Android中使用AsyncTask实现下载文件动态更新进度条功能

1. 泛型

aysnctask<params, progress, result>

params：启动任务时传入的参数，通过调用asynctask.execute(param)方法传入。

progress：后台任务执行的进度，若不用显示进度条，则不需要指定。

result：后台任务结束时返回的结果。

2. 重要方法

doinbackground(params... params)：必须重写的方法，后台任务就在这里执行，会开启一个新的线程。params为启动任务时传入的参数，参数个数不定。

onpreexecute()：在主线程中调用，在后台任务开启前的操作在这里进行，例如显示一个进度条对话框。

onpostexecute(result result)：当后台任务结束后，在主线程中调用，处理doinbackground()方法返回的结果。

onprogressupdate(progress... values)：当在doinbackground()中调用publishprogress(progress... values)时，返回主线程中调用，这里的参数个数也是不定的。

oncancelled()：取消任务。

3. 注意事项

（1）execute()方法必须在主线程中调用；

（2）asynctask实例必须在主线程中创建；

（3）不要手动调用doinbackground()、onpreexecute()、onpostexecute()、onprogressupdate()方法；

（4）注意防止内存泄漏，在doinbackground()方法中若出现对activity的强引用，可能会造成内存泄漏。

4. 下载文件动态更新进度条（未封装）

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="20dp"
  tools:context="com.studying.asynctaskdemo.mainactivity">
  <progressbar
    android:id="@+id/progressbar"
    style="?android:progressbarstylehorizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:progress="0" />
  <button
    android:id="@+id/download"
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:layout_margintop="20dp"
    android:text="@string/start_btn" />
  <textview
    android:id="@+id/status"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_margintop="20dp"
    android:text="@string/waiting" />
</linearlayout>

activity：

public class mainactivity extends activity {
  private static final string file_name = "test.pdf";//下载文件的名称
  private static final string pdf_url = "http://clfile.imooc.com/class/assist/118/1328281/asynctask.pdf";
  private progressbar mprogressbar;
  private button mdownloadbtn;
  private textview mstatus;
  @override
  protected void oncreate(bundle savedinstancestate) {
    super.oncreate(savedinstancestate);
    setcontentview(r.layout.activity_main);
    initview();
    setlistener();
  }
  private void initview() {
    mprogressbar = (progressbar) findviewbyid(r.id.progressbar);
    mdownloadbtn = (button) findviewbyid(r.id.download);
    mstatus = (textview) findviewbyid(r.id.status);
  }
  private void setlistener() {
    mdownloadbtn.setonclicklistener(new view.onclicklistener() {
      @override
      public void onclick(view v) {
        //asynctask实例必须在主线程创建
        downloadasynctask asynctask = new downloadasynctask();
        asynctask.execute(pdf_url);
      }
    });
  }
  /**
   * 泛型：
   * string：传入参数为文件下载地址
   * integer：下载过程中更新progressbar的进度
   * boolean：是否下载成功
   */
  private class downloadasynctask extends asynctask<string, integer, boolean> {
    private string mfilepath;//下载文件的保存路径
    @override
    protected boolean doinbackground(string... params) {
      if (params != null && params.length > 0) {
        string pdfurl = params[0];
        try {
          url url = new url(pdfurl);
          urlconnection urlconnection = url.openconnection();
          inputstream in = urlconnection.getinputstream();
          int contentlength = urlconnection.getcontentlength();//获取内容总长度
          mfilepath = environment.getexternalstoragedirectory() + file.separator + file_name;
          //若存在同名文件则删除
          file pdffile = new file(mfilepath);
          if (pdffile.exists()) {
            boolean result = pdffile.delete();
            if (!result) {
              return false;
            }
          }
          int downloadsize = 0;//已经下载的大小
          byte[] bytes = new byte[1024];
          int length = 0;
          outputstream out = new fileoutputstream(mfilepath);
          while ((length = in.read(bytes)) != -1) {
            out.write(bytes, 0, length);
            downloadsize += length;
            publishprogress(downloadsize / contentlength * 100);
          }
          in.close();
          out.close();
        } catch (ioexception e) {
          e.printstacktrace();
          return false;
        }
      } else {
        return false;
      }
      return true;
    }
    @override
    protected void onpreexecute() {
      super.onpreexecute();
      mdownloadbtn.settext("下载中");
      mdownloadbtn.setenabled(false);
      mstatus.settext("下载中");
      mprogressbar.setprogress(0);
    }
    @override
    protected void onpostexecute(boolean aboolean) {
      super.onpostexecute(aboolean);
      mdownloadbtn.settext("下载完成");
      mstatus.settext(aboolean ? "下载完成" + mfilepath : "下载失败");
    }
    @override
    protected void onprogressupdate(integer... values) {
      super.onprogressupdate(values);
      if (values != null && values.length > 0) {
        mprogressbar.setprogress(values[0]);
      }
    }
  }
}

5. 下载文件动态更新进度条（封装）

activity：

public class mainactivity extends activity {
  private static final string file_name = "test.pdf";
  private static final string pdf_url = "http://clfile.imooc.com/class/assist/118/1328281/asynctask.pdf";
  private progressbar mprogressbar;
  private button mdownloadbtn;
  private textview mstatus;
  @override
  protected void oncreate(bundle savedinstancestate) {
    super.oncreate(savedinstancestate);
    setcontentview(r.layout.activity_main);
    initview();
    setlistener();
  }
  private void initview() {
    mprogressbar = (progressbar) findviewbyid(r.id.progressbar);
    mdownloadbtn = (button) findviewbyid(r.id.download);
    mstatus = (textview) findviewbyid(r.id.status);
  }
  private void setlistener() {
    mdownloadbtn.setonclicklistener(new view.onclicklistener() {
      @override
      public void onclick(view v) {
        string localpath = environment.getexternalstoragedirectory() + file.separator + file_name;
        downloadhelper.download(pdf_url, localpath, new downloadhelper.ondownloadlistener() {
          @override
          public void onstart() {
            mdownloadbtn.settext("下载中");
            mdownloadbtn.setenabled(false);
            mstatus.settext("下载中");
            mprogressbar.setprogress(0);
          }
          @override
          public void onsuccess(file file) {
            mdownloadbtn.settext("下载完成");
            mstatus.settext(string.format("下载完成：%s", file.getpath()));
          }
          @override
          public void onfail(file file, string failinfo) {
            mdownloadbtn.settext("开始下载");
            mdownloadbtn.setenabled(true);
            mstatus.settext(string.format("下载失败：%s", failinfo));
          }
          @override
          public void onprogress(int progress) {
            mprogressbar.setprogress(progress);
          }
        });
      }
    });
  }
}

downloadhelper：

class downloadhelper {
  static void download(string url, string localpath, ondownloadlistener listener) {
    downloadasynctask task = new downloadasynctask(url, localpath, listener);
    task.execute();
  }
  private static class downloadasynctask extends asynctask<string, integer, boolean> {
    private string mfailinfo;
    private string murl;
    private string mfilepath;
    private ondownloadlistener mlistener;
    downloadasynctask(string murl, string mfilepath, ondownloadlistener mlistener) {
      this.murl = murl;
      this.mfilepath = mfilepath;
      this.mlistener = mlistener;
    }
    @override
    protected boolean doinbackground(string... params) {
        string pdfurl = murl;
        try {
          url url = new url(pdfurl);
          urlconnection urlconnection = url.openconnection();
          inputstream in = urlconnection.getinputstream();
          int contentlength = urlconnection.getcontentlength();
          file pdffile = new file(mfilepath);
          if (pdffile.exists()) {
            boolean result = pdffile.delete();
            if (!result) {
              mfailinfo = "存储路径下的同名文件删除失败！";
              return false;
            }
          }
          int downloadsize = 0;
          byte[] bytes = new byte[1024];
          int length;
          outputstream out = new fileoutputstream(mfilepath);
          while ((length = in.read(bytes)) != -1) {
            out.write(bytes, 0, length);
            downloadsize += length;
            publishprogress(downloadsize / contentlength * 100);
          }
          in.close();
          out.close();
        } catch (ioexception e) {
          e.printstacktrace();
          mfailinfo = e.getmessage();
          return false;
        }
      return true;
    }
    @override
    protected void onpreexecute() {
      super.onpreexecute();
      if (mlistener != null) {
        mlistener.onstart();
      }
    }
    @override
    protected void onpostexecute(boolean aboolean) {
      super.onpostexecute(aboolean);
      if (mlistener != null) {
        if (aboolean) {
          mlistener.onsuccess(new file(mfilepath));
        } else {
          mlistener.onfail(new file(mfilepath), mfailinfo);
        }
      }
    }
    @override
    protected void onprogressupdate(integer... values) {
      super.onprogressupdate(values);
      if (values != null && values.length > 0) {
        if (mlistener != null) {
          mlistener.onprogress(values[0]);
        }
      }
    }
  }
  interface ondownloadlistener{
    void onstart();
    void onsuccess(file file);
    void onfail(file file, string failinfo);
    void onprogress(int progress);
  }
}

总结

以上所述是小编给大家介绍的android中使用asynctask实现下载文件动态更新进度条功能，希望对大家有所帮助