我的代码如下。我在实现 Async Task 时遇到了麻烦，Async Task 的 android 文档对我来说有点令人困惑。谁能用清晰的步骤向我解释？
这是我的代码：
package com.sara.facebookappl;
import java.io.IOException;
import java.net.MalformedURLException;
import org.json.JSONException;
import org.json.JSONObject;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Looper;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;
import com.facebook.android.DialogError;
import com.facebook.android.Facebook;
import com.facebook.android.Facebook.DialogListener;
import com.facebook.android.FacebookError;
import com.facebook.android.Util;
public class MainActivity extends Activity implements OnClickListener, DialogListener {
Facebook fb;
ImageView button;
private SharedPreferences sp;
TextView welcome;
Button post;
@SuppressWarnings("deprecation")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
post=(Button)findViewById(R.id.button1);
String APP_ID = getString(R.string.APP_ID);
fb= new Facebook(APP_ID);
Looper.prepare();
new PostTask().execute(fb);
Looper.loop();
sp =getPreferences(MODE_PRIVATE);
String access_token=sp.getString("access_token", null);
long expires=sp.getLong("access_expires", 0);
if (access_token !=null) {
fb.setAccessToken(access_token);
}
if(expires !=0) {
fb.setAccessExpires(expires);
}
button=(ImageView)findViewById(R.id.login);
button.setOnClickListener((OnClickListener) this);
updateButtonImage();
}
private class PostTask extends AsyncTask<Facebook, Integer, String> {
ProgressDialog dialog;
protected void onPreExecute() {
dialog = new ProgressDialog(MainActivity.this);
dialog.setProgress(ProgressDialog.STYLE_HORIZONTAL);
dialog.setMax(100);
dialog.show();
}
@SuppressWarnings("deprecation")
@Override
protected String doInBackground(Facebook... params) {
if (fb.isSessionValid()) {
button.setImageResource(R.drawable.com_facebook_loginbutton_blue);
// ^logout button
JSONObject obj=null;
try {
String jsonUser= fb.request("me");
obj = Util.parseJson(jsonUser);
String name = obj.optString("name");
welcome.setText("Welcome, " + name);
}catch(FacebookError e) {
e.printStackTrace();
}catch (JSONException e) {
e.printStackTrace();
}catch (MalformedURLException e) {
e.printStackTrace();
}catch (IOException e) {
e.printStackTrace();
}
}else {
post.setVisibility(Button.VISIBLE);
button.setImageResource(R.drawable.com_facebook_loginbutton_blue);
}
return null;
}
}
protected void onProgressUpdated(Integer...progress) {
}
@SuppressWarnings("deprecation")
protected void onPostExecute(Facebook result) {
// ^logout button
try {JSONObject obj=null;
String jsonUser= fb.request("me");
obj = Util.parseJson(jsonUser);
String name = obj.optString("name");
welcome.setText("Welcome, " + name);}
catch(FacebookError e) {
e.printStackTrace();
}catch (JSONException e) {
e.printStackTrace();
}catch (MalformedURLException e) {
e.printStackTrace();
}catch (IOException e) {
e.printStackTrace();
}
}
private void updateButtonImage() {
// TODO Auto-generated method stub
post.setVisibility(Button.VISIBLE);
button.setImageResource(R.drawable.com_facebook_loginbutton_blue);
//logout button
}
@SuppressWarnings("deprecation")
public void buttonClicks(View v) {
switch (v.getId()) {
case R.id.button1:
//post
Bundle params= new Bundle();
params.putString("name", "User X");
params.putString("caption", "Rating");
params.putString("description", "User X Rated");
params.putString("link", "http://...");
fb.dialog(MainActivity.this, "feed", params, new Facebook.DialogListener() {
@Override
public void onFacebookError(FacebookError e) {
// TODO Auto-generated method stub
}
@Override
public void onError(DialogError e) {
// TODO Auto-generated method stub
}
@Override
public void onComplete(Bundle values) {
// TODO Auto-generated method stub
}
@Override
public void onCancel() {
// TODO Auto-generated method stub
}
});
break;
}
}
@SuppressWarnings("deprecation")
public void onClick(View v) {
if(fb.isSessionValid()) {
try {
fb.logout(getApplicationContext());
updateButtonImage();
//button will close our our session
}catch(MalformedURLException e) {
e.printStackTrace();
} catch(IOException e) {
e.printStackTrace();
}
}else{
//login into facebook
fb.authorize(MainActivity.this, new String[] {"email"}, new Facebook.DialogListener() {
@Override
public void onFacebookError(FacebookError e) {
// TODO Auto-generated method stub
Toast.makeText(MainActivity.this, "fbError", Toast.LENGTH_SHORT).show();
}
@Override
public void onError(DialogError e) {
// TODO Auto-generated method stub
Toast.makeText(MainActivity.this, "onError", Toast.LENGTH_SHORT).show();
}
@Override
public void onComplete(Bundle values) {
// TODO Auto-generated method stub
Editor editor=sp.edit();
editor.putString("access_token", fb.getAccessToken());
editor.putLong("access_expires", fb.getAccessExpires());
editor.commit();
updateButtonImage();
}
@Override
public void onCancel() {
// TODO Auto-generated method stub
Toast.makeText(MainActivity.this, "onCancel", Toast.LENGTH_SHORT).show();
}
});
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
@SuppressWarnings("deprecation")
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
fb.authorizeCallback(requestCode, resultCode, data);
}
@Override
public void onComplete(Bundle values) {
// TODO Auto-generated method stub
}
@Override
public void onFacebookError(FacebookError e) {
// TODO Auto-generated method stub
}
@Override
public void onError(DialogError e) {
// TODO Auto-generated method stub
}
@Override
public void onCancel() {
// TODO Auto-generated method stub
}
}
这是 logcat 错误：
12-17 19:24:04.886: E/AndroidRuntime(862): FATAL EXCEPTION: AsyncTask #1
12-17 19:24:04.886: E/AndroidRuntime(862): java.lang.RuntimeException: An error occured while executing doInBackground()
12-17 19:24:04.886: E/AndroidRuntime(862): at android.os.AsyncTask$3.done(AsyncTask.java:299)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.FutureTask.run(FutureTask.java:239)
12-17 19:24:04.886: E/AndroidRuntime(862): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.lang.Thread.run(Thread.java:856)
12-17 19:24:04.886: E/AndroidRuntime(862): Caused by: java.lang.NullPointerException
12-17 19:24:04.886: E/AndroidRuntime(862): at com.sara.facebookappl.MainActivity$PostTask.doInBackground(MainActivity.java:97)
12-17 19:24:04.886: E/AndroidRuntime(862): at com.sara.facebookappl.MainActivity$PostTask.doInBackground(MainActivity.java:1)
12-17 19:24:04.886: E/AndroidRuntime(862): at android.os.AsyncTask$2.call(AsyncTask.java:287)
12-17 19:24:04.886: E/AndroidRuntime(862): at java.util.concurrent.FutureTask.run(FutureTask.java:234)
12-17 19:24:04.886: E/AndroidRuntime(862): ... 4 more
编辑：我改变了它，我仍然得到这些日志猫错误：
12-17 20:22:22.659: E/AndroidRuntime(794): FATAL EXCEPTION: main
12-17 20:22:22.659: E/AndroidRuntime(794): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sara.facebookappl/com.sara.facebookappl.MainActivity}: java.lang.RuntimeException: Only one Looper may be created per thread
12-17 20:22:22.659: E/AndroidRuntime(794): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.os.Handler.dispatchMessage(Handler.java:99)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.os.Looper.loop(Looper.java:137)
12-17 20:22:22.659: E/AndroidRuntime(794): at android.app.ActivityThread.main(ActivityThread.java:5039)
12-17 20:22:22.659: E/AndroidRuntime(794): at java.lang.reflect.Method.invokeNative(Native Method)
12-17 20:22:22.659: E/AndroidRuntime(794): at java.lang.reflect.Method.invoke(Method.java:511)
12-17 20:22:22.659: E/AndroidRuntime(794): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
12-17 20:22:22.659: E/AndroidRuntime(794): 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 12-17 20:22:22.659: E/AndroidRuntime(794) : at dalvik.system.NativeStart.main(Native Method) 12-17 20:22:22.659: E/AndroidRuntime(794): Caused by: java.lang.RuntimeException: only one Looper may be created per thread 12-17 20 :22:22.659: E/AndroidRuntime(794): 在 android.os.Looper.prepare(Looper.java:78) 12-17 20:22:22.659: E/AndroidRuntime(794): 在 android.os.Looper。准备（Looper.java:73）12-17 20:22:22.659：E/AndroidRuntime（794）：在 com.sara.facebookappl.MainActivity.onCreate（MainActivity.java:53）12-17 20:22:22.659： E/AndroidRuntime(794): 在 android.app.Activity.performCreate(Activity.java:5104) 12-17 20:22:22.659: E/AndroidRuntime(794): 在 android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 12-17 20:22:22.659: E/AndroidRuntime(794): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)