解決如何讓AsyncTask終止操作

 受到這個的啟發終於結局了如何在AsyncTask運行中終止其操作。

單純的onCancelled(true)是不行的

下面把代碼貼出來~實現了登陸功能。

AsyncTask簡介，它使建立需要與使用者介面互動的長時間啟動並執行任務變得更簡單。相對來說AsyncTask更輕量級一些，適用於簡單的非同步處理，不需要藉助線程和Handler即可實現。

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();    }}