LruCache类的相关知识以及代码实现

一.LruCache的简单介绍

Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
 * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
 * 如果你cache的某个值需要明确释放，重写entryRemoved()
 * 如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回。
 * is limited to 4MiB of bitmaps: 默认cache大小是测量的item的数量，重写sizeof计算不同item的
 *  大小。

二.使用Lrucache来异步加载图片，防止阻塞（缓存图片）

下面是我的一个小程序，使用得到了Lrucache，总共有三个类，代码注释也是很详细的，很适合初学者学习。主要的功能就是形成一个像瀑布的图片墙。

在第三个类中有这个项目的设计思想。大家可以看看，共同进步，谢谢！！！

第一个类

package com.pangzaifei.falls;

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

/**
 * 瀑布流
 * 
 */
public class Demo extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_demo);

    }
}

第二个类

package com.pangzaifei.falls;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapFactory.Options;
import android.support.v4.util.LruCache;

/**
 * 图片资源 数据源类
 * 
 */
public class Images {
    private static Images images;
    // Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
    // 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
    // 如果你cache的某个值需要明确释放，重写entryRemoved()
    // 如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回。
    // 默认cache大小是测量的item的数量，重写sizeof计算不同item的大小。
    private LruCache<String, Bitmap> mMemoryCache;

    public Images() {
        initLrucache();// 初始化lrucache
    }

    public static Images getInstance() {
        if (images == null) {
            images = new Images();
        }
        return images;
    }

    /**
     * 初始化lrucache
     */
    private void initLrucache() {
        // 获取应用程序最大内存
        long maxSize = Runtime.getRuntime().maxMemory();
        // 设置图片缓存大小为程序最大可用内存的1/8.
        int cacheSize = (int) (maxSize / 8);
        mMemoryCache = new LruCache<String, Bitmap>(cacheSize);
    }

    /**
     * 从LruCache中获取一张图片，如果不存在就返回null 从lrucache中获得key key LruCache的键，传入图片的url地址
     * return 对应传入的bitmap对象，或者null
     */
    public Bitmap getMemoryCache(String key) {
        if (mMemoryCache != null) {
            Bitmap bitmap = mMemoryCache.get(key);
            if (bitmap != null) {
                return bitmap;
            }
        }
        return null;
    }

    /**
     * 将图片添加到lrucache中
     * 
     * @param key
     *            LruCache的键，这里传入图片的URL地址
     * @param bitmap
     *            这里指的是从网络上下载的bitmap对象。
     */
    public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
        if (getMemoryCache(key) == null) {
            mMemoryCache.put(key, bitmap);
        }
    }

    public static final String[] imageThumbs = new String[] {
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    /**
     * 将大图缩放
     * 
     * @param path
     * @param mClolumnWidth
     */
    public Bitmap decodeSimpleBitMapFromResource(String path, int mClolumnWidth) {
        //第一次解析将inJustDecodeBounds设置为true，来获取图片的大小
        final Options options = new Options();
        options.inJustDecodeBounds = true;
        Bitmap bitmap = BitmapFactory.decodeFile(path, options);
        //调用方法计算insampleSize的值
        options.inSampleSize = decodeSimpleSize(options, mClolumnWidth);
        //使用获取到的inSampleSize的值
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFile(path, options);

    }

    /**
     * 获得simpleSize
     * 
     * @param options
     * @return
     */
    private int decodeSimpleSize(Options options, int reqwidth) {
        //源图片的宽度
        int width = options.outWidth;
        int simplesize = 1;
        if (width > reqwidth) {
            //计算出实际宽度和目标宽度的比率
            simplesize = Math.round(width / reqwidth);
        }
        return simplesize;
    }
}

第三个类

package com.pangzaifei.falls;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;
import android.widget.ImageView.ScaleType;
import android.widget.LinearLayout;
import android.widget.ScrollView;
import android.widget.Toast;

/**
 * 瀑布流类          <主要用到了LruCache类，缓存图片 >
 * 
 * 原理: 1：创建3个linearlayout，设置他们的宽度，将获得的图片压缩成和3个linearlayout一样的宽度，
 * 然后根据3个linearlayout的高度来判断，将bitmap添加到哪一个linearlayout中
 * 2：翻页处理，根据手势抬起的位置和滑动的末尾处来进行翻页
 * 
 */
/*
ScrollView原理
      视图的滚动过程，其实是在不断修改原点坐标。当手指触摸后，ScrollView会暂时拦截触摸事件，
      使用一个计时器。假如在计时器到点后没有发生手指移动事件，那么ScrollView发送tracking events到被点击的subView；
      若是在计时器到点后发生了移动事件，那么ScrollView取消tracking自己促发滚动。
*/
/*首先还是讲一下实现原理，瀑布流的布局方式虽然看起来好像排列的很随意，其实它是有很科学的排列规则的。
 * 整个界面会根据屏幕的宽度划分成等宽的若干列，由于手机的屏幕不是很大，这里我们就分成三列。每当需要添加一张图片时，
 * 会将这张图片的宽度压缩成和列一样宽，再按照同样的压缩比例对图片的高度进行压缩，然后在这三列中找出当前高度最小的一列，
 * 将图片添加到这一列中。之后每当需要添加一张新图片时，都去重复上面的操作，就会形成瀑布流格局的照片墙
*/
public class PhotoFallScrollView extends ScrollView implements OnTouchListener {
    /**
     * 记录当前已加载到第几页
     */
    private static int page;
    /**
     * 每页显示多少张图片的数量
     */
    private static final int PAGE_SIZE = 8;
    private Context mContext;
    /**
     * 数据源图片
     */
    private Images mImagesThoumb;
    /**
     * task请求集合
     */
    private Set<DownLoadTask> mTasks;
    //判断是否是第一次进入
    boolean isFirstEntr = true;
    
    /**三列布局
     * 
     */
    private LinearLayout mFirstColumn;
    private LinearLayout mSecondColumn;
    private LinearLayout mThirdColumn;
    //当前第一列，第二列，第三列的高度
    private int mFirstColumnHeight;
    private int mSecondColumnHeight;
    private int mThirdColumnHeight;
    /** 
     * 每一列的宽度 
     */ 
    private int mClolumnWidth;

    private long mDelay = 5;
    /**
     * 上次滑动的最后位置
     */
    private static int lastScrollY = -1;

    /**
     * 是否已加载过一次layout，这里onLayout中的初始化只需加载一次
     */
    private boolean loadOnce;
    /**
     * 存放图片的集合
     */
    private List<ImageView> mImageViewList = new ArrayList<ImageView>();

    public PhotoFallScrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        this.mContext = context;
        init();
    }

    public PhotoFallScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.mContext = context;
        init();
    }

    public PhotoFallScrollView(Context context) {
        super(context);
        this.mContext = context;
        init();
    }

    /**
     * 初始化
     */
    private void init() {
        mImagesThoumb = Images.getInstance();
        mTasks = new HashSet<DownLoadTask>();
        setOnTouchListener(this);
    }

    /**
     * 进行一些关键性的初始化操作，获取MyScrollView的高度以及得到第一列的宽度值。
     * 并在这里开始加载第一页的图片。 
     *
     */
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);

        // 第一次进入就加载第一页的图片
        if (changed && !loadOnce) {
            mScrollViewHeight = this.getHeight();
            mScrollLayout = this.getChildAt(0);//从id的值为0处开始获取
            mFirstColumn = (LinearLayout) findViewById(R.id.first_column);
            mSecondColumn = (LinearLayout) findViewById(R.id.second_column);
            mThirdColumn = (LinearLayout) findViewById(R.id.third_column);
            //获取每一列的宽度
            mClolumnWidth = mFirstColumn.getWidth();
            loadOnce = true;
            //开始加载下一页的图片
            loadMoreImages();
        }
    }

    /**开始加载下一页的图片，每张图片都会开启一个异步线程去下载。
     * 加载图片
     */
    private void loadMoreImages() {
        if (hashSdcard()) {

            // 根据页数加载图片
            int startIndex = page * PAGE_SIZE;
            int endIndex = page * PAGE_SIZE + PAGE_SIZE;

            if (startIndex < mImagesThoumb.imageThumbs.length) {
                if (endIndex > mImagesThoumb.imageThumbs.length) {
                    endIndex = mImagesThoumb.imageThumbs.length;
                }
                for (int i = startIndex; i < endIndex; i++) {
                    String imageUrl = mImagesThoumb.imageThumbs[i].toString();
                    if (imageUrl != null && !"".equals(imageUrl)) {
                        //开始下载图片
                        downLoadData(imageUrl);
                    }
                }
                page++;
            } else {
                Toast.makeText(mContext, "没有更多图片了", 0).show();
            }
        } else {
            Toast.makeText(mContext, "无sdcard", 0).show();
        }
    }

    /**
     * 下载
     * 
     * @param imageUrl
     */
    private void downLoadData(String imageUrl) {
        DownLoadTask task = new DownLoadTask();
        mTasks.add(task);
        //执行
        task.execute(imageUrl);
    }

    /**
     *异步下载图片
     */
    public class DownLoadTask extends AsyncTask<String, String, Bitmap> {
        /** 
         * 图片的URL地址 
         */
        private String mImageUrl;

        @Override
        protected Bitmap doInBackground(String... params) {
            try {
                mImageUrl = params[0];
                Bitmap bitmapFromMemory = mImagesThoumb
                        .getMemoryCache(mImageUrl);
                if (bitmapFromMemory != null) {
                    return bitmapFromMemory;
                }
                if (hashSdcard()) {
                    Bitmap bitmap = loadImage(mImageUrl);
                    return bitmap;
                } else {
                    Toast.makeText(mContext, "无sdcard,无法获取图片", 0).show();
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            // 展示图片
            if (bitmap != null) {
                // 1.缩放图片
                // 2.新建ImageView
                // 3.找到需要的linerlayout添加imageView
                float width = bitmap.getWidth();
                float radio = width / mFirstColumn.getWidth();
                float scaleHeight = bitmap.getHeight() / radio;
                addImage(bitmap, mFirstColumn.getWidth(), scaleHeight);
            }
            mTasks.remove(this);
        }

        /** 
         * 向ImageView中添加一张图片 
         *  
         * @param bitmap 
         *            待添加的图片 
         * @param width 
         *            图片的宽度 
         * @param scaleHeight 
         *            图片的高度 
         */  
        public void addImage(Bitmap bitmap, float width, float scaleHeight) {
            // 生成缩放的iv
            ImageView iv = new ImageView(mContext);
            android.view.ViewGroup.LayoutParams params = new LayoutParams(
                    (int) width, (int) scaleHeight);
            iv.setLayoutParams(params);
            if (bitmap != null) {
                // 解决默认图片有大有小的问题
                iv.setScaleType(ScaleType.FIT_XY);
                iv.setPadding(5, 5, 5, 5);

                iv.setImageBitmap(bitmap);
                iv.setTag(R.string.iamgurl, mImageUrl);
                findColumnToAdd(iv, (int) scaleHeight).addView(iv);
                mImageViewList.add(iv);
            }
        }

    }

    /**将图片下载到SD卡缓存起来。
     * @param imageUrl 图片的URL地址。 
     * @return
     * @throws IOException
     */
    private Bitmap downLoad(String imageUrl) throws IOException {
        BufferedInputStream bis = null;
        FileOutputStream fos = null;
        BufferedOutputStream bos = null;
        HttpURLConnection conn = null;
        File imageFile = null;
        try {
            URL url = new URL(imageUrl);
            conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(10000);
            conn.setConnectTimeout(5000);
            conn.setDoInput(true);
            conn.setDoOutput(true);
            InputStream is = conn.getInputStream();
            imageFile = new File(getImagePath(imageUrl));
            bis = new BufferedInputStream(is);
            fos = new FileOutputStream(imageFile);
            bos = new BufferedOutputStream(fos);
            int len = 0;
            byte[] buffer = new byte[1024];
            while ((len = bis.read(buffer)) != -1) {
                bos.write(buffer, 0, len);
                bos.flush();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (bis != null) {
                bis.close();
            }
            if (bos != null) {
                bos.close();
            }
            if (conn != null) {
                conn.disconnect();
            }
        }
        // 如果imageFile不为null，将图片添加到memory中
        if (imageFile != null) {
            Bitmap bitmap = BitmapFactory.decodeFile(imageFile.getPath());
            mImagesThoumb.addBitmapToMemoryCache(imageUrl, bitmap);
            return bitmap;
        }
        return null;

    }

   
    /** 
     * 根据传入的URL，对图片进行加载。如果这张图片已经存在于SD卡中，则直接从SD卡里读取，否则就从网络上下载。 
     *  
     * @param imageUrl 
     *            图片的URL地址 
     * @return 加载到内存的图片。 
     * 判断图片sdcard是否有图片,如果有就用，没有就下载
     */  
    public Bitmap loadImage(String mImageUrl) throws Exception {
        File file = new File(getImagePath(mImageUrl));
        if (!file.exists()) {
            downLoad(mImageUrl);
        }

        if (mImageUrl != null) {
            // 处理本地图片，设置大小防止oom
            Bitmap bitmap = mImagesThoumb.decodeSimpleBitMapFromResource(
                    file.getPath(), mClolumnWidth);
            // Bitmap bitmap = BitmapFactory.decodeFile(file.getPath());
            if (bitmap != null) {
                mImagesThoumb.addBitmapToMemoryCache(mImageUrl, bitmap);
                return bitmap;
            }
        }
        return null;
    }

    /** 
     * 找到此时应该添加图片的一列。原则就是对三列的高度进行判断，当前高度最小的一列就是应该添加的一列。 
     *  
     * @param iv 
     * @param imageHeight 
     * @return 应该添加图片的一列 
     */ 
    private LinearLayout findColumnToAdd(ImageView iv, int imageHeight) {
        if (mFirstColumnHeight <= mSecondColumnHeight) {
            if (mFirstColumnHeight <= mThirdColumnHeight) {
                iv.setTag(R.string.border_top, mFirstColumnHeight);
                mFirstColumnHeight += imageHeight;
                iv.setTag(R.string.border_bottom, mFirstColumnHeight);
                return mFirstColumn;
            }
            iv.setTag(R.string.border_top, mThirdColumnHeight);
            mThirdColumnHeight += imageHeight;
            iv.setTag(R.string.border_bottom, mThirdColumnHeight);
            return mThirdColumn;

        } else {
            if (mSecondColumnHeight <= mThirdColumnHeight) {
                iv.setTag(R.string.border_top, mSecondColumnHeight);
                mSecondColumnHeight += imageHeight;
                iv.setTag(R.string.border_bottom, mSecondColumnHeight);
                return mSecondColumn;
            }
            iv.setTag(R.string.border_top, mThirdColumnHeight);
            mThirdColumnHeight += imageHeight;
            iv.setTag(R.string.border_bottom, mThirdColumnHeight);
            return mThirdColumn;
        }
    }

    /**获取图片的本地存储路径。 
     * 获得file地址
     * 
     * @param imageUrl 图片的URL地址。
     * @return 图片的本地存储路径。
     */
    private String getImagePath(String imageUrl) {
        int lastIndexOf = imageUrl.lastIndexOf("/");
        String imageName = imageUrl.substring(lastIndexOf + 1);
        String imageDir = Environment.getExternalStorageDirectory().getPath()
                + "/pangzaifei/";
        File file = new File(imageDir);
        if (!file.exists()) {
            file.mkdir();
        }
        String imagePath = imageDir + imageName;
        return imagePath;
    }

    /**判断手机是否有sd卡
     * 获得图片的名字
     * 
     * @param imageUrl
     */

    private boolean hashSdcard() {
        if (Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED)) {
            return true;
        }
        return false;
    }

    @Override
    /**
     * 当手势抬起时，开始每个5毫秒计算位置
     * 监听用户的触屏事件，如果用户手指离开屏幕则开始进行滚动检测。 
     */
    public boolean onTouch(View v, MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_UP) {
            // 发送handler
            Message msg = mHandler.obtainMessage();
            msg.obj = this;
            mHandler.sendMessageDelayed(msg, mDelay);
        }
        return false;
    }

    /**
     * 在Handler中进行图片可见性检测的判断，以及加载更多图片的操作
     */
    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            // 判断是否已经滑到了最低处，如果滑到了最低处，则加载更多页面，否则继续发送handler扫描
            PhotoFallScrollView scrollView = (PhotoFallScrollView) msg.obj;
            int scrollY = scrollView.getScrollY();
            //如果当前的滚动位置和上次相同，表示已停止滚动
            if (scrollY == lastScrollY) {
            // 当滚动的最底部，并且当前没有正在下载的任务时，开始加载下一页的图片  
                if (mScrollViewHeight + scrollY >= mScrollLayout.getHeight()
                        && mTasks.isEmpty()) {
                    scrollView.loadMoreImages();
                }
                scrollView.checkVisibile();
            } else {
                lastScrollY = scrollY;
                Message message = new Message();
                message.obj = scrollView;
               //5毫秒后在对滚动位置进行判断
                mHandler.sendMessageDelayed(message, mDelay);
            }
        }

    };
    /**
     * MyScrollView布局的高度。
     */
    private int mScrollViewHeight;
    /** 
     * MyScrollView下的直接子布局。 
     */  
    private View mScrollLayout;

    /**
     * 遍历imageview中的每一张图片，对图片的可见性进行检测，如果图片已经离开屏幕可见范围，
     * 则将图片替换成一张空图
     * 想不可见的变为空图片
     */
    protected void checkVisibile() {
        if (mImageViewList != null && mImageViewList.size() > 0) {
            for (int i = 0; i < mImageViewList.size(); i++) {
                ImageView iv = mImageViewList.get(i);
                int borderTop = (Integer) iv.getTag(R.string.border_top);
                int borderBottom = (Integer) iv.getTag(R.string.border_bottom);
                if (borderBottom > getScrollY()
                        && borderTop < getScrollY() + mScrollViewHeight) {
                    String imageUrl = (String) iv.getTag(R.string.iamgurl);
                    if (imageUrl != null && !"".equals(imageUrl)) {
                        Bitmap bitmap = mImagesThoumb.getMemoryCache(imageUrl);
                        if (bitmap != null) {
                            iv.setImageBitmap(bitmap);
                        } else {
                            downLoadData(imageUrl);
                        }
                    }
                } else {
                    //离开屏幕加载空图
                    iv.setImageResource(R.drawable.empty_photo);
                }

            }
        }
    }
}