我正在阅读有关AsyncTask的文章，并尝试了下面的简单程序。但这似乎并不奏效。我该怎么做呢?
public class AsyncTaskActivity extends Activity {
Button btn;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
btn = (Button) findViewById(R.id.button1);
btn.setOnClickListener((OnClickListener) this);
}
public void onClick(View view){
new LongOperation().execute("");
}
private class LongOperation extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... params) {
for(int i=0;i<5;i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
TextView txt = (TextView) findViewById(R.id.output);
txt.setText("Executed");
return null;
}
@Override
protected void onPostExecute(String result) {
}
@Override
protected void onPreExecute() {
}
@Override
protected void onProgressUpdate(Void... values) {
}
}
}
我只是试图在后台进程中5秒后更改标签。
这是我的main.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<ProgressBar
android:id="@+id/progressBar"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:indeterminate="false"
android:max="10"
android:padding="10dip">
</ProgressBar>
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Start Progress" >
</Button>
<TextView android:id="@+id/output"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Replace"/>
</LinearLayout>
异步任务POST请求示例:
List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("key1", "value1"));
params.add(new BasicNameValuePair("key1", "value2"));
new WEBSERVICEREQUESTOR(URL, params).execute();
class WEBSERVICEREQUESTOR extends AsyncTask<String, Integer, String>
{
String URL;
List<NameValuePair> parameters;
private ProgressDialog pDialog;
public WEBSERVICEREQUESTOR(String url, List<NameValuePair> params)
{
this.URL = url;
this.parameters = params;
}
@Override
protected void onPreExecute()
{
pDialog = new ProgressDialog(LoginActivity.this);
pDialog.setMessage("Processing Request...");
pDialog.setIndeterminate(false);
pDialog.setCancelable(false);
pDialog.show();
super.onPreExecute();
}
@Override
protected String doInBackground(String... params)
{
try
{
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpEntity httpEntity = null;
HttpResponse httpResponse = null;
HttpPost httpPost = new HttpPost(URL);
if (parameters != null)
{
httpPost.setEntity(new UrlEncodedFormEntity(parameters));
}
httpResponse = httpClient.execute(httpPost);
httpEntity = httpResponse.getEntity();
return EntityUtils.toString(httpEntity);
} catch (Exception e)
{
}
return "";
}
@Override
protected void onPostExecute(String result)
{
pDialog.dismiss();
try
{
}
catch (Exception e)
{
}
super.onPostExecute(result);
}
}
移动这两条线:
TextView txt = (TextView) findViewById(R.id.output);
txt.setText("Executed");
你的AsyncTask的doInBackground方法，并把它们放在onPostExecute方法。你的AsyncTask应该看起来像这样:
private class LongOperation extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... params) {
try {
Thread.sleep(5000); // no need for a loop
} catch (InterruptedException e) {
Log.e("LongOperation", "Interrupted", e);
return "Interrupted";
}
return "Executed";
}
@Override
protected void onPostExecute(String result) {
TextView txt = (TextView) findViewById(R.id.output);
txt.setText(result);
}
}
我相信它是正确执行，但你试图改变UI元素在后台线程，这不会做。
修改你的调用和AsyncTask如下:
调用类
注意:我个人建议在任何执行AsyncTask线程的地方使用onPostExecute()，而不是在扩展AsyncTask本身的类中。我认为这使代码更容易阅读，特别是如果你需要AsyncTask在多个地方处理略有不同的结果。
new LongThread() {
@Override public void onPostExecute(String result) {
TextView txt = (TextView) findViewById(R.id.output);
txt.setText(result);
}
}.execute("");
LongThread类(extends AsyncTask):
@Override
protected String doInBackground(String... params) {
for (int i = 0; i < 5; i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return "Executed";
}
带进度的AsyncTask示例示例
import android.animation.ObjectAnimator;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.animation.AccelerateDecelerateInterpolator;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.LinearInterpolator;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
public class AsyncTaskActivity extends AppCompatActivity implements View.OnClickListener {
Button btn;
ProgressBar progressBar;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn = (Button) findViewById(R.id.button1);
btn.setOnClickListener(this);
progressBar = (ProgressBar)findViewById(R.id.pbar);
}
public void onClick(View view) {
switch (view.getId()) {
case R.id.button1:
new LongOperation().execute("");
break;
}
}
private class LongOperation extends AsyncTask<String, Integer, String> {
@Override
protected String doInBackground(String... params) {
Log.d("AsyncTask", "doInBackground");
for (int i = 0; i < 5; i++) {
try {
Log.d("AsyncTask", "task "+(i + 1));
publishProgress(i + 1);
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.interrupted();
}
}
return "Completed";
}
@Override
protected void onPostExecute(String result) {
Log.d("AsyncTask", "onPostExecute");
TextView txt = (TextView) findViewById(R.id.output);
txt.setText(result);
progressBar.setProgress(0);
}
@Override
protected void onPreExecute() {
Log.d("AsyncTask", "onPreExecute");
TextView txt = (TextView) findViewById(R.id.output);
txt.setText("onPreExecute");
progressBar.setMax(500);
progressBar.setProgress(0);
}
@Override
protected void onProgressUpdate(Integer... values) {
Log.d("AsyncTask", "onProgressUpdate "+values[0]);
TextView txt = (TextView) findViewById(R.id.output);
txt.setText("onProgressUpdate "+values[0]);
ObjectAnimator animation = ObjectAnimator.ofInt(progressBar, "progress", 100 * values[0]);
animation.setDuration(1000);
animation.setInterpolator(new LinearInterpolator());
animation.start();
}
}
}
这里的概念和代码
我已经创建了一个简单的例子使用AsyncTask的Android。它从onPreExecute()， doInBackground()， publishProgress()开始，最后是onProgressUpdate()。
在这种情况下，doInBackground()作为后台线程工作，而其他线程在UI线程中工作。你不能访问doInBackground()中的UI元素。顺序和我之前提到的一样。
然而，如果你需要从doInBackground更新任何小部件，你可以从doInBackground发布进度，它会调用onProgressUpdate来更新你的UI小部件。
class TestAsync extends AsyncTask<Void, Integer, String> {
String TAG = getClass().getSimpleName();
protected void onPreExecute() {
super.onPreExecute();
Log.d(TAG + " PreExceute","On pre Exceute......");
}
protected String doInBackground(Void...arg0) {
Log.d(TAG + " DoINBackGround", "On doInBackground...");
for (int i=0; i<10; i++){
Integer in = new Integer(i);
publishProgress(i);
}
return "You are at PostExecute";
}
protected void onProgressUpdate(Integer...a) {
super.onProgressUpdate(a);
Log.d(TAG + " onProgressUpdate", "You are in progress update ... " + a[0]);
}
protected void onPostExecute(String result) {
super.onPostExecute(result);
Log.d(TAG + " onPostExecute", "" + result);
}
}
在你的活动中这样称呼它:
new TestAsync().execute();
开发人员参考资料
