Android：暂停和恢复活动中的线程

Question

编辑：我发现我在下面描述的内容仅发生在我的模拟设备（Nexus 5、目标 api 19、4.4.2 和 Intel Atom (x86) cpu）上，但不在我的物理设备上（HTC一）....

EDIT2：Edit1 是由于我没有捕捉到的 IllegalStateException 引起的。添加了一些代码以在尝试启动线程之前检查线程是否已经在运行。这与接受的答案相结合解决了我的问题。

我已经实现了一个 Activity，它在 Activity 的 onCreate 方法中启动一个新线程，如下所示：

...

private boolean running;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    running = true;
    new Thread(null, work, "myThread").start();
}

Runnable work = new Runnable() {

    @Override
    public void run() { 
        while (running) {
            //Doing work
        }
    }
};

我正在使用我的活动的 onPause 方法“暂停”我的线程，如下所示：

@Override
protected void onPause() {
    running = false;
    super.onPause();
}

所以我认为恢复它会很容易......¨

@Override
protected void onResume(){
    running = true;
    super.onResume();
}

但我的线程没有恢复。任何想法为什么？感谢您的帮助。

马库斯

Answer 1

我认为所有答案都与您的运行变量有关，因为您无法在没有synchronized block的情况下从两个不同的Threads读写变量所以我发布我自己的答案：

package com.example.threadandtoast;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.widget.Toast;

public class MainActivity extends ActionBarActivity {
    public class MonitorObject{
           public boolean running = true;
           public String message = "";
           public boolean mustBePost = true;
    }

    Thread t;
    int threadNameCounter = 0; // i use this variable to make sure that old thread is deleted 
                               // when i pause, you can see it and track it in DDMS

    Runnable work = new Runnable() {
        boolean myRunning;
        @Override
        public void run() {
            synchronized(mSync) {
               myRunning = mSync.running;
            }
            while (myRunning) {
                runOnUiThread(new Runnable() {  // in order to update the UI (create Toast)
                @Override                       // we must switch to main thread
                public void run() { 
                    // i want to read the message so i must use synchronized block
                    synchronized(mSync) { 
                    // i use this variable to post a message just for one time because i am in an infinite loop
                    // if i do not set a limit on the toast i create it infinite times 
                      if(mSync.mustBePost){       
                          Toast.makeText(MainActivity.this, mSync.message, Toast.LENGTH_SHORT).show();
                          // the message post so i must set it to false
                          mSync.mustBePost = false;
                          // if i am going to pause set mSync.running to false so at the end of infinite loop 
                          //of thread he reads it and leaves the loop
                          if(mSync.message.equals("Main Activity is going to pause")){
                              mSync.running=false;            
                          }
                      }
                    }
                }
               });

            synchronized(mSync) {
               myRunning = mSync.running;
           }
         }
       }
    };

    final MonitorObject mSync = new MonitorObject();

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }

    @Override
    protected void onPause() {
        super.onPause();
        synchronized(mSync) {
            // mSync.running = false; you can not set it here because 
            // it is possible for the thread to read it and exit the loop before he posts your message
            mSync.mustBePost=true;
            mSync.message = "Main Activity is going to pause";
        }    
    }

    @Override
    protected void onResume(){
       super.onResume();
       threadNameCounter++;
       synchronized(mSync) {
            mSync.running = true;
            mSync.mustBePost=true;
            mSync.message = "Main Activity is going to resume";
        } 
       t = new Thread(work,"My Name is " + String.valueOf(threadNameCounter));
       t.start();
    }

}

或者你可以使用这个代码：

public class MainActivity extends ActionBarActivity {

    Thread t;
    int threadNameCounter = 0; // i use this variable to make sure that old thread is deleted 
                               // when i pause, you can see it in DDMS

    String message = "";
    boolean isPost = false;

    Runnable work = new Runnable() {

        @Override
        public void run() {

            while (true) {
                runOnUiThread(new Runnable() {  
                @Override                       
                public void run() { 
                    if(!isPost){
                        Toast.makeText(MainActivity.this, message, Toast.LENGTH_SHORT).show();
                        isPost = true;
                        if( message.equals("Main Activity is going to pause")){
                            t.interrupt();
                        }

                    }
                }
               });
           if(Thread.currentThread().isInterrupted()){
               break;
           } 
         }
       }
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }

    @Override
    protected void onPause() {
        super.onPause();
        message = "Main Activity is going to pause";
        isPost = false;
    }

    @Override
    protected void onResume(){
       super.onResume();
       message = "Main Activity is going to resume";
       isPost = false;
       threadNameCounter++;
       t = new Thread(work,"My Name is " + String.valueOf(threadNameCounter));
       t.start();
    }

}

您也可以使用信号量或等待通知的方法。

我将 public String message = ""; 和 public boolean mustBePost = true; 放入 mSync 对象，但它是 没有必要，因为只有 main thread 可以访问它们。

如果你有任何问题，请询问。

Answer 2

语句running = false; 将停止执行Thread，而不是暂停它。使用两个变量：一个用于停止当前的Thread，另一个用于暂停和恢复Thread，如下所示：

boolean isThreadPause=false;
Runnable work = new Runnable() {    
    @Override
    public void run() { 
        while (running) {
            if (!isThreadPause) {
               // Doing work
            }
        }
    }
};

在Activity的onPause事件中，将isThreadPause设置为true，在onResume事件中，将isThreadPause设置为false。

Answer 3

这是因为您的 Runnable 对象在 while 循环停止时停止。你可以试试这个：

Runnable work = new Runnable() {

    @Override
    public void run() { 
        while () {
          if(running){
            //Doing work
          }
        }
    }
};