在Android开发中，有时候我们需要让一个Activity绑定一个Service，进行两者之间的通信。当Activity绑定成功以后，就可以调用Service中的public（或有对应可见级别）的方法。如果Service想把一些信息反馈给Activity，则需要将Activity的Listener传递给Service，由Service负责调用。
这样的做法是可以实现功能的，但有一些缺憾。
首先，Service需要把被调用的方法做成public的，不利于隐藏实现。
第二，调用显得凌乱，在Activity中调用了Service的方法，或者在Service中调用Activity的方法。
所以，developer文档提供了另外一种方式，使用Messenger。API Demos也给出了实现的示例，但该示例没有给出Service如何回应Activity部分的实现，只是给了一段提示性的说明文档：
https://developer.android.com/guide/components/bound-services.html
Notice that this example does not show how the service can respond to the client. If you want the service to respond, you need to also create a Messenger in the client. When the client receives the onServiceConnected() callback, it sends a Message to the service that includes the client‘s Messenger in the replyTo parameter of the send() method.
根据这个提示，我做了一些实验，走通了Service回应Activity的流程。
Activity与Service之间的通信，无论是Service向Activity发送消息，还是Activity发送消息，都通过Messenger类，这样，Service和Activity就不需要提供public的方法作为通信接口。
首先创建一个工程，实现了如下功能，有一个Activity，上面有三个Button和一个TextView，点击任何一个button，Activity的send messenger就会向Service发送一个message，这个message的replayTo，是Activity的另外一个Messenger， 叫receive messenger。service收到message以后。进行相应的处理，例如生成一个integer，float，string，把处理结果通过receive messenger发送出去，Activity就会收到这个消息。
下面是完整代码，只有3个Class，其他都是AndroidStudio自动生成的。
MainActivity
1 public class MainActivity extends AppCompatActivity {
2
3 /*
4 标记是否已经绑定Service。
5 Marking whether the service has been bound.
6 */
7 private boolean bServiceConnected;
8
9 /*
10 这个Messenger用于向Service发送Message。
11 This Messenger is used to send message to service.
12 */
13 private Messenger mSendMessenger;
14
15 /*
16 这个Messenger用于接收服务器发送的Message。
17 This Messenger is used to receive message from service.
18 */
19 private Messenger mReceiveMessenger;
20
21
22 private TextView mMessageText;
23
24 /*
25 处理从Service收到的Message。
26 Handling Messages received from service.
27 */
28 private Handler mHandler = new Handler() {
29 @Override
30 public void handleMessage(Message msg) {
31 switch (msg.what) {
32 case MessageSource.MSG_CREATE_FLOAT:
33 String strI = (String) msg.obj;
34 mMessageText.setText(strI);
35 break;
36 case MessageSource.MSG_CREATE_INT:
37 String strF = (String) msg.obj;
38 mMessageText.setText(strF);
39 break;
40 case MessageSource.MSG_CREATE_STRING:
41 String strS = (String) msg.obj;
42 mMessageText.setText(strS);
43 break;
44 default:
45 break;
46 }
47 }
48 };
49
50 @Override
51 protected void onCreate(Bundle savedInstanceState) {
52 super.onCreate(savedInstanceState);
53 setContentView(R.layout.activity_main);
54
55 mMessageText = (TextView) findViewById(R.id.message_from_service);
56
57 Button createIntBt = (Button) findViewById(R.id.let_service_create_int);
58 createIntBt.setOnClickListener(new View.OnClickListener() {
59 @Override
60 public void onClick(View view) {
61 handleButtonClick(MessageSource.MSG_CREATE_INT);
62 }
63 });
64
65 Button createFloatBt = (Button) findViewById(R.id.let_service_create_float);
66 createFloatBt.setOnClickListener(new View.OnClickListener() {
67 @Override
68 public void onClick(View view) {
69 handleButtonClick(MessageSource.MSG_CREATE_FLOAT);
70 }
71 });
72
73 Button createStringBt = (Button) findViewById(R.id.let_service_create_string);
74 createStringBt.setOnClickListener(new View.OnClickListener() {
75 @Override
76 public void onClick(View view) {
77 handleButtonClick(MessageSource.MSG_CREATE_STRING);
78 }
79 });
80 }
81
82 @Override
83 protected void onResume() {
84 super.onResume();
85 if (!bServiceConnected) {
86 bindService();
87 }
88
89 }
90
91 @Override
92 protected void onPause() {
93 super.onPause();
94 unbindService(mServiceConnection);
95 }
96
97 private void handleButtonClick(int type) {
98 if (bServiceConnected) {
99 Message msg = new Message();
100 msg.what = type;
101 msg.replyTo = mReceiveMessenger;
102 try {
103 mSendMessenger.send(msg);
104 } catch (RemoteException e) {
105 e.printStackTrace();
106 }
107 } else {
108 Toast.makeText(this, "Service has not been bound.", Toast.LENGTH_SHORT).show();
109 }
110 }
111
112 private void bindService() {
113 Intent intent = new Intent(getApplicationContext(), MainService.class);
114 bindService(intent, mServiceConnection, Context.BIND_AUTO_CREATE);
115 }
116
117 private ServiceConnection mServiceConnection = new ServiceConnection() {
118 @Override
119 public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
120 bServiceConnected = true;
121 mSendMessenger = new Messenger(iBinder);
122 mReceiveMessenger = new Messenger(mHandler);
123 }
124
125 @Override
126 public void onServiceDisconnected(ComponentName componentName) {
127 bServiceConnected = false;
128 mSendMessenger = null;
129 mReceiveMessenger = null;
130 }
131 };
132 }
MainService
1 public class MainService extends Service {
2
3 /*
4 这个Handler负责接收Activity的Message，收到一个Message时，通过获取Message的replayTo得到一个Messenger实例，
5 使用这个Messenger向Activity发送Message。
6 This Handler is in charge of receiving Messages sending from Activity. When it receiving a
7 Message, get The replayTo which is a Messenger instance from this Message. Using this Messenger
8 to send Message to Activity.
9 */
10 private Handler mActMsgHandler = new Handler() {
11 @Override
12 public void handleMessage(Message msg) {
13 switch (msg.what) {
14 case MessageSource.MSG_CREATE_INT:
15 createAndSendInt(msg.replyTo);
16 break;
17 case MessageSource.MSG_CREATE_FLOAT:
18 createAndSendFloat(msg.replyTo);
19 break;
20 case MessageSource.MSG_CREATE_STRING:
21 createAndSendString(msg.replyTo);
22 break;
23 default:
24
25 break;
26 }
27 }
28 };
29
30 /*
31 这个Messenger用于向Activity发送Message。
32 This Messenger is used to send Message to Activity.
33 */
34 private Messenger mSendMessenger = new Messenger(mActMsgHandler);
35
36 /*
37 假设有耗时的操作需要异步进行。
38 Suppose we have long-running jobs and execute asynchronously.
39 */
40 private Executor mExecutor = Executors.newCachedThreadPool();
41
42 public MainService() {
43 }
44
45 @Override
46 public IBinder onBind(Intent intent) {
47 return mSendMessenger.getBinder();
48 }
49
50 private void createAndSendInt(final Messenger messenger) {
51 mExecutor.execute(new Runnable() {
52 @Override
53 public void run() {
54 Random random = new Random();
55 int ret = random.nextInt();
56 String str = "Give you a int: " + ret;
57 send(MessageSource.MSG_CREATE_INT, str, messenger);
58
59 }
60 });
61 }
62
63 private void createAndSendFloat(final Messenger messenger) {
64 mExecutor.execute(new Runnable() {
65 @Override
66 public void run() {
67 Random random = new Random();
68 float ret = random.nextFloat();
69 String str = "Give you a float: " + ret;
70 send(MessageSource.MSG_CREATE_FLOAT, str, messenger);
71 }
72 });
73 }
74
75 private void createAndSendString(final Messenger messenger) {
76 mExecutor.execute(new Runnable() {
77 @Override
78 public void run() {
79 int len = MessageSource.STRING_SOURCE_ARRAY.length;
80 Random random = new Random();
81 int index = random.nextInt(len);
82 String ret = MessageSource.STRING_SOURCE_ARRAY[index];
83 String str = "Give you a string: " + ret;
84 send(MessageSource.MSG_CREATE_STRING, str, messenger);
85 }
86 });
87 }
88
89 private void send(int type, String str, Messenger messenger) {
90 Message msg = new Message();
91 msg.what = type;
92 msg.obj = str;
93 try {
94 messenger.send(msg);
95 } catch (RemoteException e) {
96 e.printStackTrace();
97 }
98 }
99 }
还有一个辅助的Class
MessageSource
1 public class MessageSource {
2 public static final int MSG_CREATE_INT = 1;
3 public static final int MSG_CREATE_FLOAT = 2;
4 public static final int MSG_CREATE_STRING = 3;
5
6 public static final String[] STRING_SOURCE_ARRAY = new String[]{"Awake from dreams",
7 "I find the locked tower high",
8 "Sober from wine", "I see the curtain hanging low",
9 "As last year spring grief seems to grow",
10 "Amid the falling blooms alone stand I",
11 "In the fine rain a pair of swallows fly",
12 "I still remember when I first saw pretty Ping",
13 "In silken dress embroidered with two hearts in a ring",
14 "Revealing lovesickness by touching pipa’s string",
15 "The moon shines bright just as last year",
16 "It did see her like a cloud disappear"};
17 }
应用截图和操作说明，当点击 CREATE A INTEGER按钮时，Activity就会向Service发送一个Message。Service收到这个Message以后，根据Message的what，产生了一个Integer，然后把这个integer拼接到一个字符串后面。创建一个Message，然后用收到的Message的replayTo，发送出去。Activity就收到了Service发来的Message，然后更新TextView里面的内容。
时间： 2024-10-11 22:50:29