完美的解决如何让AsyncTask终止操作

package com.isummation.exampleapp; import java.io.BufferedReader;import java.io.InputStreamReader;import java.net.URLEncoder;import java.net.UnknownHostException; import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.params.CoreProtocolPNames;import org.apache.http.protocol.BasicHttpContext;import org.apache.http.protocol.HttpContext;import org.json.JSONObject; import android.app.Activity;import android.app.Dialog;import android.app.ProgressDialog;import android.content.DialogInterface;import android.content.DialogInterface.OnCancelListener;import android.os.AsyncTask;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.EditText;import android.widget.Toast; public class UserLogin extends Activity {     private EditText etUsername;    private EditText etPassword;    private ProgressDialog progressDialog;    private static final int PROGRESSDIALOG_ID = 0;    private static final int SERVER_ERROR = 1;    private static final int NETWORK_ERROR = 2;    private static final int CANCELLED = 3;    private static final int SUCCESS = 4;    private String ServerResponse;    private LoginTask loginTask;     @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.login);         etUsername = (EditText) findViewById(R.id.txt_username);        etPassword = (EditText) findViewById(R.id.txt_password);         Button login_button = (Button) this.findViewById(R.id.login_button);        login_button.setOnClickListener(new OnClickListener() {            public void onClick(View viewParam) {                if (etUsername.getText().toString().length() == 0                        || etPassword.getText().toString().length() == 0) {                    Toast.makeText(getApplicationContext(),                            "Please enter username and password",                            Toast.LENGTH_SHORT).show();                } else {                     //Show dialog by passing id                    showDialog(PROGRESSDIALOG_ID);                }            }        });    }     protected Dialog onCreateDialog(int id) {        switch(id) {        case PROGRESSDIALOG_ID:            removeDialog(PROGRESSDIALOG_ID);             //Please note that forth parameter is true for cancelable Dialog            //Also register cancel event listener            //if the litener is registered then forth parameter has no effect            progressDialog = ProgressDialog.show(UserLogin.this, "Authenticating",                    "Please wait...", true, true, new OnCancelListener(){                         public void onCancel(DialogInterface dialog) {                            //Check the status, status can be RUNNING, FINISHED and PENDING                            //It can be only cancelled if it is not in FINISHED state                            if (loginTask != null && loginTask.getStatus() != AsyncTask.Status.FINISHED)                                loginTask.cancel(true);                        }                    });            break;        default:            progressDialog = null;        }        return progressDialog;    }     @Override    protected void onPrepareDialog(int id, Dialog dialog) {        switch (id) {        case PROGRESSDIALOG_ID:            //check if any previous task is running, if so then cancel it            //it can be cancelled if it is not in FINISHED state            if (loginTask != null && loginTask.getStatus() != AsyncTask.Status.FINISHED)                loginTask.cancel(true);            loginTask = new LoginTask(); //every time create new object, as AsynTask will only be executed one time.            loginTask.execute();        }    }     class LoginTask extends AsyncTask<Void, Integer, Void> {        @Override        protected Void doInBackground(Void... unused) {            try {                ServerResponse = null; //don't forget to make it null, as task can be called again                HttpClient httpClient = new DefaultHttpClient();                HttpContext localContext = new BasicHttpContext();                HttpGet httpGet = new HttpGet(                        getString(R.string.WebServiceURL)                                + "/cfc/iphonewebservice.cfc?returnformat=json&method=validateUserLogin&username="                                + URLEncoder.encode(etUsername.getText()                                        .toString(), "UTF-8")                                + "&password="                                + URLEncoder.encode(etPassword.getText()                                        .toString(), "UTF-8"));                httpClient.getParams().setParameter(                        CoreProtocolPNames.USER_AGENT,"Some user agent string");                 //call it just before you make server call                //calling after this statement and canceling task will no meaning if you do some update database kind of operation                //so be wise to choose correct place to put this condition                //you can also put this condition in for loop, if you are doing iterative task                 //now this very important                //if you do not put this condition and not maintaining execution, then there is no meaning of calling .cancel() method                //you should only check this condition in doInBackground() method, otherwise there is no logical meaning                if (isCancelled())                {                    publishProgress(CANCELLED); //Notify your activity that you had canceled the task                    return (null); // don't forget to terminate this method                }                HttpResponse response = httpClient.execute(httpGet,                        localContext);                 BufferedReader reader = new BufferedReader(                        new InputStreamReader(                                response.getEntity().getContent(), "UTF-8"));                ServerResponse = reader.readLine();                publishProgress(SUCCESS); //if everything is Okay then publish this message, you may also use onPostExecute() method            } catch (UnknownHostException e) {                removeDialog(PROGRESSDIALOG_ID);                e.printStackTrace();                publishProgress(NETWORK_ERROR);            } catch (Exception e) {                removeDialog(PROGRESSDIALOG_ID);                e.printStackTrace();                publishProgress(SERVER_ERROR);            }            return (null);        }         @Override        protected void onProgressUpdate(Integer... errorCode) {            switch (errorCode[0]) {            case CANCELLED:                removeDialog(PROGRESSDIALOG_ID);                Toast.makeText(getApplicationContext(), "Cancelled by user",                        Toast.LENGTH_LONG).show();                break;            case NETWORK_ERROR:                removeDialog(PROGRESSDIALOG_ID);                Toast.makeText(getApplicationContext(), "Network connection error",                        Toast.LENGTH_LONG).show();                break;            case SERVER_ERROR:                removeDialog(PROGRESSDIALOG_ID);                Toast.makeText(getApplicationContext(), "Server error",                        Toast.LENGTH_LONG).show();                break;            case SUCCESS:                removeDialog(PROGRESSDIALOG_ID);                try {                    if (ServerResponse != null) {                        JSONObject JResponse = new JSONObject(ServerResponse);                        String sMessage = JResponse.getString("MESSAGE");                        int success = JResponse.getInt("SUCCESS");                        if (success == 1) {                            //proceed further                             //you may start new activity from here                            //after that you may want to finish this activity                            UserLogin.this.finish();                             //Remember when you finish an activity, it doesn't mean that you also finish thread or AsynTask started within that activity                            //So you must implement onDestroy() method and terminate those threads.                        } else {                            //just showing invalid username password from server response                            Toast.makeText(getApplicationContext(), sMessage,                                    Toast.LENGTH_SHORT).show();                        }                    }                } catch (Exception e){                    Toast.makeText(getApplicationContext(), "Server error",                            Toast.LENGTH_LONG).show();                    e.printStackTrace();                }                break;            }        }         @Override        protected void onPostExecute(Void unused) {         }    }     @Override    protected void onDestroy(){        //you may call the cancel() method but if it is not handled in doInBackground() method        if (loginTask != null && loginTask.getStatus() != AsyncTask.Status.FINISHED)            loginTask.cancel(true);        super.onDestroy();    }}

 

受到这个的启发终于结局了如何在AsyncTask运行中终止其操作。

单纯的onCancelled(true)是不行的

下面把代码贴出来~实现了登陆功能。

AsyncTask简介，它使创建需要与用户界面交互的长时间运行的任务变得更简单。相对来说AsyncTask更轻量级一些，适用于简单的异步处理，不需要借助线程和Handler即可实现。

转自stackoverflow

 原文出处：http://www.ericyue.info/archive/stop-asynctask#more-1117