我目前正在通过一本书学习android应用程序开发,我目前正在努力学习一些表面上看起来相当简单的东西。界面由一个按钮和一个文本视图组成。这个练习的目的是演示在执行耗时的任务时如何使用单独的线程。
我试过调试 values
的数组 onProgressUpdate
方法确实包含传递给它的值,并且在尝试为其设置文本时似乎总是发生崩溃 myTextView
. 如果有任何帮助,我将不胜感激。
代码:
package com.ebookfrenzy.asyncdemo;
import androidx.appcompat.app.AppCompatActivity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private TextView myTextView;
private String tag = "Test";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void buttonClick(View view) {
AsyncTask task = new MyTask().execute();
}
private class MyTask extends AsyncTask<String,Integer,String> {
@Override
protected void onPreExecute() {
}
@Override
protected void onProgressUpdate(Integer... values) {
Log.i(tag,"publish: " + Integer.toString(values[0]));
myTextView.setText("Counter = " + values[0]);
}
@Override
protected void onPostExecute(String result) {
myTextView.setText(result);
}
@Override
protected String doInBackground(String... strings) {
int i = 0;
while (i <= 20) {
Log.i(tag,"before publish: " + Integer.toString(i));
publishProgress(i);
try {
Thread.sleep(1000);
i++;
} catch(Exception e) {
}
}
return "Button Pressed";
}
}
}
logcat输出(由我为故障排除添加的log.i项创建)通常如下所示,但如果警告消息提示应用程序不断崩溃,则“发布前”项会不断填充i,每次递增,直到我关闭警告消息:
2020-12-13 23:17:04.499 31729-31772/com.ebookfrenzy.asyncdemo I/Test: before publish: 0
2020-12-13 23:17:04.503 31729-31729/com.ebookfrenzy.asyncdemo I/Test: publish: 0
logcat的错误部分:
--------- beginning of crash
2020-12-13 23:19:47.635 353-353/com.ebookfrenzy.asyncdemo E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.ebookfrenzy.asyncdemo, PID: 353
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.ebookfrenzy.asyncdemo.MainActivity$MyTask.onProgressUpdate(MainActivity.java:34)
at com.ebookfrenzy.asyncdemo.MainActivity$MyTask.onProgressUpdate(MainActivity.java:25)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:715)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2020-12-13 23:19:47.639 353-353/com.ebookfrenzy.asyncdemo I/Process: Sending signal. PID: 353 SIG: 9
1条答案
按热度按时间gt0wga4j1#
我想
myTextView
必须为空。试试这个。
你应该初始化
myTextView
在onCreate()
方法。