5

在我的一个应用程序中,我有一个需要执行一些后台任务的场景。为此,我正在使用异步任务。我也在使用自定义进度对话框。下面是自定义进度对话框的布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/layout_root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_gravity="center_vertical|center_horizontal"
    android:orientation="vertical" >

    <ProgressBar
        android:layout_width="60dp"
        android:layout_height="60dp"
        android:indeterminateDrawable="@drawable/progressloader" 
        android:layout_gravity="center"/>

    <TextView
        android:id="@+id/progressMessage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@color/black"
        android:textSize="18sp"
        android:text="Please wait...." />

</LinearLayout>

一切正常,但是当我尝试将文本设置为 TextView 时,我得到了 java NullPointerException。

异步任务代码

private class InitialSetup extends AsyncTask<String, Integer, Long> {

        ProgressDialog dialog = new ProgressDialog(getParent(),R.style.progressdialog);


        @Override
        protected void onPreExecute() {
            dialog.show();
            dialog.setContentView(R.layout.progressbar);

        }

        @Override
        protected Long doInBackground(String... urls) {
                    //    txtView.setText("Testing");    here I am getting the error
            fetchDetails();

            return 0;
        }

        @Override
        protected void onPostExecute(Long result) {

            if (this.dialog.isShowing()) {
                this.dialog.dismiss();
            }

            populateUI(getApplicationContext());
        }
    }

主要活动

public class SummaryActivity extends Activity {


final TextView txtView = (TextView)findbyid(R.id.progressMessage);
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.accountsummary);

              new InitialSetup().execute("");

    }
}
4

4 回答 4

14

如果我理解正确,您TextView要设置的文本可以在 xml 文件 progressbar.xml (即R.layout.progressbar)中找到。这TextView可以在设置内容视图后获得(使用setContentView())。在您的代码中,您在此调用之前设置了它,而 mussharapp 的代码,他提前调用了它。即,他在setContentView(R.layout.accountsummary)不包含TextView. 因此,该变量txtView将为 NULL,您将获得一个NullPointerException.

你应该做的是:

  • onPreExecute在调用之后设置变量 txtView setContentView
  • 根据 Paresh Mayani 的解释:使用 runOnUiThread 方法。

对于代码,请看下面:

private class InitialSetup extends AsyncTask<String, Integer, Long> {

        ProgressDialog dialog = new ProgressDialog(getParent(),R.style.progressdialog);
        // The variable is moved here, we only need it here while displaying the
        // progress dialog.
        TextView txtView;

        @Override
        protected void onPreExecute() {
            dialog.show();
            dialog.setContentView(R.layout.progressbar);
            // Set the variable txtView here, after setContentView on the dialog
            // has been called! use dialog.findViewById().
            txtView = dialog.findViewById(R.id.progressMessage); 
        }

        @Override
        protected Long doInBackground(String... urls) {
            // Already suggested by Paresh Mayani:
            // Use the runOnUiThread method.
            // See his explanation.
            runOnUiThread(new Runnable() {
               @Override
               public void run() {
                  txtView.setText("Testing");       
               }
            });

            fetchDetails();
            return 0;
        }

        @Override
        protected void onPostExecute(Long result) {

            if (this.dialog.isShowing()) {
                this.dialog.dismiss();
            }

            populateUI(getApplicationContext());
        }
    }
于 2012-04-17T18:16:02.087 回答
12

是的,因为您试图在 doInBackground() 方法中设置 TextView,这是不允许的,

为什么不允许?因为只有一个线程在运行,即 UI 主线程,并且不允许从线程进程更新 UI。在此处阅读更多信息:无痛穿线

所以有一个解决方案,如果你想在doInBackground()方法里面设置TextView,在runOnUiThread方法里面做UI更新操作

否则,建议在 onPostExecute() 方法中执行所有 UI 显示/更新相关操作,而不是 AsyncTask 类的 doInBackground() 方法。

于 2012-04-17T18:02:57.550 回答
1
(TextView)findViewByid(R.id.progressMessage);

只能在命令 setContentView() 之后执行。

TextView txtView;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.accountsummary);
    **txtView = (TextView)findbyid(R.id.progressMessage);**


    new InitialSetup().execute("");

}

此外,您只能在主 UI 线程中更改 UI 元素。doInBackground() 不在主 UI 线程中。在 onPostExecute 中进行 UI 更改

public class InitialSetup extends AsyncTask<String, Integer, Long> {

        private Activity activity;
        ProgressDialog progressDialog;

        public InitialSetup(Activity activity) {
            this.activity = activity;
        }




        @Override
        protected void onPreExecute() {
            progressDialog = new ProgressDialog(activity);
            progressDialog.setMessage("Starting task....");
            progressDialog.show();    
        }

        @Override
        protected Long doInBackground(String... urls) {
            // do something

            //        

            return 0;
        }

        @Override
        protected void onPostExecute(Long result) {
            progressDialog.dismiss();
             //Perform all UI changes here
            **textView.setText("Text#2");**
        }
    }
于 2012-04-17T16:28:50.620 回答
1

解释是正确的:除了创建 UI 的线程之外,您不得在任何线程中进行 UI 更改。但是 AsyncTask 有一个方法叫做

onProgressUpdate()

它总是会在 UI 线程中运行。因此,根据 dennisg 的修改,您的代码应如下所示:

private class InitialSetup extends AsyncTask<String, String, Long> {

    ProgressDialog dialog = new ProgressDialog(getParent(),R.style.progressdialog);
    // The variable is moved here, we only need it here while displaying the
    // progress dialog.
    TextView txtView;

    @Override
    protected void onPreExecute() {
        dialog.show();
        dialog.setContentView(R.layout.progressbar);
        // Set the variable txtView here, after setContentView on the dialog
        // has been called! use dialog.findViewById().
        txtView = dialog.findViewById(R.id.progressMessage); 
    }

    @Override
    protected Long doInBackground(String... urls) {
        publishProgress("Testing");

        fetchDetails();

        return 0;
    }

    @Override
    protected void onPostExecute(Long result) {

        if (this.dialog.isShowing()) {
            this.dialog.dismiss();
        }

        populateUI(getApplicationContext());
    }

    @Override
    protected void onProgressUpdate(String... update) {
        if (update.length > 0)
            txtView.setText(update[0]); 
    }
}

注意onProgressUpdate的参数类型是AsyncTask中给出的第二种类型!

额外:为了使您的代码更健壮,您应该在设置文本之前检查进度对话框是否仍然存在。

于 2013-05-30T11:07:52.543 回答