java - 带有进度对话框和按钮的 ASyncTask

Question

我是初学者，我参加了考试。我完成了所有任务，但遇到问题 -

public class HttpTask extends AsyncTask<Integer, String, String> {####

ProgressDialog dialog;
Context context;
public HttpTask(Activity activity) {
    //init progress dialog
    dialog = new ProgressDialog(context);****
    dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
}

protected void onPreExecute() {
    // show progress dialog
    dialog.setMessage("Loading...");
    dialog.setCancelable(false);
}

protected String doInBackground(Integer... params) {
    //freeze system to 5 seconds
        try {
            int seconds = params[0]*5;####
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    return null;
}

@Override
protected void onPostExecute(final String success) {
    // if there is progress dialog hide it
    dialog.dismiss();
}

}

当我尝试编译它时它崩溃了(我用 * 符号显示了哪里有问题):

08-03 10:43:10.873  29441-29441/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:142)
            at android.app.AlertDialog.<init>(AlertDialog.java:98)
            at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
            at net.joerichard.androidtest.main.f.HttpTask.<init>(HttpTask.java:26)
            at net.joerichard.androidtest.main.f.F_Networking_Activity$1.onClick(F_Networking_Activity.java:27)
            at android.view.View.performClick(View.java:4107)
            at android.view.View$PerformClick.run(View.java:17166)
            at android.os.Handler.handleCallback(Handler.java:615)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:155)
            at android.app.ActivityThread.main(ActivityThread.java:5559)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1074)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:841)
            at dalvik.system.NativeStart.main(Native Method)
08-03 10:43:10.913      754-877/? E/EmbeddedLogger﹕ App crashed! Process: net.joerichard.androidtest
08-03 10:43:10.913      754-877/? E/EmbeddedLogger﹕ App crashed! Package: net.joerichard.androidtest v1 (1.0)
08-03 10:43:10.913      754-877/? E/EmbeddedLogger﹕ Application Label: AndroidTest

这是主要 Activity 的类。

public class F_Networking_Activity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_f__networking_);

    // bDownload: start HttpTask

    Button bDownload = (Button) findViewById(R.id.bDownload);
    bDownload.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            HttpTask task = new HttpTask(F_Networking_Activity.this);****
            task.execute();
        }
    });

}

感谢您的回答。现在我有另一个问题(我显示了第二个问题的#标志)

08-03 11:28:18.292      754-877/? E/EmbeddedLogger﹕ App crashed! Process: net.joerichard.androidtest'
08-03 11:28:18.292      754-877/? E/EmbeddedLogger﹕ App crashed! Package: net.joerichard.androidtest v1 (1.0)
08-03 11:28:18.292      754-877/? E/EmbeddedLogger﹕ Application Label: AndroidTest
08-03 11:28:18.292  30544-30726/? E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:299)
        at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
        at java.util.concurrent.FutureTask.run(FutureTask.java:137)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
        at java.lang.Thread.run(Thread.java:864)
 Caused by: java.lang.ArrayIndexOutOfBoundsException: length=0; index=0
        at   net.joerichard.androidtest.main.f.HttpTask.doInBackground(HttpTask.java:40)
        at net.joerichard.androidtest.main.f.HttpTask.doInBackground(HttpTask.java:20)
        at     android.os.AsyncTask$2.call(AsyncTask.java:287)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
        at     java.util.concurrent.FutureTask.run(FutureTask.java:137)
        at     android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
        at java.lang.Thread.run(Thread.java:864) 

Answer 1

实际上，你的 context 是 null 因为你没有初始化它。

在您的HttpTask 中添加一行:

public HttpTask(Activity activity) {
    this.context = activity;
    dialog = new ProgressDialog(context);
    dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
}

并将 Context 更改为 Activity，如下所示:

Activity context;

现在在你类(class)的任何地方调用这个context。