【并发编程】线程间通信的方式

管道流Pipes

“管道”是java.io包的一部分，它是Java的特性，而不是Android特有的。一条“管道”为两个线程建立一个单向的通道。生产者负责写数据，消费者负责读取数据。

public class PipeExampleActivity extends Activity {

    private static final String TAG = "PipeExampleActivity";
    private EditText editText;

    PipedReader r;
    PipedWriter w;

    private Thread workerThread;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        r = new PipedReader();
        w = new PipedWriter();

        try {
            w.connect(r);
        } catch (IOException e) {
            e.printStackTrace();
        }

        setContentView(R.layout.activity_pipe);
        editText = (EditText) findViewById(R.id.edit_text);
        editText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int start, int count, int after) {
            }

            @Override
            public void onTextChanged(CharSequence charSequence, int start, int before, int count) {
                try {
                    if(count > before) {
                        w.write(charSequence.subSequence(before, count).toString());
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void afterTextChanged(Editable editable) {
            }
        });

        workerThread = new Thread(new TextHandlerTask(r));
        workerThread.start();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        workerThread.interrupt();
        try {
            r.close();
            w.close();
        } catch (IOException e) {
        }
    }

    private static class TextHandlerTask implements Runnable {
        private final PipedReader reader;

        public TextHandlerTask(PipedReader reader){
            this.reader = reader;
        }
        @Override
        public void run() {
            while(!Thread.currentThread().isInterrupted()){
                try {
                    int i;
                    while((i = reader.read()) != -1){
                        char c = (char) i;
                        
                        Log.d(TAG, "char = " + c);
                    }

                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

对EditText设置一个TextWatcher监听，一旦EditText的内容发生改变，就向“管道”中输入字符，它就是所谓的生产者。

同时有一个工作线程负责从管道中读取字符，它就是所谓的消费者。这样，就实现了UI线程和工作线程之间的数据通信。

共享内存

多个线程共享同一份内存。就是说，一个变量可以同时被多个线程所访问。这里要特别注意同步和原子操作的问题。

synchronized(this) {
    while(isConditionFullfilled == false) {
        wait();
    }


    notify();
}

如果觉得使用wait/notify不容易控制，可以使用Java提供的BlockingQueue，它是一个阻塞队列。

public class ConsumerProducer {
    private final int LIMIT = 10;
    private BlockingQueue blockingQueue = new LinkedBlockingQueue(LIMIT);
    
    public void produce() throws InterruptedException {
        int value = 0;
        while (true) {
            blockingQueue.put(value++);
        }
    }
    
    public void consume() throws InterruptedException {
        while (true) {
            int value = blockingQueue.take();
        }
    }

}

使用Hander和Message

一个线程对应一个Looper，一个Looper持有一个MessageQueue，一个Looper可以与多个Handler绑定，一个MessageQueue中可以包含多个Message。

public class HandlerExampleActivity extends Activity {

    private final static int SHOW_PROGRESS_BAR = 1;
    private final static int HIDE_PROGRESS_BAR = 0;
    private BackgroundThread mBackgroundThread;

    private TextView mText;
    private Button mButton;
    private ProgressBar mProgressBar;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_handler_example);

        mBackgroundThread = new BackgroundThread();
        mBackgroundThread.start();

        mText = (TextView) findViewById(R.id.text);
        mProgressBar = (ProgressBar) findViewById(R.id.progress);
        mButton = (Button) findViewById(R.id.button);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mBackgroundThread.doWork();
            }
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mBackgroundThread.exit();
    }

    private final Handler mUiHandler = new Handler() {
        public void handleMessage(Message msg) {

            switch(msg.what) {
                case SHOW_PROGRESS_BAR:
                    mProgressBar.setVisibility(View.VISIBLE);
                    break;
                case HIDE_PROGRESS_BAR:
                    mText.setText(String.valueOf(msg.arg1));
                    mProgressBar.setVisibility(View.INVISIBLE);
                    break;
            }
        }
    };

    private class BackgroundThread extends Thread {

        private Handler mBackgroundHandler;

        public void run() {
            Looper.prepare();
            mBackgroundHandler = new Handler();
            Looper.loop();
        }

        public void doWork() {
            mBackgroundHandler.post(new Runnable() {
                @Override
                public void run() {
                    Message uiMsg = mUiHandler.obtainMessage(SHOW_PROGRESS_BAR, 0,
                            0, null);
                    mUiHandler.sendMessage(uiMsg);

                    Random r = new Random();
                    int randomInt = r.nextInt(5000);
                    SystemClock.sleep(randomInt);

                    uiMsg = mUiHandler.obtainMessage(HIDE_PROGRESS_BAR, randomInt,
                            0, null);
                    mUiHandler.sendMessage(uiMsg);
                }
            });
        }

        public void exit() {
            mBackgroundHandler.getLooper().quit();
        }
    }
}

参考资料

http://developer.android.com/reference/java/io/PipedInputStream.html

http://developer.android.com/reference/java/io/PipedOutputStream.html

https://engineering.purdue.edu/~smidkiff/ece563/files/java.pdf

http://developer.android.com/reference/android/os/Handler.html