android - 无法从 AsyncTask 返回值;使用接口和委托;
问题描述
我正在 AsyncTask 中进行 HTTP 连接。我将查询的字符串结果返回给调用代码。在那里,我将 EditText "HTTPResult" 的值设置为返回的结果字符串。但是 HTTPResult 是空的。这意味着没有任何内容返回给调用代码。但是,我在 AsyncTask 中获取了结果字符串,并且可以在 AsyncTask 的 protected void onPostExecute(String bitmap) 方法中设置 HTTPResult。然后我使用了接口和委托,但没有得到结果。
代码:
public class RegisterActivity extends AppCompatActivity implements AsyncResponse {
Button btn_Connect;
EditText busNumber;
EditText text;
EditText HTTPResult;
String myServer="http://swulj.atwebpages.com/hi.php";
String myResult, token;
HTTPConnection1 conn = new HTTPConnection1();
RegisterActivity obj = this;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_register);
btn_Connect = (Button) findViewById(R.id.button_Connect);
busNumber = (EditText) findViewById(R.id.Bus_number);
text = (EditText) findViewById(R.id.text);
HTTPResult = (EditText) findViewById(R.id.HTTPResult);
btn_Connect.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// it was the 1st button
Toast.makeText(getApplicationContext(),"You download is resumed2",Toast.LENGTH_LONG).show();
conn.delegate = obj;
conn.execute(myServer, (String) busNumber.getText().toString());
//sleep(50000);
HTTPResult.setText(myResult);
/*StringTokenizer st = new StringTokenizer(myResult);
token = st.nextToken("<br>");
text.setText(token);
while (st.hasMoreTokens()) {
token = st.nextToken("<br>");
busNumber.setText(token);
}*/
}
});
}
@Override
public void processFinish(String output){
//Here you will receive the result fired from async class
//of onPostExecute(result) method.
myResult = output;
}
class HTTPConnection1 extends AsyncTask<String, Void, String> {
String result;
String url;
String busNumber;
public AsyncResponse delegate = null;
@Override
protected String doInBackground(String... params) {
url = params[0];
busNumber = params[1];
HttpClient httpclient = new DefaultHttpClient();
HttpPost httppost = new HttpPost(url);
try {
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("id", busNumber));
nameValuePairs.add(new BasicNameValuePair("message", "msg"));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
HttpResponse httpResponse = httpclient.execute(httppost);
InputStream inputStream = httpResponse.getEntity().getContent();
//HTTPResult.setText("result3");
InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
//HTTPResult.setText("result4");
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
//HTTPResult.setText("result5");
StringBuilder stringBuilder = new StringBuilder();
//HTTPResult.setText("result6");
String bufferedStrChunk = null;
while ((bufferedStrChunk = bufferedReader.readLine()) != null) {
stringBuilder.append(bufferedStrChunk);
}
result = stringBuilder.toString();
//result= "Sandeep";
} catch (ClientProtocolException e) {
result = "ClientProtocolException";
// TODO Auto-generated catch block
} catch (IOException e) {
result = "IOException";
// TODO Auto-generated catch block
}
return result;
}
@Override
protected void onPostExecute(String bitmap) {
super.onPostExecute(bitmap);
//HTTPResult.setText(result);
delegate.processFinish(bitmap);
}
}
}
public interface AsyncResponse {
void processFinish(String output);
}
我的动机不是将文本设置为 HTTPResult,而是设置字符串 myResult;我正在使用 HTTPResult 测试值
解决方案
此问题是因为 AsyncTask 在后台线程中执行任务,因此当您尝试在 EditText 中设置结果时,AsyncTask 可能尚未完成,因此您的 myResult 将具有要修复的默认值,您可以在 onPostExecute 方法中更新 EditText以及这条线在 onPostExecute 中运行良好的原因
HTTPResult.setText(bitmap);
编辑:您可以在 onPostExecute 中更新 myResult 但问题是要知道 AsyncTask 何时完成,因此您可以通过使用接口作为回调来解决此问题
检查此问题以查找如何将接口用作 AsyncTask 的回调的示例
这是一个关于如何使用接口创建回调的好教程
https://www.tutorialspoint.com/Callback-using-Interfaces-in-Java
编辑 2: : 将 setText 从 onCreate 移动到您的 processFinish 函数,或者您可以在 asynctask 完成时检查日志或 toast 以更新 ui 并将值推送到您的回调
@Override
public void processFinish(String output){
myResult = output;
HTTPResult.setText(myResult);
}