<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:orientation="horizontal" >

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="城市："
            android:textSize="23sp" />

        <EditText 
            android:id="@+id/city"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:inputType="text" />
    </LinearLayout>

    <Button
        android:id="@+id/query"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:text="查询" 
        android:textSize="23sp" />
    
    <TextView
        android:id="@+id/result"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

package com.rainsong.asynctaskdemo;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {
    EditText et_city;
    Button btn_query;
    TextView tv_result;
    QueryTask task;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        et_city = (EditText)findViewById(R.id.city);
        tv_result = (TextView)findViewById(R.id.result);
        btn_query = (Button)findViewById(R.id.query);

        btn_query.setOnClickListener(new OnClickListener() {
            public void onClick(View view) {
                String city = et_city.getText().toString();
                if (city.length() < 1) {
                    Toast.makeText(MainActivity.this, "请输入城市名",
                            Toast.LENGTH_LONG).show();
                    return;
                }
                task = new QueryTask(MainActivity.this, tv_result);
                task.execute(city);
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

package com.rainsong.asynctaskdemo;

import java.io.IOException;
import java.net.URLEncoder;
import java.util.ArrayList;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

import android.content.Context;
import android.os.AsyncTask;
import android.widget.TextView;
import android.widget.Toast;

public class QueryTask extends AsyncTask<String, Void, String> {
    Context context;
    TextView tv_result;

    private static final String JUHE_URL_ENVIRONMENT_AIR_PM = 
                                    "http://web.juhe.cn:8080/environment/air/pm";
    private static final String JUHE_APPKEY = "你申请的APPKEY值";

    public QueryTask(Context context, TextView tv_result) {
        // TODO Auto-generated constructor stub
        super();
        this.context = context;
        this.tv_result = tv_result; 
    }

    @Override
    protected String doInBackground(String... params) {
        String city = params[0];

        ArrayList<NameValuePair> headerList = new ArrayList<NameValuePair>();
        headerList.add(new BasicNameValuePair("Content-Type", "text/html; charset=utf-8"));

        String targetUrl = JUHE_URL_ENVIRONMENT_AIR_PM;

        ArrayList<NameValuePair> paramList = new ArrayList<NameValuePair>();
        paramList.add(new BasicNameValuePair("key", JUHE_APPKEY));
        paramList.add(new BasicNameValuePair("dtype", "json"));
        paramList.add(new BasicNameValuePair("city", city));

        for (int i = 0; i < paramList.size(); i++) {
            NameValuePair nowPair = paramList.get(i);
            String value = http://www.mamicode.com/nowPair.getValue();>

API知识点
public abstract class
AsyncTask
extends Object

android.os.AsyncTask<Params, Progress, Result>

Class Overview
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.

Public Constructors
AsyncTask()
Creates a new asynchronous task.

static void execute(Runnable runnable)
Convenience version of execute(Object) for use with a simple Runnable object.

abstract Result doInBackground(Params... params)
Override this method to perform a computation on a background thread.

void onCancelled(Result result)
Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.

void onCancelled()
Applications should preferably override onCancelled(Object).

void onPostExecute(Result result)
Runs on the UI thread after doInBackground(Params...).

void onPreExecute()
Runs on the UI thread before doInBackground(Params...).

void onProgressUpdate(Progress... values)
Runs on the UI thread after publishProgress(Progress...) is invoked.

final void publishProgress(Progress... values)
This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running.

Android AsyncTask异步任务