Android Service进阶教程

以你之姓@ 2022-08-22 15:18 129阅读 0赞

转载地址：http://blog.csdn.net/vanpersie\_9987/article/details/51398817

绑定式Service在CS结构中扮演着Server的角色。绑定式Service允许其他组件（如Activity）绑定该Service、发送请求、接收响应、甚至IPC通信（ interprocess communication）。绑定式Service通常服务于其他应用程序的组件、且没有明确的后台的概念（does not run in the background indefinitely）。

--------------------

本文将介绍bound Service的相关内容，包括其创建、与其他应用组件如何绑定 等。有关Service的基础内容，您可以参考我翻译的官方文档：[《Android官方文档之Services》][Android_Services]；如需访问bound Service的官方原文，您可以点击这个链接：[《Bound Services》][Bound Services]。

--------------------

# 绑定式Service基础（The Basics） #

--------------------

绑定式Service是一个继承于Service的类。它可以与其他应用交互。为了实现绑定Service，您必须重写[onBind()][onBind]方法。该方法返回一个[IBinder][]接口，此接口是绑定式Service与其它应用组件交互的桥梁。

--------------------

其它应用组件可调用[`bindService()`][bindService]方法绑定Service。该方法需要传入的参数中包含一个实现了[`ServiceConnection`][ServiceConnection]接口的对象。该对象监控着组件与Service的绑定状态（which monitors the connection with the service）。`bindService()`方法并不返回数据，而一旦系统创建了组件与Service的连接，`ServiceConnection`接口中的方法[`onServiceConnected()`][onServiceConnected]将被回调，此时实现了`IBinder`接口的对象将传递至组件中，这样便实现了Service与绑定组件的通信（to deliver the IBinder that the client can use to communicate with the service）。

--------------------

Service可同时与多个组件绑定。然而Service仅在绑定的第一个组件时回调`onBind()`方法以获得`IBinder`接口对象，之后与该Service绑定的组件都传递的是同一个`IBinder`接口对象，而且并不再回调`onBind()`方法。

--------------------

当Service与绑定它的最后一个组件解绑时，系统将该Service 销毁（destroy），当然若Service还使用start方式启动过（调用`startService()`方法启动），则该Service并不会destroy。

--------------------

创建bound Service时，最重要的就是实现`onBind()`回调方法中的返回接口`IBinder`，下面将介绍几种不同实现`IBinder`接口的方式。

--------------------

# 创建绑定式Service（Creating a Bound Service） #

--------------------

以下列举了三种实现`IBinder`接口的方式：

*  继承[Binder类][Binder]（Extending the Binder class）：`Binder`是一个实现了`IBinder`接口的类。若Service只允许被本应用所在的进程访问（这是大多数情况），您需要继承`Binder`类，并将该对象作为`onBind()`方法的返回值。这样，与Service绑定的组件就可以通过该返回对象访问`Binder`的继承类中的public方法、甚至是Service中的方法（to directly access public methods available in either the Binder implementation or even the Service）。
    
    若在您的应用程序中，Service仅作为一个在后台工作的组件，那么这种方式最好不过了。除非您需要Service进行跨进程通信。

--------------------

*  使用Messenger（Using a Messenger）：如需要使用`IBinder`进行跨进程通信，您应当为Service创建一个[`Messenger`][Messenger]对象。这样，Service可以定义一个[`Handler`][Handler]对象以接受不同类型的[`Message`][Message]。`Handler`是`Messenger`的基础，它可以在客户端与`IBinder`共享（This Handler is the basis for a Messenger that can then share an IBinder with the client），并允许使用`Message`对象向Service端发送指令（allowing the client to send commands to the service using Message objects）。除此之外，亦可以在client端定义`Messenger`，这样Service端可以回传信息。

--------------------

*  使用AIDL（Using AIDL）：AIDL（Android Interface Definition Language）是Android接口定义语言的缩写。大多数应用程序并不应该使用AIDL创建bound Service，因为这需要应用程序有处理多线程的能力，而这会使应用程序变得复杂，在本文中并不打算具体介绍AIDL，如需访问有关AIDL的官方原文，您可以点击这个链接：[《Android Interface Definition Language (AIDL)》][Android Interface Definition Language _AIDL]。

--------------------

## 继承[Binder类][Binder]（Extending the Binder class） ##

--------------------

若您的Service仅是应用程序内部使用，并不需要跨进程通信，那么可以继承`Binder`类。这样，与Service绑定的组件可以直接访问Service中的public方法。

--------------------

！请注意：这种继承`Binder`类的方式仅适用于Service与绑定的组件处于同一应用程序或进程的情况，当然这也是最普遍的情况。举例来说，在播放音乐应用程序中，可以使用这种方式将一个Activity与Service绑定，而Service用于在后台播放音乐。

--------------------

创建方式：

1.  在Service类中创建一个继承于`Binder`的内部类。在Service类中定义public方法，以便client端可以访问。在继承于`Binder`的内部类中返回该Service实例。将该内部类实例作为`onBind()`返回参数。
2.  在客户端中的`onServiceConnected()`回调方法中接受`Binder`对象，并访问Service中的public方法。

--------------------

示例如下：

--------------------

在上例中，`LocalBinder`提供了`getService()`方法以获得`LocalService`实例。这样，client端可以通过该实例访问Service中pubic方法。比如，client端可以访问`LocalService`中的public方法`getRandomNumber()`，如下所示：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">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;
 }
 };
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li></ul>

--------------------

首先，在Activity的`onStart()`回调方法中调用`bindService()`绑定`LocalService`，这时，`LocalService`中的`onCreate()`与`onBind()`依次回调；接着，`ServiceConnection` 中的`onServiceConnected()`方法回调，表示组件与Service已绑定，这时可以通过回传给`onServiceConnected()`中的`IBinder`接口对象获得`LocalService`实例，一旦获得了该实例，便可以调用`LocalService`中的public方法，如`getRandomNumber()`方法。

--------------------

！请注意：Service应在合适的时候与组件解除绑定，本例中应在`onStop()`中解除与Service的绑定。

--------------------

## 使用Messenger绑定Service（Using a Messenger） ##

--------------------

当Service需要进行IPC通信时，应在Service中使用`Messenger`。使用`Messenger`的方式如下：

*  继承`Handler`类，并实现回调方法`handleMessage()`，每当client端访问Service中的方法时，`handleMessage()`都将回调（receives a callback for each call from a client）。
 *  需在Service中创建一个`Messenger`对象，构造该对象需传入一个`Handler`参数。
 *  调用`Messenger`的`getBinder()`返回一个IBinder对象，将该对象作为`onBind()`回调方法的返回值。
 *  client端通过`onServiceConnected()`回传的IBinder参数，构造`Messenger`对象，并将`Message`信息传入`Messenger`对象，发送给Service。
 *  Service在`Handler`的`handleMessage()`方法中接收`Message`信息。

--------------------

按照如此方式，client端并没有显式调用Service中的方法，而是传递了`Message`对象，并在Service的Handler中接收。

--------------------

以下是Service端示例：

<code class="hljs scala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">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();
 }
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li></ul>

--------------------

以下是client端接收示例：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">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;
 }
 }
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li></ul>

--------------------

本例中并未包含Service端向client端发送消息的逻辑，如需要Service答复client发送的消息，需在client端也创建一个`Messenger`对象，当`onServiceConnected()`方法被回调时，在`send()`方法中传入`replyTo`参数。

--------------------

# 绑定一个Service（Binding to a Service） #

--------------------

绑定Service是一个异步过程（The binding is asynchronous）：应用程序中的组件调用`bindService()`绑定一个Service，`bindService()`立即返回；接着系统回调Service的`onBind()`方法，而client并不会接收到`IBinder`参数，为了接收该参数，需要创建一个`ServiceConnection`实例，并将该实例传入`bindService()`中，系统会将`IBinder`回传至`ServiceConnection`的回调方法中（The ServiceConnection includes a callback method that the system calls to deliver the IBinder）。

--------------------

！请注意：只有activities、services、content providers可以绑定Service， broadcast receiver不能绑定Service（you cannot bind to a service from a broadcast receiver）。

--------------------

所以，绑定Service应按如下步骤：

*  实现`ServiceConnection`接口；
    
     *  实现`onServiceConnected()`方法：当client与Service建立绑定时，系统回调该方法，并将`onBind()`返回的`IBinder`参数回传至该方法中；
     *  实现`onServiceDisconnected()`方法：当绑定的Service意外终止时（ unexpectedly lost），系统回调该方法，如Service被进程kill或Service崩溃（crashed）。系统若回调`unBindService()`方法，将不会回调`onServiceDisconnected()`方法。

--------------------

*  调用`bindService()`，并传入`ServiceConnection`的实现类对象；

--------------------

*  当系统回调`onServiceConnected()`时，表示client与Service已绑定，此时可以访问Service中的public方法。

--------------------

*  系统回调`unbindService()`，解除绑定。
    
     *  ！请注意：当client与Service绑定时，系统destroy掉client端，这将破坏绑定状态（destruction causes the client to unbind）。好的做法是只要client与Service交互完成，就解除绑定（to unbind the client as soon as it is done interacting with the service）。

--------------------

下面的代码片段演示了如何绑定Service：

--------------------

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">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;
 }
 };</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

--------------------

下面演示了启动绑定的方式：

<code class="hljs actionscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Intent intent = new Intent(this, LocalService.class);
 bindService(intent, mConnection, Context.BIND_AUTO_CREATE);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

--------------------

其中第三个参数表示绑定的模式，通常为[`BIND_AUTO_CREATE`][BIND_AUTO_CREATE]，表示当Service还尚未处于alive状态时创建该Service。其它可用的参数为[`BIND_DEBUG_UNBIND`][BIND_DEBUG_UNBIND]、[`BIND_NOT_FOREGROUND`][BIND_NOT_FOREGROUND]，若不打算指定模式，可传入0。

--------------------

## 需要额外注意的地方（Additional notes） ##

--------------------

*  当连接错误时，系统会抛出[`DeadObjectException`][DeadObjectException]异常，这也是在client端调用Service中的方法时可能抛出的唯一异常（This is the only exception thrown by remote methods）。
 *  `binding 和 unbinding`应成对出调用。
    
     *  若当Activity在前台处于运行状态时，需要与绑定的Service交互，那么应在`onStart()`方法中`bindService()`，在`onStop()`中`unbindService()`。
     *  若当Activity在后台处于stop状态时，那么应在`onCreate()`方法中`bindService()`，在`onDestroy()`中`unbindService()`。此时系统将更易kill该Service。

--------------------

！请注意：请不要在`onResume() 和 onPause()`方法中绑定、解绑Service，因为这两个生命周期回调方法经常被回调，频繁的绑定与解绑会降低程序的执行效率。

--------------------

# 管理Bound Service的生命周期（Managing the Lifecycle of a Bound Service） #

--------------------

当Service不再与任何Client绑定时，系统将回收该Service（除非Service也用Start方式启动了（将回调`onStartCommand()`方法）），您无需手动管理一个纯bound Service的生命周期（you don’t have to manage the lifecycle of your service if it’s purely a bound service），系统会自动管理。

--------------------

无论Service绑定了多少个client，若您还回调了`onStartCommand()`方法，那么必须显式stop该Service，可以通过在Service中调用`stopSelf()`方法、或在其他组件中调用`stopService()`stop该Service。

--------------------

若通过两种方式（start、bound）同时启动了一个Service，那么如果希望Service在下一次绑定该client时回调[`onRebind()`][onRebind]方法，应在`onUnbind()`方法中返回true。按照这种方式，再次与该Service绑定的client仍可以在`onServiceConnected()`方法中接收到回传的`IBinder` 参数。如下图所示：

--------------------

![这里写图片描述][20160514133235113]

[Android_Services]: http://blog.csdn.net/vanpersie_9987/article/details/51360245
[Bound Services]: https://developer.android.com/guide/components/bound-services.html
[onBind]: https://developer.android.com/reference/android/app/Service.html#onBind%28android.content.Intent%29
[IBinder]: https://developer.android.com/reference/android/os/IBinder.html
[bindService]: https://developer.android.com/reference/android/content/Context.html#bindService%28android.content.Intent,%20android.content.ServiceConnection,%20int%29
[ServiceConnection]: https://developer.android.com/reference/android/content/ServiceConnection.html
[onServiceConnected]: https://developer.android.com/reference/android/content/ServiceConnection.html#onServiceConnected%28android.content.ComponentName,%20android.os.IBinder%29
[Binder]: https://developer.android.com/reference/android/os/Binder.html
[Messenger]: https://developer.android.com/reference/android/os/Messenger.html
[Handler]: https://developer.android.com/reference/android/os/Handler.html
[Message]: https://developer.android.com/reference/android/os/Message.html
[Android Interface Definition Language _AIDL]: https://developer.android.com/guide/components/aidl.html
[BIND_AUTO_CREATE]: https://developer.android.com/reference/android/content/Context.html#BIND_AUTO_CREATE
[BIND_DEBUG_UNBIND]: https://developer.android.com/reference/android/content/Context.html#BIND_DEBUG_UNBIND
[BIND_NOT_FOREGROUND]: https://developer.android.com/reference/android/content/Context.html#BIND_NOT_FOREGROUND
[DeadObjectException]: https://developer.android.com/reference/android/os/DeadObjectException.html
[onRebind]: https://developer.android.com/reference/android/app/Service.html#onRebind%28android.content.Intent%29
[20160514133235113]: /images/20220722/da879dca9d87459c821c5490832f44ae.png