Handler实现线程之间的通信-下载文件动态更新进度条

1. 原理

每一个线程对应一个消息队列MessageQueue，实现线程之间的通信，可通过Handler对象将数据装进Message中，再将消息加入消息队列，而后线程会依次处理消息队列中的消息。

2. Message

初始化：一般使用Message.obtain()方法获取一个消息对象，该方法会检查Message对象池中是否存在可重复利用的对象，若无，才会new一个新对象。

what：相当于Message的标识符，区别于其它消息。

arg1、arg2：int类型，可传递整数。

obj：object类型，可传递任意对象。

3. 发送消息

在子线程中可调用主线程的handler.sendMessage(msg)进行发送消息，经过一系列方法调用，会触发handler的handleMessage方法，从而进行消息处理。

发送消息的主要方法：

handler.sendMessage(Message msg);
handler.sendMessageAtTime(Message msg, int time);
handler.sendMessageDelayed(Message msg, int time);

sendMessageAtTime()和sendMessageDelayed()区别在于前者是在指定时间发送消息，可配合SystemClock.uptimeMillis()使用；而后者则是延时发送消息。

除了SendMessage()方法以外，还可以通过post()方法发送消息：

handler.post(Runnable r);
handler.postDelayed(Runnable r, int time);

sendMessage()与post()的区别：http://blog.csdn.net/u013168615/article/details/47024073

4. 内存泄漏

http://www.cnblogs.com/xujian2014/p/5025650.html

5. 通过Handler对象实现下载文件动态更新进度条

AndroidManifest加入权限声明：

布局：

Activity：

public class DownloadActivity extends Activity {

    private static final int DOWNLOAD_MESSAGE_CODE = 100001;
    private static final int DOWNLOAD_MESSAGE_FAIL_CODE = 100002;
    private static final String APP_URL = "http://clfile.imooc.com/class/assist/119/1328281/Android%20Studio%20教辅%20.pdf";
    private MyHandler mHandler;
    private ProgressBar mProgressBar;

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

        findViewById(R.id.download).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //开启子线程
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        download(APP_URL);
                    }
                }).start();
            }
        });
        mProgressBar = (ProgressBar) findViewById(R.id.progress_bar);
        mHandler = new MyHandler(this);
    }

    private void download(String appUrl) {
        try {
            URL url = new URL(appUrl);
            URLConnection conn = url.openConnection();

            InputStream in = conn.getInputStream();
            int contentLength = conn.getContentLength();//获取文件总大小

            String downloadPath = Environment.getExternalStorageDirectory() + File.separator + "imooc" + File.separator;
            File file = new File(downloadPath);
            if (!file.exists()) {
                file.mkdir();
            }

            String fileName = downloadPath + "test.pdf";
            File apkFile = new File(fileName);
            if (apkFile.exists()) {
                apkFile.delete();
            }

            int downloadSize = 0;//记录已经下载的大小
            byte[] bytes = new byte[1024];
            int length = 0;

            OutputStream out = new FileOutputStream(fileName);
            while ((length = in.read(bytes)) != -1) {
                out.write(bytes, 0, length);
                downloadSize += length;

                Message msg = Message.obtain();
                msg.obj = downloadSize / contentLength * 100;//progress的值为0到100，因此得到的百分数要乘以100
                msg.what = DOWNLOAD_MESSAGE_CODE;
                mHandler.sendMessage(msg);
            }

            in.close();
            out.close();

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

    private void notifyDownloadFailed() {
        Message msg = Message.obtain();
        msg.what = DOWNLOAD_MESSAGE_FAIL_CODE;
        mHandler.sendMessage(msg);
    }

    private static class MyHandler extends Handler{

        private WeakReference weakReference;

        MyHandler(DownloadActivity activity) {
            this.weakReference = new WeakReference<>(activity);//以弱引用的形式传递Activity，避免内存泄漏
        }

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            DownloadActivity activity = weakReference.get();
            //消息处理
            switch (msg.what) {
                case DOWNLOAD_MESSAGE_CODE:
                    activity.mProgressBar.setProgress((Integer) msg.obj);
                    break;
                case DOWNLOAD_MESSAGE_FAIL_CODE:
                    Toast.makeText(activity, "下载失败！", Toast.LENGTH_SHORT).show();
                    break;
            }
        }
    }
}