService = binder.getService();
mBound = true;
}
// Called when the connection with the service disconnects unexpectedly
public void onServiceDisconnected(ComponentName className) {
Log.e(TAG, "onServiceDisconnected");
mBound = false;
}
};
</pre>
<p>With this {@link android.content.ServiceConnection}, the client can bind to a service by passing
this it to {@link android.content.Context#bindService bindService()}. For example:</p>
<pre>
Intent intent = new Intent(this, LocalService.class);
bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
</pre>
<ul>
<li>The first parameter of {@link android.content.Context#bindService bindService()} is an
{@link android.content.Intent} that explicitly names the service to bind (thought the intent
could be implicit).</li>
<li>The second parameter is the {@link android.content.ServiceConnection} object.</li>
<li>The third parameter is a flag indicating options for the binding. It should usually be {@link
android.content.Context#BIND_AUTO_CREATE} in order to create the service if its not already alive.
Other possible values are {@link android.content.Context#BIND_DEBUG_UNBIND}
and {@link android.content.Context#BIND_NOT_FOREGROUND}, or {@code 0} for none.</li>
</ul>
<h3>Additional notes</h3>
<p>Here are some important notes about binding to a service:</p>
<ul>
<li>You should always trap {@link android.os.DeadObjectException} exceptions, which are thrown
when the connection has broken. This is the only exception thrown by remote methods.</li>
<li>Objects are reference counted across processes. </li>
<li>You should usually pair the binding and unbinding during
matching bring-up and tear-down moments of the client's lifecycle. For example:
<ul>
<li>If you only need to interact with the service while your activity is visible, you
should bind during {@link android.app.Activity#onStart onStart()} and unbind during {@link
android.app.Activity#onStop onStop()}.</li>
<li>If you want your activity to receive responses even while it is stopped in the
background, then you can bind during {@link android.app.Activity#onCreate onCreate()} and unbind
during {@link android.app.Activity#onDestroy onDestroy()}. Beware that this implies that your
activity needs to use the service the entire time it's running (even in the background), so if
the service is in another process, then you increase the weight of the process and it becomes
more likely that the system will kill it.</li>
</ul>
<p class="note"><strong>Note:</strong> You should usually <strong>not</strong> bind and unbind
during your activity's {@link android.app.Activity#onResume onResume()} and {@link
android.app.Activity#onPause onPause()}, because these callbacks occur at every lifecycle transition
and you should keep the processing that occurs at these transitions to a minimum. Also, if
multiple activities in your application bind to the same service and there is a transition between
two of those activities, the service may be destroyed and recreated as the current activity unbinds
(during pause) before the next one binds (during resume). (This activity transition for how
activities coordinate their lifecycles is described in the <a
href="{@docRoot}guide/components/activities.html#CoordinatingActivities">Activities</a>
document.)</p>
</ul>
<p>For more sample code, showing how to bind to a service, see the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/RemoteService.html">{@code
RemoteService.java}</a> class in <a
href="{@docRoot}resources/samples/ApiDemos/index.html">ApiDemos</a>.</p>
<h2 id="Lifecycle">Managing the Lifecycle of a Bound Service</h2>
<div class="figure" style="width:588px">
<img src="{@docRoot}images/fundamentals/service_binding_tree_lifecycle.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The lifecycle for a service that is started
and also allows binding.</p>
</div>
<p>When a service is unbound from all clients, the Android system destroys it (unless it was also
started with {@link android.app.Service#onStartCommand onStartCommand()}). As such, you don't have
to manage the lifecycle of your service if it's purely a bound
service—the Android system manages it for you based on whether it is bound to any clients.</p>
<p>However, if you choose to implement the {@link android.app.Service#onStartCommand
onStartCommand()} callback method, then you must explicitly stop the service, because the
service is now considered to be <em>started</em>. In this case, the service runs until the service
stops itself with {@link android.app.Service#stopSelf()} or another component calls {@link
android.content.Context#stopService stopService()}, regardless of whether it is bound to any
clients.</p>
<p>Additionally, if your service is started and accepts binding, then when the system calls
your {@link android.app.Service#onUnbind onUnbind()} method, you can optionally return
{@code true} if you would like to receive a call to {@link android.app.Service#onRebind
onRebind()} the next time a client binds to the service (instead of receiving a call to {@link
android.app.Service#onBind onBind()}). {@link android.app.Service#onRebind
onRebind()} returns void, but the client still receives the {@link android.os.IBinder} in its
{@link android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback.
Below, figure 1 illustrates the logic for this kind of lifecycle.</p>
<p>For more information about the lifecycle of an started service, see the <a
href="{@docRoot}guide/components/services.html#Lifecycle">Services</a> document.</p>
|