深入Android Handler與線程間通信ITC的詳解

2020-04-11 12:22:50

字體：大中小

來源：轉載

供稿：網(wǎng)友

在《Android Handler之消息循環(huán)的深入解析》中談到了Handler是用于操作線程內部的消息隊列，所以Handler可以用來線程間通信ITC，這種方式更加安全和高效，可以大大減少同步的煩惱，甚至都可以不用syncrhonized。
線程間通訊ITC
正常情況下函數(shù)調用棧都會生存在同一個線程內，想要把執(zhí)行邏輯交換到其他線程可以新建一個Thread，然后start()。另外一種方法就是用ITC，也即用消息隊列來實現(xiàn)，線程需要把執(zhí)行邏輯交到其他線程時就向另外的線程的消息隊列發(fā)送一個消息，發(fā)送消息后函數(shù)就此結束返回，調用棧也停止。當消息隊列中有了消息時，線程會被喚醒來執(zhí)行處理消息，從而把執(zhí)行邏輯從一個線程轉到另外一個線程。這就實現(xiàn)了線程間的通信ITC，與進行間通訊IPC有十分類似的思想。

通常的做法都是，在主線程創(chuàng)建一個Handler，然后在新建線程中使用此Handler與主線程通訊。因為主線程的消息隊列已經(jīng)建好，所以直接創(chuàng)建Handler即可，新建的線程就可以直接使用。
有些情況，需要在多線程之間進行通信，這就要為每個線程都創(chuàng)建MessageQueue和Handler，只要線程能訪問其他線程的Handler就可以與之通信。

要正確的創(chuàng)建Handler，因為Handler要與線程綁定，所以在初始化Handler的時候就要注意：
如果給Handler指定Looper對象new Handler(Looper)，那么此Handler便綁定到Looper對象所在的線程中，Handler的消息處理回調會在那個線程中執(zhí)行。
如果創(chuàng)建線程時不指定Looper對象，那么此Handler綁定到創(chuàng)建此Handler的線程內，消息回調處理會在那個線程中執(zhí)行，所以像下面的例子，如果這樣寫：

復制代碼代碼如下:

private class CookServer extends Thread {
       private Handler mHandler = new Handler() {
               public void handleMessage(Message msg) {
                     ....
                }
        };

那么，此mHandler會與創(chuàng)建此CookerServer的線程綁定，handleMessage也會運行于其中。顯然，如果是主線程調用new CookServer()，那么mHandler其實是運行在主線程中的。正確的寫法應該是：

復制代碼代碼如下:

private class CookServer extends Thread {
       public void run() {
             Looper.prepare();
                 // or new Handler(Looper.myLooper())
                 private Handler mHandler = new Handler() {
                       public void handleMessage(Message msg) {
                     ....
                }
        };

HandlerThread
如果要在一個線程中使用消息隊列和Handler，Android API中已經(jīng)有封裝好了的一個類HandlerThread，這個類已經(jīng)做好了Looper的初始化工作，你需要做的就是重寫其onLooperPrepared()方法，在其中創(chuàng)建Handler：

復制代碼代碼如下:

private class DeliverServer extends HandlerThread {
      private Handler mHandler;
      public DeliverServer(String name) {
           super(name);
      }
      @Override
      public void onLooperPrepared() {
            mHandler = new Handler(getLooper()) {
                    public void handleMessage(Message msg) {
                        .....
                    }
             };
       }
}

實例
此實例模擬了一個網(wǎng)絡訂餐系統(tǒng)，客戶點擊“Submit order"來產(chǎn)生一個定單，主線程中負責收集定單，然后交由CookServer來制作，CookServer在制作完成后會交由DeliverServer來把食物運送到客戶，至此一個定單完成，同時CookServer和DeliverServer會更新狀態(tài)。

復制代碼代碼如下:

/**
 * How to attach an Handler to a Thread:
 * If you specify Looper object to Handler, i.e. new Handler(Looper), then the handler is attached to the thread owning
 * the Looper object, in which handleMessage() is executed.
 * If you do not specify the Looper object, then the handler is attached to the thread calling new Handler(), in which
 * handleMessage() is executed.
 * In this example, for class CookServer or DeliverServer, if you write this way:
 *     private class CookServer extends Thread {
  private Handler mHandler;
  private Looper mLooper;

  public CookServer() {
   mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
     ....
    }
       start();
  }
 * then mHandler is attached to thread calling new CookServer(), which is the main thread, so mHandler.handleMessage() will
 * be executed in main thread.
 * To attach mHandler to its own thread, you must put it in run(), or after mLooper is created. For our example, providing
 * mLooper or not won't matter, because new Handler() is called in run(), which is in a new thread.
 */
public class HandlerITCDemo extends ListActivity {
    private static final int COOKING_STARTED = 1;
    private static final int COOKING_DONE = 2;
    private static final int DELIVERING_STARTED = 3;
    private static final int ORDER_DONE = 4;

    private ListView mListView;
    private static final String[] mFoods = new String[] {
 "Cubake",
 "Donut",
 "Eclaire",
 "Gingerbread",
 "Honeycomb",
 "Ice Cream Sanwitch",
 "Jelly Bean",
    };
    private ArrayList<String> mOrderList;
    private TextView mGeneralStatus;
    private Button mSubmitOrder;
    private static Random mRandomer = new Random(47);
    private int mOrderCount;
    private int mCookingCount;
    private int mDeliveringCount;
    private int mDoneCount;

    private Handler mMainHandler = new Handler() {
 @Override
 public void handleMessage(Message msg) {
     switch (msg.what) {
     case COOKING_STARTED:
  mCookingCount++;
  break;
     case COOKING_DONE:
  mCookingCount--;
  break;
     case DELIVERING_STARTED:
  mDeliveringCount++;
  break;
     case ORDER_DONE:
  mDeliveringCount--;
  mDoneCount++;
     default:
  break;
     }
     mGeneralStatus.setText(makeStatusLabel());
 }
    };

    private CookServer mCookServer;
    private DeliverServer mDeliverServer;

    @Override
    protected void onDestroy() {
 super.onDestroy();
 if (mCookServer != null) {
     mCookServer.exit();
     mCookServer = null;
 }
 if (mDeliverServer != null) {
     mDeliverServer.exit();
     mDeliverServer = null;
 }
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 mListView = getListView();
 mOrderList = new ArrayList<String>();
 mGeneralStatus = new TextView(getApplication());
 mGeneralStatus.setText(makeStatusLabel());
 mSubmitOrder = new Button(getApplication());
 mSubmitOrder.setText("Submit order");
 mSubmitOrder.setOnClickListener(new View.OnClickListener() {
     public void onClick(View v) {
  String order = mFoods[mRandomer.nextInt(mFoods.length)];
  mOrderList.add(order);
  mOrderCount = mOrderList.size();
  mGeneralStatus.setText(makeStatusLabel());
  setAdapter();
  mCookServer.cook(order);
     }
 });
 mListView.addHeaderView(mGeneralStatus);
 mListView.addFooterView(mSubmitOrder);
 setAdapter();
 mCookServer = new CookServer();
 mDeliverServer = new DeliverServer("deliver server");
    }

    private String makeStatusLabel() {
 StringBuilder sb = new StringBuilder();
 sb.append("Total: ");
 sb.append(mOrderCount);
 sb.append("    Cooking: ");
 sb.append(mCookingCount);
 sb.append("    Delivering: ");
 sb.append(mDeliveringCount);
 sb.append("    Done: ");
 sb.append(mDoneCount);
 return sb.toString();
    }

    private void setAdapter() {
 final ListAdapter adapter = new ArrayAdapter<String>(getApplication(), android.R.layout.simple_list_item_1, mOrderList);
 setListAdapter(adapter);
    }

    private class CookServer extends Thread {
 private Handler mHandler;
 private Looper mLooper;

 public CookServer() {
     start();
 }

 @Override
 public void run() {
     Looper.prepare();
     mLooper = Looper.myLooper();
     mHandler = new Handler(mLooper, new Handler.Callback() {
  public boolean handleMessage(Message msg) {
      new Cooker((String) msg.obj);
      return true;
  }
     });
     Looper.loop();
 }

 public void cook(String order) {
     if (mLooper == null || mHandler == null) {
  return;
     }
     Message msg = Message.obtain();
     msg.obj = order;
     mHandler.sendMessage(msg);
 }

 public void exit() {
     if (mLooper != null) {
  mLooper.quit();
  mHandler = null;
  mLooper = null;
     }
 }
    }

    private class Cooker extends Thread {
 private String order;
 public Cooker(String order) {
     this.order = order;
     start();
 }

 @Override
 public void run() {
            mMainHandler.sendEmptyMessage(COOKING_STARTED);
            SystemClock.sleep(mRandomer.nextInt(50000));
            mDeliverServer.deliver(order);
            mMainHandler.sendEmptyMessage(COOKING_DONE);
 }
    }

    private class DeliverServer extends HandlerThread {
 private Handler mHandler;

 public DeliverServer(String name) {
     super(name);
     start();
 }

 @Override
 protected void onLooperPrepared() {
     super.onLooperPrepared();
     mHandler = new Handler(getLooper(), new Handler.Callback() {
  public boolean handleMessage(Message msg) {
      new Deliver((String) msg.obj);
      return true;
  }
     });
 }
 public void deliver(String order) {
     if (mHandler == null || getLooper() == null) {
  return;
     }
     Message msg = Message.obtain();
     msg.obj = order;
     mHandler.sendMessage(msg);
 }

 public void exit() {
     quit();
     mHandler = null;
 }
    }

    private class Deliver extends Thread {
 private String order;
 public Deliver(String order) {
     this.order = order;
     start();
 }

 @Override
 public void run() {
     mMainHandler.sendEmptyMessage(DELIVERING_STARTED);
     SystemClock.sleep(mRandomer.nextInt(50000));
     mMainHandler.sendEmptyMessage(ORDER_DONE);
 }
    }
}