Android AsyncTask 详解及注意事项

AsyncTask是Android提供的轻量级的异步类，它使创建异步任务变得更加简单，不再需要编写任务线程和Handler实例即可完成相同的任务。

AsyncTask定义了三种泛型类型 Params，Progress和Result。

• Params 启动任务执行的输入参数，比如HTTP请求的URL。
• Progress 后台任务执行的百分比。
• Result 后台执行任务最终返回的结果，比如String。

例如：

class RetrieveCategoryTask extends AsyncTask<String, Void, List<String>>

根据AsyncTask源码：

public abstract class AsyncTask<Params, Progress, Result> 

这里的String, Void, List<String>分别对应Params, Progress, Result

一般使用AsyncTask至少需要实现以下2个方法：

protected abstract Result doInBackground(Params... var1);//耗时操作，例如网络请求任务。这里相当于一个子线程

   protected void onPostExecute(Result result) {//可以在这里处理doInBackground得到的数据，能够对UI进行操作，属于UI主线程
        throw new RuntimeException("Stub!");
    }

当然如果有必要的话还可以实现下面几个方法：

• onProgressUpdate(Progress…)   可以使用进度条增加用户体验度。 此方法在主线程执行，用于显示任务执行的进度。
• onPreExecute()        这里是最终用户调用Excute时的接口，当任务执行之前开始调用此方法，可以在这里显示进度对话框。
• onCancelled()             用户调用取消时，会调用此方法

使用AsyncTask类，以下是几条必须遵守的准则：

• Task的实例必须在UI thread中创建；
• execute方法必须在UI thread中调用；
• 不要手动的调用onPreExecute(), onPostExecute(Result)，doInBackground(Params...), onProgressUpdate(Progress...)这几个方法；
• 该task只能被执行一次，否则多次调用时将会出现异常；

需要注意的是Android为了安全考虑，不允许在主线程即UI线程进行耗时操作。例如HTTP请求等。

如果在UI中使用了耗时操作的话，Android Studio本身是不会报错的。只有在APP执行到相应的耗时操作位置时才会停止运行。手机或模拟器上会出现“很抱歉，XXX已停止运行”同时Android Studio logcat输出“

E/AndroidRuntime: FATAL EXCEPTION: main
Process：.....

 java.lang.RuntimeException.....

”

下面给出一个范例：

package idv.ron.texttojson_android;

import android.app.ProgressDialog;
import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;

import com.google.gson.Gson;
import com.google.gson.JsonObject;
import com.google.gson.reflect.TypeToken;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.lang.reflect.Type;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.List;

public class SearchActivity extends ActionBarActivity {
    private final static String TAG = "SearchActivity";
    private ProgressDialog progressDialog;
    private AsyncTask retrieveCategoryTask, retrieveBookTask;
    private Spinner spCategory;
    private ListView lvBook;

    class RetrieveCategoryTask extends AsyncTask<String, Void, List<String>> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressDialog = new ProgressDialog(SearchActivity.this);
            progressDialog.setMessage("Loading...");
            progressDialog.show();
        }

        @Override
        protected List<String> doInBackground(String... params) {
            String url = params[0];
            String jsonIn;
            JsonObject jsonObject = new JsonObject();
            jsonObject.addProperty("param", "category");
            try {
                jsonIn = getRemoteData(url, jsonObject.toString());
            } catch (IOException e) {
                Log.e(TAG, e.toString());
                return null;
            }

            Gson gson = new Gson();
            Type listType = new TypeToken<List<String>>() {
            }.getType();

            return gson.fromJson(jsonIn, listType);
        }

        @Override
        protected void onPostExecute(List<String> items) {
            ArrayAdapter<String> adapter = new ArrayAdapter<>(SearchActivity.this,
                    android.R.layout.simple_list_item_1, items);
            adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
            spCategory.setAdapter(adapter);
            progressDialog.cancel();
        }
    }

    public class RetrieveBookTask extends
            AsyncTask<String, Integer, List<Book>> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressDialog = new ProgressDialog(SearchActivity.this);
            progressDialog.setMessage("Loading...");
            progressDialog.show();
        }

        @Override
        protected List<Book> doInBackground(String... params) {
            String url = params[0];
            String category = params[1];
            String jsonIn;
            JsonObject jsonObject = new JsonObject();
            jsonObject.addProperty("param", category);
            try {
                jsonIn = getRemoteData(url, jsonObject.toString());
            } catch (IOException e) {
                Log.e(TAG, e.toString());
                return null;
            }

            Gson gson = new Gson();
            Type listType = new TypeToken<List<Book>>() {
            }.getType();
            return gson.fromJson(jsonIn, listType);
        }

        @Override
        protected void onPostExecute(List<Book> result) {
            showResult(result);
            progressDialog.cancel();
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.search_activity);
        spCategory = (Spinner) findViewById(R.id.spCategory);
        lvBook = (ListView) findViewById(R.id.lvBook);
        if (networkConnected()) {
            retrieveCategoryTask = new RetrieveCategoryTask().execute(Common.URL);
        } else {
            showToast(this, R.string.msg_NoNetwork);
        }
    }

    // check if the device connect to the network
    private boolean networkConnected() {
        ConnectivityManager conManager =
                (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = conManager.getActiveNetworkInfo();
        return networkInfo != null && networkInfo.isConnected();
    }


    private String getRemoteData(String url, String jsonOut) throws IOException {
        StringBuilder jsonIn = new StringBuilder();
        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        connection.setDoInput(true); // allow inputs
        connection.setDoOutput(true); // allow outputs
        connection.setUseCaches(false); // do not use a cached copy
        connection.setRequestMethod("POST");
        connection.setRequestProperty("charset", "UTF-8");
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(connection.getOutputStream()));
        bw.write(jsonOut);
        Log.d(TAG, "jsonOut: " + jsonOut);
        bw.close();

        int responseCode = connection.getResponseCode();

        if (responseCode == 200) {
            BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line;
            while ((line = br.readLine()) != null) {
                jsonIn.append(line);
            }
        } else {
            Log.d(TAG, "response code: " + responseCode);
        }
        connection.disconnect();
        Log.d(TAG, "jsonIn: " + jsonIn);
        return jsonIn.toString();
    }

    public void onSearchClick(View v) {
        Object item = spCategory.getSelectedItem();
        if (item == null || item.toString().trim().length() <= 0) {
            showToast(this, R.string.msg_NoCategoryFound);
        } else {
            String category = item.toString().trim();
            if (networkConnected()) {
                retrieveBookTask = new RetrieveBookTask().execute(Common.URL, category);
            } else {
                showToast(this, R.string.msg_NoNetwork);
            }
        }
    }

    public void showResult(List<Book> result) {
        final BookListAdapter adapter = new BookListAdapter(this, result);
        lvBook.setAdapter(adapter);
        lvBook.setOnItemClickListener(new OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                                    int position, long id) {
                adapter.expand(position);
                lvBook.setItemChecked(position, true);
            }
        });
    }

    private class BookListAdapter extends BaseAdapter {
        private LayoutInflater layoutInflater;
        private List<Book> bookList;
        private boolean[] bookDetailExpanded;

        public BookListAdapter(Context context, List<Book> bookList) {
            this.layoutInflater = LayoutInflater.from(context);
            this.bookList = bookList;
            this.bookDetailExpanded = new boolean[bookList.size()];
        }

        @Override
        public int getCount() {
            return bookList.size();
        }

        @Override
        public Object getItem(int position) {
            return bookList.get(position);
        }

        @Override
        public long getItemId(int position) {
            return bookList.get(position).getId();
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            if (convertView == null) {
                convertView = layoutInflater.inflate(
                        R.layout.book_listview_item, parent, false);
            }
            TextView tvBookTitle = (TextView) convertView
                    .findViewById(R.id.tvBookTitle);
            TextView tvBookDetail = (TextView) convertView
                    .findViewById(R.id.tvBookDetail);
            Book book = bookList.get(position);

            tvBookTitle.setText(book.getName() + "  $" + book.getPrice());
            tvBookDetail.setText("Author: " + book.getAuthor() + "  Type: "
                    + book.getType());
            tvBookDetail
                    .setVisibility(bookDetailExpanded[position] ? View.VISIBLE
                            : View.GONE);
            return convertView;
        }

        public void expand(int position) {
            // 被點擊的資料列才會彈出內容，其他資料列的內容會自動縮起來
            // for (int i=0; i<newsExpanded.length; i++) {
            // newsExpanded[i] = false;
            // }
            // newsExpanded[position] = true;

            bookDetailExpanded[position] = !bookDetailExpanded[position];
            notifyDataSetChanged();
        }

    }

    @Override
    protected void onPause() {
        if (retrieveCategoryTask != null) {
            retrieveCategoryTask.cancel(true);
            retrieveCategoryTask = null;
        }

        if (retrieveBookTask != null) {
            retrieveBookTask.cancel(true);
            retrieveBookTask = null;
        }

        super.onPause();
    }

    private void showToast(Context context, int messageId) {
        Toast.makeText(context, messageId, Toast.LENGTH_SHORT).show();
    }
}