利用LruCache为GridView加载大量本地图片完整示例

MainActivity如下:

package cc.testlrucache;

import android.os.Bundle;
import android.widget.GridView;
import android.app.Activity;

public class MainActivity extends Activity {
    private GridView mGridView;
    private GridViewAdapter mGridViewAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        init();
    }

    private void init(){
        mGridView = (GridView) findViewById(R.id.gridView);
        mGridViewAdapter = new GridViewAdapter(this, 0, ImagesPath.IMAGES_PATH, mGridView);
        mGridView.setAdapter(mGridViewAdapter);
    }

}

GridViewAdapter如下:

package cc.testlrucache;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.support.v4.util.LruCache;
import android.view.LayoutInflater;
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 android.widget.AbsListView.OnScrollListener;
/**
 *   LruCache的流程分析:
 *
 *   从第一次进入应用的情况下开始
 * 1 依据图片的Path从LruCache缓存中取图片.
 *   若图片存在缓存中,则显示该图片;否则显示默认图片
 * 2 因为是第一次进入该界面所以会执行:
 *   loadBitmaps(firstVisibleItem, visibleItemCount);
 *
 *   从loadBitmaps()方法作为切入点,继续往下梳理
 *
 * 3 尝试从LruCache缓存中取图片.如果在显示即可,否则进入4
 * 4 从SDCrad读取图片,并且将读取后的图片保存到LruCache缓存中
 *
 * 5 在停止滑动时,会调用loadBitmaps(firstVisibleItem, visibleItemCount)
 *   显示目前GridView可见Item的图片
 */
public class GridViewAdapter extends ArrayAdapter<String> {
    private GridView mGridView;
    //图片缓存类
    private LruCache<String, Bitmap> mLruCache;
    //GridView中可见的第一张图片的下标
    private int mFirstVisibleItem;
    //GridView中可见的图片的数量
    private int mVisibleItemCount;
    //记录是否是第一次进入该界面
    private boolean isFirstEnterThisActivity = true;

    public GridViewAdapter(Context context, int textViewResourceId,String[] objects, GridView gridView) {
        super(context, textViewResourceId, objects);
        mGridView = gridView;
        mGridView.setOnScrollListener(new ScrollListenerImpl());
        //应用程序最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        //设置图片缓存大小为maxMemory的1/3
        int cacheSize = maxMemory/3;

        mLruCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getRowBytes() * bitmap.getHeight();
            }
        };

    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        String path = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(R.layout.gridview_item, null);
        } else {
            view = convertView;
        }
        ImageView imageView = (ImageView) view.findViewById(R.id.imageView);
        //为该ImageView设置一个Tag,防止图片错位
        imageView.setTag(path);
        //为该ImageView设置显示的图片
        setImageForImageView(path, imageView);
        return view;
    }

    /**
     * 为ImageView设置图片(Image)
     * 1 从缓存中获取图片
     * 2 若图片不在缓存中则为其设置默认图片
     */
    private void setImageForImageView(String imagePath, ImageView imageView) {
        Bitmap bitmap = getBitmapFromLruCache(imagePath);
        if (bitmap != null) {
            imageView.setImageBitmap(bitmap);
        } else {
            imageView.setImageResource(R.drawable.default_image);
        }
    }

    /**
     * 将图片存储到LruCache
     */
    public void addBitmapToLruCache(String key, Bitmap bitmap) {
        if (getBitmapFromLruCache(key) == null) {
            mLruCache.put(key, bitmap);
        }
    }

    /**
     * 从LruCache缓存获取图片
     */
    public Bitmap getBitmapFromLruCache(String key) {
        return mLruCache.get(key);
    }

   /**
    * 为GridView的item加载图片
    * @param firstVisibleItem GridView中可见的第一张图片的下标
    * @param visibleItemCount GridView中可见的图片的数量
    */
    private void loadBitmaps(int firstVisibleItem, int visibleItemCount) {
        try {
            for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {
                String imagePath = ImagesPath.IMAGES_PATH[i];
                Bitmap bitmap = getBitmapFromLruCache(imagePath);
                if (bitmap == null) {
                    System.out.println("--->XXXXX 图片"+imagePath+"不在缓存中"+",所以从SDCard读取");
                    bitmap=BitmapFactory.decodeFile(imagePath);
                    ImageView imageView = (ImageView) mGridView.findViewWithTag(imagePath);
                    if (imageView != null && bitmap != null) {
                        imageView.setImageBitmap(bitmap);
                    }
                    //将从SDCard读取的图片添加到LruCache中
                    addBitmapToLruCache(imagePath, bitmap);
                } else {
                    System.out.println("--->OOOOO 图片在缓存中="+imagePath+",从缓存中取出即可");
                    //依据Tag找到对应的ImageView显示图片
                    ImageView imageView = (ImageView) mGridView.findViewWithTag(imagePath);
                    if (imageView != null && bitmap != null) {
                        imageView.setImageBitmap(bitmap);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private class ScrollListenerImpl implements OnScrollListener{
        /**
         * 通过onScrollStateChanged获知:每次GridView停止滑动时加载图片
         * 但是存在一个特殊情况:
         * 当第一次入应用的时候,此时并没有滑动屏幕的操作即不会调用onScrollStateChanged,但应该加载图片.
         * 所以在此处做一个特殊的处理.
         * 即代码:
         * if (isFirstEnterThisActivity && visibleItemCount > 0) {
         *      loadBitmaps(firstVisibleItem, visibleItemCount);
         *      isFirstEnterThisActivity = false;
         *    }
         *
         * ------------------------------------------------------------
         *
         * 其余的都是正常情况.
         * 所以我们需要不断保存:firstVisibleItem和visibleItemCount
         * 从而便于中在onScrollStateChanged()判断当停止滑动时加载图片
         *
         */
        @Override
        public void onScroll(AbsListView view, int firstVisibleItem,int visibleItemCount, int totalItemCount) {
            mFirstVisibleItem = firstVisibleItem;
            mVisibleItemCount = visibleItemCount;
            if (isFirstEnterThisActivity && visibleItemCount > 0) {
                System.out.println("---> 第一次进入该界面");
                loadBitmaps(firstVisibleItem, visibleItemCount);
                isFirstEnterThisActivity = false;
            }
        }

        /**
         *  GridView停止滑动时下载图片
         *  其余情况下取消所有正在下载或者等待下载的任务
         */
        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {
            if (scrollState == SCROLL_STATE_IDLE) {
                System.out.println("---> GridView停止滑动  mFirstVisibleItem="+mFirstVisibleItem+",mVisibleItemCount="+mVisibleItemCount);
                loadBitmaps(mFirstVisibleItem, mVisibleItemCount);
            }
        }
    }
}

ImagesPath如下:

package cc.testlrucache;

public class ImagesPath {

    public final static String DIR="/mnt/sdcard/Test/";
    public final static String[] IMAGES_PATH = new String[] {
        DIR+"a.jpg",
        DIR+"b.jpg",
        DIR+"c.jpg",
        DIR+"d.jpg",
        DIR+"e.jpg",
        DIR+"f.jpg",
        DIR+"g.jpg",
        DIR+"h.jpg",
        DIR+"i.jpg",
        DIR+"j.jpg",
        DIR+"k.jpg",
        DIR+"l.jpg",
        DIR+"m.jpg",
        DIR+"n.jpg",
        DIR+"o.jpg",
        DIR+"p.jpg",
        DIR+"q.jpg",
        DIR+"r.jpg",
        DIR+"s.jpg",
        DIR+"t.jpg",
        DIR+"w.jpg",
        DIR+"x.jpg",
        DIR+"y.jpg",
        DIR+"z.jpg",
        DIR+"za.jpg",
        DIR+"zb.jpg",
        DIR+"zc.jpg",
        DIR+"zd.jpg",
        DIR+"ze.jpg",
        DIR+"zf.jpg",
        DIR+"zg.jpg",
        DIR+"zh.jpg",
        DIR+"zi.jpg",
        DIR+"zj.jpg",
        DIR+"zk.jpg",
        DIR+"zl.jpg",
        DIR+"zm.jpg"
};
}

main.xml如下:

<RelativeLayout 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" >

   <GridView
       android:id="@+id/gridView"
       android:layout_width="fill_parent"
       android:layout_height="fill_parent"
       android:verticalSpacing="10dip"
       android:listSelector="@android:color/transparent"
       android:numColumns="3"
    />

</RelativeLayout>

gridview_item.xml如下:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" >

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="90dip"
        android:layout_height="90dip"
        android:src="@drawable/default_image"
        android:layout_centerInParent="true"
        />

</RelativeLayout>