android开发步步为营之88:基于LruCache和AsyncTask的网络相册开发

        哈哈，这个题目有点像当年读研时候的论文题目，基于XX技术的XX系统的开发。本文主要是想讲一下AsyncTask，AsyncTask在android 3.0之前的版本使用它的execute()方法，默认会开启一个只有cpu个数*2+1个线程的线程池，所以项目里面到处使用的话就会造成，某些页面加载数据的时候，需要等待线程池中有空闲的线程才能执行。android 3.0之后，google索性将使用execute()方法来执行异步任务的时候，直接改成一条条顺序执行了，这样已经没有了多线程的意义，看下源代码：

   public final AsyncTask<Params, Progress, Result> execute(Params... params) {        return executeOnExecutor(sDefaultExecutor, params);    }

   public static final Executor SERIAL_EXECUTOR = new SerialExecutor();    private static final int MESSAGE_POST_RESULT = 0x1;    private static final int MESSAGE_POST_PROGRESS = 0x2;    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

   private static class SerialExecutor implements Executor {        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();        Runnable mActive;        public synchronized void execute(final Runnable r) {            mTasks.offer(new Runnable() {                public void run() {                    try {                        r.run();                    } finally {                        scheduleNext();                    }                }            });            if (mActive == null) {                scheduleNext();            }        }        protected synchronized void scheduleNext() {            if ((mActive = mTasks.poll()) != null) {                THREAD_POOL_EXECUTOR.execute(mActive);            }        }    }

        可以看到默认的执行器是顺序执行任务的，所以我们推荐使用AsyncTask的executeOnExecutor(executor,params)方法，这个方法可以创建的自己的线程池，有3种线程池

   //单个任务建议用   Executor mSingeThreadExecutor = Executors.newSingleThreadExecutor();   //不设线程个数，建议还是别这样用，免得占用cpu过长时间   Executor mCacheExecutor = Executors.newCachedThreadPool();   //开启固定个数线程的线程池   Executor mFixedExecutor = Executors.newFixedThreadPool(6);

       另外本文使用了LruCache, support v4的包里包含了android.support.v4.util.LruCache，LRU==Least Recently Used 近期最少使用算法,根据分配给它的内存空间，内存不足的时候将新加入的数据替换近期最少使用的数据，这样一方面通过缓存技术，加快页面的加载速度，另外一方面，自动管理内存不用开发人员去操心。
        好的，给出全部的代码：

package com.figo.study.activity;import android.app.Activity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.AsyncTask;import android.os.Bundle;import android.support.v4.util.LruCache;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.BaseAdapter;import android.widget.GridView;import android.widget.ImageView;import com.figo.study.R;import java.io.InputStream;import java.net.HttpURLConnection;import java.net.URL;import java.util.ArrayList;import java.util.concurrent.Executor;import java.util.concurrent.Executors;public class AsyncTaskActivity extends Activity {    //单个任务建议用//    Executor mSingeThreadExecutor = Executors.newSingleThreadExecutor();    //不设线程个数，建议还是别这样用，免得占用cpu过长时间//    Executor mCacheExecutor = Executors.newCachedThreadPool();    //开启固定个数线程的线程池    Executor mFixedExecutor = Executors.newFixedThreadPool(6);    //内存自动回收    private LruCache<String, Bitmap> mLruCache;    private GridView mGridView;    private ArrayList<String> alImgUrls = new ArrayList<String>();    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_async_task);        initView();        for (int a = 0; a < 80; a++) {            alImgUrls.add("http://www.onegreen.org/desk/Upload_desk/200902/20090205181316946.jpg");        }    }    private void initView() {        mGridView = (GridView) findViewById(R.id.gv_album);        int maxMemory = (int) Runtime.getRuntime().maxMemory();        int cacheSize = maxMemory / 10;        mLruCache = new LruCache<String, Bitmap>(cacheSize) {            @Override            protected int sizeOf(String key, Bitmap value) {                return value.getByteCount();            }        };        mGridView.setAdapter(new PictureAdapter());    }    class PictureAdapter extends BaseAdapter {        @Override        public Object getItem(int position) {            return null;        }        @Override        public int getCount() {            return alImgUrls.size();        }        @Override        public long getItemId(int position) {            return position;        }        @Override        public View getView(int position, View convertView, ViewGroup parent) {            ViewHolder holder = null;            if (convertView == null) {                holder = new ViewHolder();                //从xml页面实例化视图                convertView = LayoutInflater.from(AsyncTaskActivity.this).inflate(R.layout.layout_item_image, null);                //填充我们定义的容器里面的控件                holder.img = (ImageView) convertView.findViewById(R.id.img_show);                convertView.setTag(holder);            } else {                //直接从视图里面获取控件容器                holder = (ViewHolder) convertView.getTag();            }            //不再推荐这么写了，因为execute使用的是默认线程池，android 3.0之后，execute是按顺序一个个执行的            new DownloadPicTask(holder.img).execute(alImgUrls.get(position));            new DownloadPicTask(holder.img).executeOnExecutor(mFixedExecutor, alImgUrls.get(position));            return convertView;        }    }    // 视图容器    public final class ViewHolder {        public ImageView img;    }    //下载图片异步任务    class DownloadPicTask extends AsyncTask<String, Integer, Bitmap> {        ImageView mImg;        public  DownloadPicTask(ImageView imgView)        {            mImg=imgView;        }        @Override        protected void onPreExecute() {            super.onPreExecute();        }        @Override        protected void onPostExecute(Bitmap bitmap) {            if(mImg!=null)            {                mImg.setImageBitmap(bitmap);            }        }        @Override        protected void onProgressUpdate(Integer... values) {            super.onProgressUpdate(values);        }        @Override        protected Bitmap doInBackground(String... params) {            try {                return getBitmap(params[0].toString());            } catch (Exception e) {                return null;            }        }    }    //获取网络图片    public Bitmap getBitmap(String path) throws Exception {        if(mLruCache.get(path)!=null)        {            return mLruCache.get(path);        }else {            URL url = new URL(path);            HttpURLConnection conn = (HttpURLConnection) url.openConnection();            conn.setConnectTimeout(5 * 1000);            conn.setRequestMethod("GET");            InputStream inStream = conn.getInputStream();            if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {                Bitmap bitmap = BitmapFactory.decodeStream(inStream);                mLruCache.put(path,bitmap);                return  bitmap;            }        }        return null;    }}

         你会发现运行速度是杠杠的！运行效果如下：