从Async Task更新TextView,它使用自定义程序对话框

Question

在我的一个应用程序中,我有一个场景,我需要做一些后台任务.为此,我使用异步任务.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,但当我尝试将文本设置为TextView时,我得到java NullPointerException.

AsyncTask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("");

    }
}

Answer 1

如果我理解正确,您TextView可以在xml文件progressbar.xml(即R.layout.progressbar)中找到要设置文本的内容.TextView一旦设置了内容视图(使用setContentView()),就可以获得这一点.在您的代码中,您在此调用之前设置它和mussharapp的代码,他正在提前调用它.也就是说,他在setContentView(R.layout.accountsummary)不包含电话的电话之后拨打电话TextView.因此,变量txtView将为NULL,您将得到一个NullPointerException.

你应该做的是以下几点:

• 调用onPreExecute后设置变量txtView in setContentView.
• 基于Paresh Mayani的解释:使用runOnUiThread方法.

对于代码,请查看以下内容:

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());
        }
    }

Answer 2

是的,因为你试图在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Thread运行是UI主线程,并且不允许从线程进程更新UI.在这里阅读更多信息:无痛线程

因此,有一个解决方案,如果你想设置的doInBackground()方法中TextView的,做UI更新内部操作runOnUiThread方法.

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.