来自AsyncTask中URL的InputStream [英] InputStream from URL in AsyncTask

问题描述

尝试从URL获取文件到InputStream元素(使用AsyncTask)，然后使用它，但到目前为止没有成功.这些参数是否适合AsyncTask?我可以像在代码中一样检查InputStream还是错误，并且它总是返回null?我还应该进行其他更改吗?

Trying to get a file from URL into InputStream element (with AsyncTask) and then use it, but so far unsuccessful. Are the parameters right for AsyncTask? Can I check InputStream like I did in my code or is it wrong and it will always return null? Any other changes I should make?

我的代码:

String url = "https://www.example.com/file.txt";
InputStream stream = null;

public void load() {
    DownloadTask downloadTask = new DownloadTask();
    downloadTask.execute(url);
    if (stream == null) {
        //fail, stream == null
    } else {
        //success
    }
}

class DownloadTask extends AsyncTask <String, Integer, InputStream> {

@Override
protected void onPreExecute() {

}

@Override
protected InputStream doInBackground(String... params){

    try {
        URL url = new URL(url);
        stream = url.openStream();

    } catch (MalformedURLException e) {
            e.printStackTrace();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return stream;
}

@Override
protected void onPostExecute(InputStream result) {
    super.onPostExecute(result);
}
}

我修好了.

public void load(){
    //some code
    DownloadTask dt=new DownloadTask();
    dt.execute("www.example.com");
}

private class DownloadTask extends AsyncTask<String,Integer,Void>{
    InputStream text;

    protected Void doInBackground(String...params){
        URL url;
        try {
            url = new URL(params[0]);
            HttpURLConnection con=(HttpURLConnection)url.openConnection();
            InputStream is=con.getInputStream();
            text = is;
        }catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    protected void onPostExecute(Void result){
        //use "text"
    }
}

推荐答案

您以错误的方式使用了 AsyncTask .在 doInBackground(String ... params)方法内执行所有操作，例如下载或服务器请求.

you are using AsyncTask in the wrong way. Do all operations like download or server requests inside doInBackground(String... params) method.

非常重要的一点是，您应该在 onPostExecute(..)方法内更新UI.从UI-Thread调用此方法.

Very important to know, that you should update your UI inside the onPostExecute(..) method. This method get called from UI-Thread.

这是一个示例(伪代码):

Here is an example (pseudo code):

class DownloadTask extends AsyncTask <String, Integer, MyDownloadedData> {

    private MyDownloadedData getFromStream(InputStream stream){
        // TODO
        return null;
    }

    @Override
    protected void onPreExecute() {
    }

    @Override
    protected MyDownloadedData doInBackground(String... params){

        try {
            URL url = new URL(url);
            InputStream stream = url.openStream();

            MyDownloadedData data = getFromStream(stream);
            return data;
        } catch (Exception e) {
            // do something
        }
        return stream;
    }

    @Override
    protected void onPostExecute(MyDownloadedData data) {
        //
        // update your UI in this method. example:
        //
        someLabel.setText(data.getName());
    }

    protected void onProgressUpdate(Integer... progress) {
        //...
    }


}

这篇关于来自AsyncTask中URL的InputStream的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！