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：

4. 内存泄漏

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：

<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" />

布局：

<?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<downloadactivity> 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;
 }
 }
 }
}

总结

以上所述是小编给大家介绍的handler实现线程之间的通信下载文件动态更新进度条，希望对大家有所帮助