AsyncTask Android 示例

好的，您正在try 通过另一个线程访问GUI.这基本上不是好的做法.

AsyncTask在另一个线程中执行doInBackground()中的所有操作，该线程无权访问视图所在的GUI.

preExecute()和postExecute()允许您在这个新线程中进行繁重的提升之前和之后访问GUI，您甚至可以将长操作的结果传递给postExecute()，然后显示任何处理结果.

查看以下几行，稍后更新TextView:

TextView txt = findViewById(R.id.output);
txt.setText("Executed");

把他们放进onPostExecute()号房.

然后，您将看到文本视图文本在doInBackground完成后更新.

我注意到你的onClick监听器没有判断 Select 了哪个视图.我发现最简单的方法是通过switch语句.我有一个完整的课堂，下面是所有的建议，以避免混淆.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.provider.Settings.System;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class AsyncTaskActivity extends Activity implements OnClickListener {

    Button btn;
    AsyncTask<?, ?, ?> runningTask;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = findViewById(R.id.button1);

        // Because we implement OnClickListener, we only
        // have to pass "this" (much easier)
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        // Detect the view that was "clicked"
        switch (view.getId()) {
        case R.id.button1:
            if (runningTask != null)
                runningTask.cancel(true);
            runningTask = new LongOperation();
            runningTask.execute();
            break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Cancel running task(s) to avoid memory leaks
        if (runningTask != null)
            runningTask.cancel(true);
    }

    private final class LongOperation extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // We were cancelled; stop sleeping!
                }
            }
            return "Executed";
        }

        @Override
        protected void onPostExecute(String result) {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed"); // txt.setText(result);
            // You might want to change "executed" for the returned string
            // passed into onPostExecute(), but that is up to you
        }
    }
}