Handler實現線程之間的通信-下載文件動態更新進度條

1. 原理

每一個線程對應一個消息隊列MessageQueue，實現線程之間的通信，可通過Handler對象將數據裝進Message中，再將消息加入消息隊列，而後線程會依次處理消息隊列中的消息。

2. Message

初始化：一般使用Message.obtain()方法獲取一個消息對象，該方法會檢查Message對象池中是否存在可重覆利用的對象，若無，才會new一個新對象。

what：相當於Message的標識符，區別於其它消息。

arg1、arg2：int類型，可傳遞整數。

obj：object類型，可傳遞任意對象。

3. 發送消息

在子線程中可調用主線程的handler.sendMessage(msg)進行發送消息，經過一系列方法調用，會觸發handler的handleMessage方法，從而進行消息處理。

發送消息的主要方法：

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

sendMessageAtTime()和sendMessageDelayed()區別在於前者是在指定時間發送消息，可配合SystemClock.uptimeMillis()使用；而後者則是延時發送消息。

除了SendMessage()方法以外，還可以通過post()方法發送消息：

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

sendMessage()與post()的區別：http://blog.csdn.net/u013168615/article/details/47024073

4. 記憶體泄漏

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

5. 通過Handler對象實現下載文件動態更新進度條

AndroidManifest加入許可權聲明：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

佈局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="10dp"
    tools:context="com.studying.network.DownloadActivity">

    <ProgressBar
        android:id="@+id/progress_bar"
        style="?android:progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:max="100" />

    <Button
        android:id="@+id/download"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:text="@string/download" />

</LinearLayout>

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教輔%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) {
                //開啟子線程
                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();//獲取文件總大小

            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;//記錄已經下載的大小
            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的值為0到100，因此得到的百分數要乘以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<DownloadActivity> weakReference;

        MyHandler(DownloadActivity activity) {
            this.weakReference = new WeakReference<>(activity);//以弱引用的形式傳遞Activity，避免記憶體泄漏
        }

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            DownloadActivity activity = weakReference.get();
            //消息處理
            switch (msg.what) {
                case DOWNLOAD_MESSAGE_CODE:
                    activity.mProgressBar.setProgress((Integer) msg.obj);
                    break;
                case DOWNLOAD_MESSAGE_FAIL_CODE:
                    Toast.makeText(activity, "下載失敗！", Toast.LENGTH_SHORT).show();
                    break;
            }
        }
    }
}