android - 当按钮按下两次android时应用程序停止

Question

我最近开始 android 开发并面临不幸应用程序已停止的错误。该应用程序非常简单，它在按下按钮时从 twitter 获取提要并将其显示在文本框中。多次按下按钮时会发生错误。这是我的代码:

package com.example.newsfeed;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class NewsFeed extends Activity {

    AsyncTask<Void, Void, Void> mTask;
    String jsonString;
    String url = "https://api.twitter.com/1/statuses/user_timeline.json?include_entities=true&include_rts=true&screen_name=50cent&count=3";
    Button b;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_news_feed);
        b = (Button) findViewById(R.id.getFeedBtn);
        final TextView tv = (TextView) findViewById(R.id.newsfeedEdit);

        mTask = new AsyncTask<Void, Void, Void> () {

            @Override
            protected Void doInBackground(Void... arg0) {
                // TODO Auto-generated method stub
                try {
                    jsonString = getJsonFromServer(url);
                } catch(IOException e) {
                    e.printStackTrace();
                }
                return null;
            }

            protected void onPostExecute(Void result) {
                super.onPostExecute(result);
                try {
                    JSONArray jArray = new JSONArray(jsonString);
                    String text="";


                    for( int i=0; i<jArray.length(); i++ ) {
                        JSONObject e = (JSONObject) jArray.getJSONObject(i);

                        JSONObject f = (JSONObject) jArray.getJSONObject(i).get("user");

                        text = text + "User : "+f.getString("name")+"\n";

                        text = text + "ID : "+e.getString("id")+"\n";
                        text = text + "SOURCE: "+e.getString("source")+"\n";
                        text = text + "TEXT : "+e.getString("text")+"\n";
                    }
                    tv.setText(text);
                } catch (JSONException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        };

        b.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                mTask.execute();
            }
        });
    }

    public static String getJsonFromServer(String url) throws IOException {
        BufferedReader inputStream = null;

        URL jsonUrl = new URL(url);
        URLConnection dc = jsonUrl.openConnection();

        dc.setConnectTimeout(5000);
        dc.setReadTimeout(5000);

        inputStream = new BufferedReader(new InputStreamReader(
                dc.getInputStream()));

        String jsonResult = inputStream.readLine();
        return jsonResult;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_news_feed, menu);
        return true;
    }

}

这是我的日志:

03-12 11:01:17.172: E/AndroidRuntime(10297): FATAL EXCEPTION: main
03-12 11:01:17.172: E/AndroidRuntime(10297): java.lang.IllegalStateException: Cannot execute task: the task has already been executed (a task can be executed only once)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:578)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.os.AsyncTask.execute(AsyncTask.java:534)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at com.example.newsfeed.NewsFeed$2.onClick(NewsFeed.java:83)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.view.View.performClick(View.java:4202)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.view.View$PerformClick.run(View.java:17340)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.os.Handler.handleCallback(Handler.java:725)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.os.Handler.dispatchMessage(Handler.java:92)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.os.Looper.loop(Looper.java:137)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at android.app.ActivityThread.main(ActivityThread.java:5039)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at java.lang.reflect.Method.invokeNative(Native Method)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at java.lang.reflect.Method.invoke(Method.java:511)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
03-12 11:01:17.172: E/AndroidRuntime(10297):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

在 onClick方法你必须检查你的任务是否已经在运行。如果您尝试多次启动一个任务，那么您的应用程序将会崩溃。我建议您将任务的声明分离到一个私有(private)类中，然后在 onClick 方法中将该类实例化(在您检查它是否正在运行之后)。

您可以引用AsyncTask documentation更多细节。那里也有例子。

如果随后单击按钮，我修改了您的代码以重新创建任务。找到它here .您将不得不对其进行一些修改，因为一直取消它是没有意义的，但我还是决定举一个例子来说明如何做到这一点。