1. 把socket連結和心跳功能都放在一個Service中，為什麼要放在Service中？

一般我們這種socket幾乎是跟app的生命週期一樣長，甚至更長。不管在不在Service中去完成操作，我們都得開非同步執行緒，雖然Service並不是非同步操作，但是為了提升我們任務的優先順序，我們最好是放在Service中，因為Service是由Android系統管理的，並且擁有比較高的優先順序，執行緒是java中的非同步任務載體，可以說Android系統不太認識執行緒。放在Service中可以很大程度上避免任務被回收或者關閉

2. 為什麼需要心跳機制？

由於移動裝置的網路的複雜性，經常會出現網路斷開，如果沒有心跳包的檢測，客戶端只會在需要傳送資料的時候才知道自己已經斷線，會延誤，甚至丟失伺服器傳送過來的資料。我們可以每隔3秒鐘或者每間隔1秒鐘去判斷一下socket是否斷開，沒斷開就讀取資料，斷開了就重新連線socket。

3. Service與Activity之間怎麼通訊？

3.1 Activity呼叫bindService (Intent service, ServiceConnection conn, int flags)方法，得到Service物件的一個引用，這樣Activity可以直接呼叫到Service中的方法，如果要主動通知Activity，我們可以利用回撥方法

3.2 Service向Activity傳送訊息，可以使用廣播，當然Activity要註冊相應的接收器。比如Service要向多個Activity傳送同樣的訊息的話，用這種方法就更好

4.原始碼

4.1 Service程式碼

package danxx.library.socket;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.ref.WeakReference;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Arrays;

import android.app.Service;
import android.content.Intent;
import
 
 android.os.Handler;
import android.os.IBinder;
import android.os.RemoteException;
import android.support.v4.content.LocalBroadcastManager;
import android.util.Log;

/**
 * Created by dawish on 2017/7/24.
 *  由於移動裝置的網路的複雜性，經常會出現網路斷開，如果沒有心跳包的檢測，
 *  客戶端只會在需要傳送資料的時候才知道自己已經斷線，會延誤，甚至丟失伺服器傳送過來的資料。
 */

public class BackService extends Service {
    private static final String TAG = "danxx";
    /**心跳頻率*/
    private static final long HEART_BEAT_RATE = 3 * 1000;
    /**伺服器ip地址*/
    public static final String HOST = "192.168.123.27";// "192.168.1.21";//
    /**伺服器埠號*/
    public static final int PORT = 9800;
    /**伺服器訊息回覆廣播*/
    public static final String MESSAGE_ACTION="message_ACTION";
    /**伺服器心跳回復廣播*/
    public static final String HEART_BEAT_ACTION="heart_beat_ACTION";
    /**讀執行緒*/
    private ReadThread mReadThread;

    private LocalBroadcastManager mLocalBroadcastManager;
    /***/
    private WeakReference<Socket> mSocket;

    // For heart Beat
    private Handler mHandler = new Handler();
    /**心跳任務，不斷重複呼叫自己*/
    private Runnable heartBeatRunnable = new Runnable() {

        @Override
        public void run() {
            if (System.currentTimeMillis() - sendTime >= HEART_BEAT_RATE) {
                boolean isSuccess = sendMsg("HeartBeat");//就傳送一個\r\n過去 如果傳送失敗，就重新初始化一個socket
                if (!isSuccess) {
                    mHandler.removeCallbacks(heartBeatRunnable);
                    mReadThread.release();
                    releaseLastSocket(mSocket);
                    new InitSocketThread().start();
                }
            }
            mHandler.postDelayed(this, HEART_BEAT_RATE);
        }
    };

    private long sendTime = 0L;
    /**
     * aidl通訊回撥
     */
    private IBackService.Stub iBackService = new IBackService.Stub() {

        /**
         * 收到內容傳送訊息
         * @param message 需要傳送到伺服器的訊息
         * @return
         * @throws RemoteException
         */
        @Override
        public boolean sendMessage(String message) throws RemoteException {
            return sendMsg(message);
        }
    };

    @Override
    public IBinder onBind(Intent arg0) {
        return iBackService;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        new InitSocketThread().start();
        mLocalBroadcastManager=LocalBroadcastManager.getInstance(this);

    }
    public boolean sendMsg(final String msg) {
        if (null == mSocket || null == mSocket.get()) {
            return false;
        }
        final Socket soc = mSocket.get();
            if (!soc.isClosed() && !soc.isOutputShutdown()) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            OutputStream os = soc.getOutputStream();
                            String message = msg + "\r\n";
                            os.write(message.getBytes());
                            os.flush();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }).start();
                sendTime = System.currentTimeMillis();//每次傳送成資料，就改一下最後成功傳送的時間，節省心跳間隔時間
            } else {
                return false;
            }
        return true;
    }

    private void initSocket() {//初始化Socket
        try {
            Socket so = new Socket(HOST, PORT);
            mSocket = new WeakReference<Socket>(so);
            mReadThread = new ReadThread(so);
            mReadThread.start();
            mHandler.postDelayed(heartBeatRunnable, HEART_BEAT_RATE);//初始化成功後，就準備傳送心跳包
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * 心跳機制判斷出socket已經斷開後，就銷燬連線方便重新建立連線
     * @param mSocket
     */
    private void releaseLastSocket(WeakReference<Socket> mSocket) {
        try {
            if (null != mSocket) {
                Socket sk = mSocket.get();
                if (!sk.isClosed()) {
                    sk.close();
                }
                sk = null;
                mSocket = null;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    class InitSocketThread extends Thread {
        @Override
        public void run() {
            super.run();
            initSocket();
        }
    }

    // Thread to read content from Socket
    class ReadThread extends Thread {
        private WeakReference<Socket> mWeakSocket;
        private boolean isStart = true;

        public ReadThread(Socket socket) {
            mWeakSocket = new WeakReference<Socket>(socket);
        }

        public void release() {
            isStart = false;
            releaseLastSocket(mWeakSocket);
        }

        @Override
        public void run() {
            super.run();
            Socket socket = mWeakSocket.get();
            if (null != socket) {
                try {
                    InputStream is = socket.getInputStream();
                    byte[] buffer = new byte[1024 * 4];
                    int length = 0;
                    while (!socket.isClosed() && !socket.isInputShutdown()
                            && isStart && ((length = is.read(buffer)) != -1)) {
                        if (length > 0) {
                            String message = new String(Arrays.copyOf(buffer,
                                    length)).trim();
                            Log.e(TAG, message);
                            //收到伺服器過來的訊息，就通過Broadcast傳送出去
                            if(message.equals("ok")){//處理心跳回復
                                Intent intent=new Intent(HEART_BEAT_ACTION);
                                mLocalBroadcastManager.sendBroadcast(intent);
                            }else{
                                //其他訊息回覆
                                Intent intent=new Intent(MESSAGE_ACTION);
                                intent.putExtra("message", message);
                                mLocalBroadcastManager.sendBroadcast(intent);
                            }
                        }
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mHandler.removeCallbacks(heartBeatRunnable);
        mReadThread.release();
        releaseLastSocket(mSocket);
    }
}

4.2 Activity程式碼

package com.danxx.views;

import android.content.BroadcastReceiver;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.os.RemoteException;
import android.support.v4.content.LocalBroadcastManager;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.lang.ref.WeakReference;
import java.util.concurrent.Executors;

import danxx.library.socket.BackService;
import danxx.library.socket.IBackService;

/**
 * Created by daish on 2017/7/24.
 */

public class ActivityHeartSocket extends AppCompatActivity {

    private TextView mResultText;
    private EditText mEditText;
    private Intent mServiceIntent;

    private IBackService iBackService;

    private ServiceConnection conn = new ServiceConnection() {

        @Override
        public void onServiceDisconnected(ComponentName name) {
            iBackService = null;

        }

        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            iBackService = IBackService.Stub.asInterface(service);
        }
    };

    class MessageBackReciver extends BroadcastReceiver {
        private WeakReference<TextView> textView;

        public MessageBackReciver(TextView tv) {
            textView = new WeakReference<TextView>(tv);
        }

        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            TextView tv = textView.get();
            if (action.equals(BackService.HEART_BEAT_ACTION)) {
                if (null != tv) {
                    Log.i("danxx", "Get a heart heat");
                    tv.setText("Get a heart heat");
                }
            } else {
                Log.i("danxx", "Get a heart heat");
                String message = intent.getStringExtra("message");
                tv.setText("伺服器訊息:"+message);
            }
        };
    }
    private MessageBackReciver mReciver;

    private IntentFilter mIntentFilter;

    private LocalBroadcastManager mLocalBroadcastManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_heart_socket);

        mLocalBroadcastManager = LocalBroadcastManager.getInstance(this);

        mResultText = (TextView) findViewById(R.id.resule_text);
        mEditText = (EditText) findViewById(R.id.content_edit);

        mReciver = new MessageBackReciver(mResultText);

        mServiceIntent = new Intent(this, BackService.class);

        mIntentFilter = new IntentFilter();
        mIntentFilter.addAction(BackService.HEART_BEAT_ACTION);
        mIntentFilter.addAction(BackService.MESSAGE_ACTION);

    }

    @Override
    protected void onStart() {
        super.onStart();
        mLocalBroadcastManager.registerReceiver(mReciver, mIntentFilter);
        bindService(mServiceIntent, conn, BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        unbindService(conn);
        mLocalBroadcastManager.unregisterReceiver(mReciver);
    }

    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.send:
                String content = mEditText.getText().toString();
                try {
                    boolean isSend = iBackService.sendMessage(content);//Send Content by socket
                    Toast.makeText(this, isSend ? "success" : "fail",
                            Toast.LENGTH_SHORT).show();
                    mEditText.setText("");
                } catch (RemoteException e) {
                    e.printStackTrace();
                }
                break;

            default:
                break;
        }
    }

}

5.效果圖

5.1 golang服務端

5.2 Android客戶端