在其他类中获取AsyncTask的输出。

Question

我正在学习android开发，我需要检查一个文件是否存在于服务器上。

我使用了以下代码

public class CheckReportExists extends AsyncTask<String, Void, Boolean>{

    Boolean fileExists = false;

    public CheckReportExists() {

    }

    public Boolean CheckReportExists1(String download_url){
        execute(download_url);
        return fileExists;
    }

    @Override
    protected void onPreExecute() {
        //display progress dialog.

    }
    @Override
    protected Boolean doInBackground(String... params) {
        try {
            HttpURLConnection.setFollowRedirects(false);
            HttpURLConnection con =  (HttpURLConnection) new URL(params[0]).openConnection();
            con.setRequestMethod("HEAD");
            int response = con.getResponseCode();
            if(response == HttpURLConnection.HTTP_OK){
                fileExists = true;
            }
        } catch(Exception e){

        }
        return fileExists;
    }

    @Override
    protected void onPostExecute(Boolean result) {
        // dismiss progress dialog and update ui
        //super.onPostExecute(result);
    }

}

我使用以下代码来调用它

     CheckReportExists cre = new CheckReportExists();
     Boolean fileExists = cre.CheckReportExists1(download_url);
     if(fileExists) {
          builder.setPositiveButton("Download Report", new DialogInterface.OnClickListener() {
          @Override
          public void onClick(DialogInterface dialog, int which) {
                 new DownloadTask(Results.this, download_url);
          }
          });
      }else{
            builder.setPositiveButton("Report not ready yet", new DialogInterface.OnClickListener() {
                   @Override
                    public void onClick(DialogInterface dialog, int which) {

                    }
             });
      }

但这段代码在我得到 AlertDialog 与 "报告尚未准备好 "按钮，即使文件存在于服务器上。

谢谢你。

Answer 1

类似这样的东西。

 CheckReportExists cre = new CheckReportExists() {
     @Override
     protected void onPostExecute(Boolean result) {
        super.onPostExecute(result);
        // RIGHT HERE is where you can add your code to handle the result
     }
 };
 // CheckReportExists1 is no longer a good name because it's not
 // returning a value.  It just starts the process.
 cre.CheckReportExists1(download_url);

Answer 2

CheckReportExists1() 总是会返回一个 false 值；这是因为它在UI线程中运行，不会等到 AsyncTask 完成其后台工作。所以它总是返回它的初始分配值，即 false

为了解决这个问题，你可以创建一个带有回调方法的监听器接口，这个回调方法需要一个 Boolean 表示该文件是否存在。并在后台工作结束后，每当你真正确定文件是否存在时，就触发这个回调。

所以用以下方法修改你的代码。

监听器接口：

interface OutputListener {
    void checkFile(Boolean exists);
}

AsyncTask:

public class CheckReportExists extends AsyncTask<String, Void, Boolean>{

    Boolean fileExists = false;
    OutputListener listener;

    public CheckReportExists() {

    }

    public void CheckReportExists1(String download_url, OutputListener outputListener){
        execute(download_url);
        listener = outputListener;
    }

    @Override
    protected void onPreExecute() {
        //display progress dialog.

    }
    @Override
    protected Boolean doInBackground(String... params) {
        try {
            HttpURLConnection.setFollowRedirects(false);
            HttpURLConnection con =  (HttpURLConnection) new URL(params[0]).openConnection();
            con.setRequestMethod("HEAD");
            int response = con.getResponseCode();
            if(response == HttpURLConnection.HTTP_OK){
                fileExists = true;
            }
        } catch(Exception e){

        }
        return fileExists;
    }

    @Override
    protected void onPostExecute(Boolean result) {
        if (listener != null)
            listener.checkFile(result);

        // dismiss progress dialog and update ui
        //super.onPostExecute(result);
    }

}

AsyncTask的用法：

CheckReportExists cre = new CheckReportExists();
cre.CheckReportExists1(download_url, new OutputListener() {
            @Override
            public void checkFile(Boolean exists) {
                if(fileExists) {
                    builder.setPositiveButton("Download Report", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                             new DownloadTask(Results.this, download_url);
                        }
                    });
                } else {
                    builder.setPositiveButton("Report not ready yet", new DialogInterface.OnClickListener() {
                           @Override
                            public void onClick(DialogInterface dialog, int which) {

                            }
                     });
                }

            }
        });