android照片墙的实现

转载自 http://blog.csdn.net/guolin_blog/article/details/9526203

由于每个android应用程序都有内存限制，所以如果加载很多图片的话，一定会出现OOM异常

这就需要使用LruCache内存缓存技术了，LruCache是在support-v4 jar包中，所以想运用此技术需要把jar包加进去。

LruCache的技术原理就是吧最近使用的对象使用强引用保存在LinkedHashMap中，对那些很少使用的在缓存值达到预定值之前从内存中移除

以前经常使用的软引用 在2.3之后已经不再推荐使用了，因为android的垃圾回收机制更倾向于软引用的回收，使得这种技术不再变得可考

package com.qunzheng.photowalldemo;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.ImageView;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by QunZheng on 2015/5/17.
 */
public class PhotoWallAdapter extends ArrayAdapter implements AbsListView.OnScrollListener {
    private static final String TAG = "PhotoWallAdapter";
    private GridView mPhotoWall;
    //定义内存缓存
    private MyLruCache memoryCache;
    private List<DownloadBitmapTask> tasks;

    private int firstVisibleItem;
    private int visibleItemCount;

    private boolean firstLoad = true;

    public PhotoWallAdapter(Context context, int resource, String[] objects, GridView mPhotoWall) {
        super(context, resource, objects);

        //获取应用程序的最大可用内存，并且分配1/8的内存给memoryCache
        long maxMemory = Runtime.getRuntime().maxMemory();
        memoryCache = new MyLruCache((int)(maxMemory / 8));

        tasks = new ArrayList<>();
        this.mPhotoWall = mPhotoWall;
        //设置照片的墙的滚动监听事件
        this.mPhotoWall.setOnScrollListener(this);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        String imageUrl = (String) getItem(position);
        View view = null;
        if (convertView == null) {
            view = View.inflate(getContext(), R.layout.griditem_photo_wall, null);
        } else {
            view = convertView;
        }
        ImageView imageView = (ImageView) view.findViewById(R.id.photo);
        imageView.setTag(imageUrl);
        setImageView(imageView,imageUrl);

        return view;
    }

    private void setImageView(ImageView imageView, String imageUrl) {
        Bitmap bitmap = memoryCache.getBitmapFromMemoryCache(imageUrl);
        if (bitmap == null) {
            imageView.setImageResource(R.drawable.empty_photo);
        }else{
            imageView.setImageBitmap(bitmap);
        }
    }

    private void loadBitmap(int firstVisibleItem, int visibleItemCount) {
        for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {
            String imageUrl = Images.imageThumbUrls[i];

            ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
            Bitmap bitmap = memoryCache.getBitmapFromMemoryCache(imageUrl);
            if (bitmap == null) {
                //开启后台下载任务
                DownloadBitmapTask task = new DownloadBitmapTask();
                tasks.add(task);
                task.execute(imageUrl);

            }else {
                imageView.setImageBitmap(bitmap);
            }
        }
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        Log.i(TAG, "滚动条的状态发生改变:" + scrollState);
        if (scrollState == SCROLL_STATE_IDLE) {
            //加载图片
            loadBitmap(firstVisibleItem, visibleItemCount);
        } else {
            cancleTasks();
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        this.firstVisibleItem = firstVisibleItem;
        this.visibleItemCount = visibleItemCount;

        if (firstLoad && visibleItemCount > 0) {
            loadBitmap(firstVisibleItem,visibleItemCount);
            firstLoad = false;
        }

        Log.i(TAG,"firstVisibleItem:" + firstVisibleItem);
        Log.i(TAG,"visibleItemCount:" + visibleItemCount);
    }

    /**
     *
     */
    public void cancleTasks() {
        if (tasks != null) {
            for (DownloadBitmapTask task : tasks) {
                task.cancel(false);
            }
        }
    }

    /**
     * 下载图片的任务
     */
    private class DownloadBitmapTask extends AsyncTask<String,Void,Bitmap>{
        String imageUrl;
        @Override
        protected Bitmap doInBackground(String... params) {
        imageUrl = params[0];
        Bitmap bitmap = downloadBitmap(imageUrl);
        Log.i(TAG, bitmap.toString());
        if(bitmap != null){
            //吧下载的bitmap添加到内存缓存中
            memoryCache.addBitmapToMemoryCache(imageUrl,bitmap);
        }
        return bitmap;
    }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
            if (imageView != null && bitmap != null) {
                imageView.setImageBitmap(bitmap);
            }

            tasks.remove(this);
        }

        private Bitmap downloadBitmap(String imageUrl) {
            HttpURLConnection conn = null;
            Bitmap bitmap = null;
            try {
                URL url = new URL(imageUrl);
                conn = (HttpURLConnection) url.openConnection();
                conn.setConnectTimeout(5 * 1000);
                conn.setReadTimeout(5 * 1000);

//                conn.connect();
                InputStream in = conn.getInputStream();
                bitmap = BitmapFactory.decodeStream(in);
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                if (conn != null) {
                    conn.disconnect();
                }
            }
            return bitmap;
        }
    }
}