• 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;
            }
        }
    }
    

      

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  • 原文地址:https://www.cnblogs.com/zhengqun/p/4510394.html
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