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 Studio 教辅 .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;
}
}
}
}