Android系统开发入门-8.java服务回调

上一小节我们添加了一个 java service，也写了一个 client 来做测试。但只是单向的从 client 调用 service，实际项目中我们经常是需要双向调用的。某些事件发生的时候，service 端需要通知 client 来处理。这节我们就来学习一下如何从 service 端回调 client。为了简单起见，我们直接在上一小节的代码基础上进行修改。

1. 使用 aidl 定义callback接口

在 frameworks/base/core/java/android/pure 目录下创建 ICallback.aidl

package android.pure;
interface ICallback {
    void onMessage(in String message);
}

在 frameworks/base/Android.bp framework-defaults 模块中添加我们刚刚加的 aidl 文件

"core/java/android/pure/ICallback.aidl",

然后进入 framework/base 目录执行 mm -j 命令编译 framework.jar 模块。
编译成功后，会在 out/soong/.intermediates/frameworks/base/framework/android_common/gen/aidl/frameworks/base/core/java/android/pure 目录生成 ICallback.java 这个文件。

2. 添加注册回调接口

修改 IHelloService.aidl 如下

package android.pure;
import android.pure.ICallback;
interface IHelloService {
    void hello(in String name);
    void registerCallback(in int pid, in ICallback callback);
    void unregisterCallback(in int pid);
}

HelloService.java 修改如下：

package com.android.server;

import android.os.RemoteException;
import android.pure.IHelloService;
import android.pure.ICallback;
import android.util.Log;

import java.util.HashMap;
import java.util.Map;

public class HelloService extends IHelloService.Stub {
    private final String TAG = "HelloService";
    private Map<Integer, ICallback> mClients;

    public HelloService() {
        Log.d(TAG, "create hello service");
        mClients = new HashMap<>();
    }

    @Override
    public void hello(String name) {
        Log.d(TAG, "hello " + name);
        try {
            for (ICallback callback : mClients.values()) {
                callback.onMessage("message from service");
            }
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void registerCallback(int pid, ICallback callback) {
        mClients.put(pid, callback);
        Log.d(TAG, "registerCallback client's size = " + mClients.size());
    }

    @Override
    public void unregisterCallback(int pid) {
        mClients.remove(pid);
        Log.d(TAG, "unregisterCallback client's size = " + mClients.size());
    }
}

我们把 client 传递过来的 callback 保存到 mClients 集合中，当 client 调用 hello 接口时， 遍历 mclients 把消息传递给 client。

3. client 注册回调接口

修改 PureSettings apk 中的 MainActivity

package com.pure.settings;

import android.app.Activity;
import android.os.Bundle;
import android.os.RemoteException;
import android.os.ServiceManager;
import android.pure.ICallback;
import android.pure.IHelloService;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

    private static final String TAG = "PureSettings";
    private IHelloService service = null;

    private Button button;

    private ICallback callback = new ICallback.Stub() {

        @Override
        public void onMessage(String message) throws RemoteException {
            Log.d(TAG, "onMessage:" + message);
        }
    };

    private void test() {
        Log.d(TAG, "test");
        try {
            service.hello("qiushao");
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        service = IHelloService.Stub.asInterface(ServiceManager.getService("HelloService"));
        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                test();
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        try {
            service.registerCallback(android.os.Process.myPid(), callback);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        try {
            service.unregisterCallback(android.os.Process.myPid());
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }
}

4. 编译验证

整编系统，运行虚拟机，打开 PureSettings 测试，查看 logcat

pure:/ # logcat -s HelloService:D PureSettings:D
02-02 22:57:37.913  1769  1769 D HelloService: create hello service
02-02 22:57:53.498  1769  3058 D HelloService: registerCallback client's size = 1
02-02 22:57:56.587  3580  3580 D PureSettings: test
02-02 22:57:56.587  1769  3058 D HelloService: hello qiushao
02-02 22:57:56.588  3580  3580 D PureSettings: onMessage:message from service
02-02 22:58:01.160  1769  3503 D HelloService: unregisterCallback client's size = 0

从 log 上看，client 收到了 service 传递过来的消息。