Android使用LruCache、DiskLruCache实现图片缓存+图片瀑布流

写在前面

PS:本文中的例子来源于官网地址:Caching Bitmaps,源码地址(自备梯子):Caching Bitmaps Demo,并在其基础上稍微改变了一下。

PPS:本文仅用于学习利用LruCache、DiskLruCache图片缓存策略、实现瀑布流和Matix查看大图缩放移动等功能,如果想用到项目中,建议用更成熟的框架,如glidepicasso 等。

在开始本文之前,请先了解下LruCache和DiskLruCache的用法,不了解的可以先看下这两篇:
1、Android使用磁盘缓存DiskLruCache
2、Android内存缓存LruCache源码解析

先上效果图:

嗯,效果还是不错的~代码已上传Github:LruCache、DiskLruCache实现图片缓存

### 图片瀑布流
这个用RecycleView来实现已经很简单了,直接上代码:

 recycler_view = (RecyclerView) findViewById(R.id.recycler_view);
 recycler_view.setLayoutManager(new StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL));

首先初始化StaggeredGridLayoutManager,这里设置显示方式是竖直方向3列,然后通过recycleView的setLayoutManager设置好,接着看RecyclerView.Adapter中的处理:

public class WaterFallAdapter extends RecyclerView.Adapter {

  private int DATA_SIZE = Constant.imageUrls.length;
  private List hList;//定义一个List来存放图片的height

  public WaterFallAdapter(Context mContext) {
     this.mContext = mContext;
     hList = new ArrayList<>();
     for (int i = 0; i < DATA_SIZE; i++) {
         //每次随机一个高度并添加到hList中
         int height = new Random().nextInt(200) + 300;//[100,500)的随机数
         hList.add(height);
     }
    }

 @Override
  public void onBindViewHolder(final MyHolder holder, int position) {
      //通过setLayoutParams(params)来设置图片的宽高信息
      int width = DpUtil.getScreenSizeWidth(mContext) / 3;
      RecyclerView.LayoutParams params = new RecyclerView.LayoutParams(width, hList.get(position));
      holder.imageView.setLayoutParams(params);
   }

}

在WaterFallAdapter构造方法中将随机高度添加到存放图片height的hList中,并在onBindViewHolder()中取出图片的宽高并通过setLayoutParams(params)来设置图片的宽高信息,经过上面的代码,一个图片的瀑布流效果就出来了,so easy~

图片缓存

下面接着看本文的重点,实现图片的缓存策略:

先看怎么使用的:

 private static final String IMAGE_CACHE_DIR = "thumbs";//图片缓存目录

 @Override
 protected void onCreate(@Nullable Bundle savedInstanceState) {
   ImageCache.ImageCacheParams cacheParams = new   ImageCache.ImageCacheParams(this, IMAGE_CACHE_DIR);
   cacheParams.setMemCacheSizePercent(0.25f);// Set memory cache to 25% of app memory

   // The ImageFetcher takes care of loading images into our ImageView children asynchronously
   mImageFetcher = new ImageFetcher(this, (int) DpUtil.dp2px(this, 100));
   mImageFetcher.setLoadingImage(R.mipmap.img_default_bg);
   mImageFetcher.addImageCache(cacheParams);
 }

上面初始化了缓存大小和下载时ImageView加载默认的图片等操作,然后在RecyclerView的onBindViewHolder()开始加载图片:

@Override
 public void onBindViewHolder(final MyHolder holder, int position) { 
   .............其他操作.............
  mImageFetcher.loadImage(Constant.imageUrls[position], holder.imageView); 
  }

用起来还是挺简单的,里面主要的几个核心类:

接下来就依次分析一下各个类的作用:

ImageCache.java:
public class ImageCache {

  // Default memory cache size in kilobytes
  private static final int DEFAULT_MEM_CACHE_SIZE = 1024 * 5; // 5KB
  // Default disk cache size in bytes
  private static final int DEFAULT_DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB

  //内存缓存核心类,用于缓存已经下载好的图片
  private DiskLruCache mDiskLruCache;
  //磁盘缓存核心类,用于缓存图片到外部存储中
  private LruCache mMemoryCache;
  //ImageCacheParams 是ImageCache 的内部类,用于设置图片缓存缓存的各个参数
  private ImageCacheParams mCacheParams;
  //使用Set保存数据可以确保没有重复元素,使用软引用SoftReference关联Bitmap,当内存不足时Bitmap会被回收
  private Set> mReusableBitmaps;

  //实现一个单例模式
  private volatile static ImageCache imageCache;
  public static ImageCache getInstance(ImageCacheParams cacheParams) {
      if (imageCache == null) {
          synchronized (ImageCache.class) {
              if (imageCache == null) {
                  imageCache = new ImageCache(cacheParams);
              }
          }
      }
      return imageCache;
  }
 private ImageCache(ImageCacheParams cacheParams) {
     init(cacheParams);
  }

 private void init(ImageCacheParams cacheParams) {
     mCacheParams = cacheParams;
     if (Utils.hasHoneycomb()) {
         mReusableBitmaps =
                 Collections.synchronizedSet(new HashSet>());
     }
      //初始化LruCache并覆写entryRemoved()和sizeOf()方法 
     mMemoryCache = new LruCache(mCacheParams.memCacheSize) {
        @Override
        protected void entryRemoved(boolean evicted, String key,
                                        BitmapDrawable oldValue, BitmapDrawable newValue) {
              if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
                  // The removed entry is a recycling drawable, so notify it
                  // that it has been removed from the memory cache
                  ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
            } else {
                  // The removed entry is a standard BitmapDrawable
                  if (Utils.hasHoneycomb()) {
                      // We're running on Honeycomb or later, so add the bitmap
                      // to a SoftReference set for possible use with inBitmap later
                      mReusableBitmaps.add(new SoftReference(oldValue.getBitmap()));
                  }
              }
          }

       @Override
       protected int sizeOf(String key, BitmapDrawable value) {
           final int bitmapSize = getBitmapSize(value) / 1024;
           return bitmapSize == 0 ? 1 : bitmapSize;
       }
   };

   // By default the disk cache is not initialized here as it should be initialized
   // on a separate thread due to disk access.
   if (cacheParams.initDiskCacheOnCreate) {
      // Set up disk cache
       initDiskCache();
    }
 }

//初始化DiskLruCache,因为操作外部存储比较耗时,所以这部分最好放在子线程中执行
 public void initDiskCache() {
    // Set up disk cache
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache == null || mDiskLruCache.isClosed()) {
            File diskCacheDir = mCacheParams.diskCacheDir;
            if (mCacheParams.diskCacheEnabled && diskCacheDir != null) {
                if (!diskCacheDir.exists()) {
                    diskCacheDir.mkdirs();
                }
                if (getUsableSpace(diskCacheDir) > mCacheParams.diskCacheSize) {
                    try {
                        mDiskLruCache = DiskLruCache.open(
                                    diskCacheDir, 1, 1, mCacheParams.diskCacheSize);
                        } catch (final IOException e) {
                        mCacheParams.diskCacheDir = null;
                        Log.e(TAG, "initDiskCache - " + e);
                    }
                }
            }
        }
        mDiskCacheStarting = false;
        mDiskCacheLock.notifyAll();
    }
 }

//将图片添加到内存缓存和外部缓存中
public void addBitmapToCache(String data, BitmapDrawable value) {
    if (data == null || value == null) {
        return;
    }
    // Add to memory cache
    if (mMemoryCache != null) {
        if (RecyclingBitmapDrawable.class.isInstance(value)) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been added into the memory cache
            ((RecyclingBitmapDrawable) value).setIsCached(true);
        }
        mMemoryCache.put(data, value);
    }
    synchronized (mDiskCacheLock) {
        //Add to disk cache
        if (mDiskLruCache != null) {
            final String key = hashKeyForDisk(data);
            OutputStream out = null;
            try {
                DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                if (snapshot == null) {
                    final DiskLruCache.Editor editor = mDiskLruCache.edit(key);
                    if (editor != null) {
                        out = editor.newOutputStream(DISK_CACHE_INDEX);
                        value.getBitmap().compress(
                                mCacheParams.compressFormat, mCacheParams.compressQuality, out);
                        editor.commit();
                        out.close();
                    }
                } else {
                    snapshot.getInputStream(DISK_CACHE_INDEX).close();
                }
            } catch (Exception e) {
                Log.e(TAG, "addBitmapToCache - " + e);
            } finally {
                try {
                    if (out != null) {
                        out.close();
                    }
                } catch (IOException e) {
                }
            }
        }
    }
 }

 //清除缓存,该方法也应该在子线程中调用
 public void clearCache() {
     if (mMemoryCache != null) {
         mMemoryCache.evictAll();
         if (BuildConfig.DEBUG) {
             Log.d(TAG, "Memory cache cleared");
         }
     }
     synchronized (mDiskCacheLock) {
         mDiskCacheStarting = true;
         if (mDiskLruCache != null && !mDiskLruCache.isClosed()) {
             try {
                 mDiskLruCache.delete();
                } catch (Exception e) {
                    Log.e(TAG, "clearCache - " + e);
                }
                mDiskLruCache = null;
                initDiskCache();
            }
        }
    }

//从内存缓存中取出图片
 public BitmapDrawable getBitmapFromMemCache(String data) {
     BitmapDrawable memValue = null;
     if (mMemoryCache != null) {
         memValue = mMemoryCache.get(data);
     }
     return memValue;
 }

//从外部存储中取出图片
 public Bitmap getBitmapFromDiskCache(String data) {
     final String key = hashKeyForDisk(data);
     Bitmap bitmap = null;
     synchronized (mDiskCacheLock) {
            while (mDiskCacheStarting) {
             try {
                 mDiskCacheLock.wait();
             } catch (Exception e) {
             }
         }
         if (mDiskLruCache != null) {
             InputStream inputStream = null;
             try {
                 final DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                 if (snapshot != null) {
                     inputStream = snapshot.getInputStream(DISK_CACHE_INDEX);
                     if (inputStream != null) {
                         FileDescriptor fd = ((FileInputStream) inputStream).getFD();
                         // Decode bitmap, but we don't want to sample so give
                         // MAX_VALUE as the target dimensions
                         bitmap = ImageResizer.decodeSampledBitmapFromDescriptor(
                                    fd, Integer.MAX_VALUE, Integer.MAX_VALUE, this);
                     }
                 }
             } catch (Exception e) {
                 Log.e(TAG, "getBitmapFromDiskCache - " + e);
             } finally {
                 try {
                     if (inputStream != null) {
                         inputStream.close();
                     }
                 } catch (Exception e) {
                 }
             }
         }
     }
     return bitmap;
  }

  public static class ImageCacheParams {
    public int memCacheSize = DEFAULT_MEM_CACHE_SIZE;
    public int diskCacheSize = DEFAULT_DISK_CACHE_SIZE;
    public File diskCacheDir;
    public Bitmap.CompressFormat compressFormat = DEFAULT_COMPRESS_FORMAT;
    public int compressQuality = DEFAULT_COMPRESS_QUALITY;
    public boolean memoryCacheEnabled = DEFAULT_MEM_CACHE_ENABLED;
    public boolean diskCacheEnabled = DEFAULT_DISK_CACHE_ENABLED;
    public boolean initDiskCacheOnCreate = DEFAULT_INIT_DISK_CACHE_ON_CREATE;

    public ImageCacheParams(Context context, String diskCacheDirectoryName) {
         //外存缓存目录
        diskCacheDir = getDiskCacheDir(context, diskCacheDirectoryName);
        }

    //设置图片内存缓存和应用最大内存的比例
    public void setMemCacheSizePercent(float percent) {
        memCacheSize = Math.round(percent * Runtime.getRuntime().maxMemory() / 1024);
      }
  }

}

ImageCache这个类主要是初始化了LruCache和DiskLruCache用来缓存图片到内存和外存中去,并从内存外存中取出图片及清除缓存等操作

ImageWorker.java:
public abstract class ImageWorker {

    //初始化ImageCache和外存缓存
    public void addImageCache(ImageCache.ImageCacheParams cacheParams) {
        mImageCacheParams = cacheParams;
        mImageCache = ImageCache.getInstance(mImageCacheParams);
        new CacheAsyncTask().execute(MESSAGE_INIT_DISK_CACHE);
    }

    //加载本地图片,如果没有去服务器下载该图片
    public void loadImage(Object data, ImageView imageView, OnImageLoadedListener listener) {
        if (data == null) return;
        BitmapDrawable bitmapDrawable = null;
        if (mImageCache != null) {
            bitmapDrawable = mImageCache.getBitmapFromMemCache(String.valueOf(data));
        }
        if (bitmapDrawable != null) {
            // Bitmap found in memory cache
            imageView.setImageDrawable(bitmapDrawable);
            if (listener != null) {
                listener.onImageLoaded(true);
            }
        } else if (cancelPotentialWork(data, imageView)) {
            final BitmapWorkerTask task = new BitmapWorkerTask(data, imageView, listener);
            final AsyncDrawable asyncDrawable = new AsyncDrawable(mResources, mLoadingBitmap, task);
            imageView.setImageDrawable(asyncDrawable);

            // NOTE: This uses a custom version of AsyncTask that has been pulled from the
            // framework and slightly modified. Refer to the docs at the top of the class
            // for more info on what was changed.
            task.executeOnExecutor(AsyncTask.DUAL_THREAD_EXECUTOR);
        }
    }

}

ImageWorker处理ImageView加载Bitmap时的耗时操作,如处理内存缓存和外存缓存的使用,ImageWorker中的两个主要方法:addImageCache()loadImage() 方法。

1、addImageCache()中获得了ImageCache的单例对象,并开启CacheAsyncTask在新线程中初始化缓存:

 protected class CacheAsyncTask extends AsyncTask {
     @Override
     protected Void doInBackground(Object... params) {
         int param = (int) params[0];
         switch (param) {
             case MESSAGE_CLEAR:
                 clearCacheInternal();
                 break;
             case MESSAGE_INIT_DISK_CACHE:
                 initDiskCacheInternal();
                 break;
             case MESSAGE_FLUSH:
                 flushCacheInternal();
                 break;
             case MESSAGE_CLOSE:
                 closeCacheInternal();
                 break;
         }
         return null;
     }
 }

 //清除缓存
 protected void clearCacheInternal() {
     if (mImageCache != null) {
         mImageCache.clearCache();
     }
 }
 //初始化缓存
 protected void initDiskCacheInternal() {
     if (mImageCache != null) {
         mImageCache.initDiskCache();
     }
 }
 //强制将缓存刷新到文件系统中
 protected void flushCacheInternal() {
     if (mImageCache != null) {
         mImageCache.flush();
     }
 }
 //关闭缓存
 protected void closeCacheInternal() {
     if (mImageCache != null) {
         mImageCache.close();
         mImageCache = null;
     }
 }

2、loadImage()方法中,首先通过mImageCache.getBitmapFromMemCache尝试从内存中取出图片,如果内存中有,直接取出来显示,流程结束;如果内存中没有,就调用到了cancelPotentialWork(),来看这个方法:

  public static boolean cancelPotentialWork(Object data, ImageView imageView) {
      final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
      if (bitmapWorkerTask != null) {
          final Object bitmapData = bitmapWorkerTask.mData;
          if (bitmapData == null || !bitmapData.equals(data)) {
              bitmapWorkerTask.cancel(true);
            } else {
              // The same work is already in progress.
              return false;
          }
      }
      return true;
  }

 //获得与ImageView关联的BitmapWorkerTask ,如果没有返回Null
  private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
      if (imageView != null) {
          final Drawable drawable = imageView.getDrawable();
          if (drawable instanceof AsyncDrawable) {
              final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
              return asyncDrawable.getBitmapWorkerTask();
          }
      }
      return null;
  }

首先通过getBitmapWorkerTask()获得与ImageView关联的BitmapWorkerTask(以下简称task),如果作用在ImageView上的task存在并且task.mData(图片的URL)没有改变,那么task接着执行,直到下载完图片;如果task.mData变化了,那么取消之前作用在ImageView上的task,重新去new一个Task下载新的图片。其实是这样:当我们不滑动界面的时候,task.mData(图片的URL)是不会改变的,但是当我们滑动界面的时候,比如现在RecycleView一屏显示5个ItemView,往上滑动时,第一个ItemView不可见并且此时第一个ItemView的图片还没有下载完成,第6个ItemView会复用之前第一个ItemView,如果此时继续执行第一个ItemView对应的task,那么这时候第6个ItemView显示的就是第一个ItemView应该显示的图片了,这就是我们经常遇到的显示错位问题。所以这里要判断task.mData(图片的URL)是否一致,如果不一致,需要取消之前的task,重启一个新的task,并把新的task关联给ImageView。看看task里面都干了些什么:

private class BitmapWorkerTask extends AsyncTask {
    private Object mData;
    private final WeakReference imageViewWeakReference;
    private final OnImageLoadedListener mOnImageLoadedListener;
    /**
     * Background processing.
     */
    public BitmapWorkerTask(Object data, ImageView imageView) {
        mData = data;
        imageViewWeakReference = new WeakReference(imageView);
        mOnImageLoadedListener = null;
    }

    public BitmapWorkerTask(Object data, ImageView imageView, OnImageLoadedListener listener) {
        mData = data;
        //弱引用关联一个ImageView
        imageViewWeakReference = new WeakReference<>(imageView);
        mOnImageLoadedListener = listener;
    }

    @Override
    protected BitmapDrawable doInBackground(Void... params) {  
        final String dataString = String.valueOf(mData);
        Bitmap bitmap = null;
        BitmapDrawable drawable = null;
        // Wait here if work is paused and the task is not cancelled
        synchronized (mPauseWorkLock) {
            while (mPauseWork && !isCancelled()) {
                try {
                    mPauseWorkLock.wait();
                } catch (Exception e) {
                }
            }
        }

        if (mImageCache != null && !isCancelled() && getAttachedImageView() != null
                    && !mExitTasksEarly) {
            //从外部存储去取图片
            bitmap = mImageCache.getBitmapFromDiskCache(dataString);
        }

        if (bitmap == null && !isCancelled() && getAttachedImageView() != null
                    && !mExitTasksEarly) {
              //如果外存中也没有图片,那么就去服务器上下载
              bitmap = processBitmap(mData);
          }

       if (bitmap != null) {
           if (Utils.hasHoneycomb()) {
               // Running on Honeycomb or newer, so wrap in a standard BitmapDrawable
               drawable = new BitmapDrawable(mResources, bitmap);
           } else {
               // Running on Gingerbread or older, so wrap in a RecyclingBitmapDrawable
               // which will recycle automagically
               drawable = new RecyclingBitmapDrawable(mResources, bitmap);
           }
           if (mImageCache != null) {
                //把下载的图片缓存到内存和外存中去
               mImageCache.addBitmapToCache(dataString, drawable);
           }
       }
       return drawable;
   }

   @Override
   protected void onPostExecute(BitmapDrawable value) {
       boolean success = false;
       // if cancel was called on this task or the "exit early" flag is set then we're done
       if (isCancelled() || mExitTasksEarly) {
           value = null;
       }
       final ImageView imageView = getAttachedImageView();
       if (value != null && imageView != null) {
           success = true;
           //给ImageView设置图片
           setImageDrawable(imageView, value);
       }
       if (mOnImageLoadedListener != null) {
           //执行回调
           mOnImageLoadedListener.onImageLoaded(success);
       }
   }

   @Override
   protected void onCancelled(BitmapDrawable value) {
       super.onCancelled(value);
       synchronized (mPauseWorkLock) {
           mPauseWorkLock.notifyAll();
       }
   }

   private ImageView getAttachedImageView() {
       final ImageView imageView = imageViewWeakReference.get();
       final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
       if (this == bitmapWorkerTask) {
           return imageView;
       }
       return null;
   }
 }

BitmapWorkerTask 的工作很明确,首先开启一个新线程,先确认外存中是否有相应缓存图片,如果有,直接拿出来使用并返回结果;如果没有,去服务器上下载该图片,并将该图片缓存到内存和外存中去,最后onPostExecute()中加载图片并执行结果回调。

ImageResizer.java (extends ImageWorker):
 public class ImageResizer extends ImageWorker {

  protected int mImageWidth;
  protected int mImageHeight;

  protected ImageResizer(Context context, int imageWidth, int imageHeight) {
     super(context);
     setImageSize(imageWidth, imageHeight);
  }

  //设置图片的宽高
  private void setImageSize(int imageWidth, int imageHeight) {
      this.mImageWidth = imageWidth;
      this.mImageHeight = imageHeight;
  }

  @Override
  protected Bitmap processBitmap(Object data) {
       //覆写父类的方法,加载本地图片
      return processBitmap(Integer.parseInt(String.valueOf(data)));
  }

  private Bitmap processBitmap(int resId) {
      return decodeSampledBitmapFromResource(mResources, resId, mImageWidth,
                mImageHeight, getImageCache());
  }

  //从本地resource中解码bitmap并采样源bitmap至指定大小的宽高
  public static Bitmap decodeSampledBitmapFromResource(Resources res,
                                                   int resId, int reqWidth, int reqHeight, ImageCache cache) {
     // First decode with inJustDecodeBounds=true to check dimensions
     final BitmapFactory.Options options = new BitmapFactory.Options();
     options.inJustDecodeBounds = true;
     BitmapFactory.decodeResource(res, resId, options);
     // Calculate inSampleSize
     options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
     // If we're running on Honeycomb or newer, try to use inBitmap
     if (Utils.hasHoneycomb()) {
         addInBitmapOptions(options, cache);
     }
     // Decode bitmap with inSampleSize set
     options.inJustDecodeBounds = false;
     return BitmapFactory.decodeResource(res, resId, options);
    }
}

 //从本地文件中解码bitmap并采样源bitmap至指定大小的宽高
 public static Bitmap decodeSampledBitmapFromFile(String filename,
                                                     int reqWidth, int reqHeight, ImageCache cache) {
     // First decode with inJustDecodeBounds=true to check dimensions
     final BitmapFactory.Options options = new BitmapFactory.Options();
     options.inJustDecodeBounds = true;
     BitmapFactory.decodeFile(filename, options);
     // Calculate inSampleSize
     options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
     // If we're running on Honeycomb or newer, try to use inBitmap
     if (Utils.hasHoneycomb()) {
         addInBitmapOptions(options, cache);
     }
     // Decode bitmap with inSampleSize set
     options.inJustDecodeBounds = false;
     return BitmapFactory.decodeFile(filename, options);
  }

 //从文件输入流中解码bitmap并采样源bitmap至指定大小的宽高
 public static Bitmap decodeSampledBitmapFromDescriptor(
            FileDescriptor fileDescriptor, int reqWidth, int reqHeight, ImageCache cache) {
     // First decode with inJustDecodeBounds=true to check dimensions
     final BitmapFactory.Options options = new BitmapFactory.Options();
     options.inJustDecodeBounds = true;
     BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);
     // Calculate inSampleSize
     options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
     // Decode bitmap with inSampleSize set
     options.inJustDecodeBounds = false;
     // If we're running on Honeycomb or newer, try to use inBitmap
     if (Utils.hasHoneycomb()) {
         addInBitmapOptions(options, cache);
     }

     return BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);
 }

//计算采样率大小
 public static int calculateInSampleSize(BitmapFactory.Options options,
                                            int reqWidth, int reqHeight) {
      // Raw height and width of image
      final int height = options.outHeight;
      final int width = options.outWidth;
      //采样率
      int inSampleSize = 1;
      //判断图片的宽高之一是否大于所需宽高
      if (height > reqHeight || width > reqWidth) {
          //图片宽高之一大于所需宽高,那么宽高都除以2
          final int halfHeight = height / 2;
          final int halfWidth = width / 2;
          //循环宽高除以采样率的值,如果大于所需宽高,采样率inSampleSize翻倍
          //PS:采样率每变大2倍,图片大小缩小至原来大小的1/4
          while ((halfHeight / inSampleSize) > reqHeight
                    && (halfWidth / inSampleSize) > reqWidth) {
              inSampleSize *= 2;
          }
      //下面逻辑是为了处理一些不规则图片,如一张图片的height特别大,远大于width,此时图片的大小很大,
      //此时加到内存中去依然不合适,算出总此时图片总像素大小totalPixels
      long totalPixels = width * height / inSampleSize;
       //如果图片总像素大于所需总像素的2倍,继续扩大采样率
      final long totalReqPixelsCap = reqHeight * reqWidth * 2;
      while (totalPixels > totalReqPixelsCap) {
          inSampleSize *= 2;
          totalPixels /= 2;
         }
      }
     //返回最终的采样率inSampleSize
     return inSampleSize;
  }

 //如果SDK>11,通过设置options.inBitmap尝试复用ImageCache中mReusableBitmaps的缓存bitmap,
 //这样可以避免频繁的申请内存和销毁内存
 private static void addInBitmapOptions(BitmapFactory.Options options, ImageCache cache) {
     // inBitmap only works with mutable bitmaps(可变的位图) so force the decoder to
     // return mutable bitmaps.
     options.inMutable = true;
     if (cache != null) {
         // Try and find a bitmap to use for inBitmap
         Bitmap inBitmap = cache.getBitmapFromReusableSet(options);
         if (inBitmap != null) {
             options.inBitmap = inBitmap;
         }
     }
  }

}

ImageResizer继承自ImageWorker,ImageResizer类可以调整bitmap到指定宽高,主要用到了BitmapFactory.Options这个内部类来处理加载图片(后面介绍),当本地图片很大时,不能直接加载到内存中去,需要经过ImageResizer处理一下。下面介绍一下BitmapFactory.Options的常用方法:

BitmapFactory用来解码创建一个Bitmap,Options是BitmapFactory的一个静态内部类,是解码Bitmap时的各种参数控制:

public class BitmapFactory {
  private static final int DECODE_BUFFER_SIZE = 16 * 1024;

  public static class Options {
      public boolean inJustDecodeBounds;
      public int inSampleSize;
      public Bitmap.Config inPreferredConfig = Bitmap.Config.ARGB_8888;
      public int outWidth;
      public int outHeight;
      public String outMimeType;
      public boolean inMutable;
      public Bitmap inBitmap;
      ............其他参数............
   }
}
  • inJustDecodeBounds:如果设置为true,则不会把bitmap加载到内存中,但是依然可以得到bitmap的宽高,当需要知道bitmap的宽高而又不想把bitmap加载到内存中去的时候,就可以通过设置这个属性来实现
  • inSampleSize:采样率,如果value > 1,解码器将会按(1/inSampleSize)的比例减小宽高,然后返回一个压缩后的图片到内存中,例如: inSampleSize == 4,返回一个宽高都是原图片1/4的压缩图片,图片被压缩到原来的1/16,;如果inSampleSize的value <= 1,都会被当成1处理,即保持原样。
  • inPreferredConfig :设置色彩模式,默认值是ARGB_8888,在这个模式下,一个像素点占用4bytes空间;如果对透明度不做要求的话,可以采用RGB_565模式,这个模式下一个像素点占用2bytes。
  • outWidth:bitmap的宽度,如果inJustDecodeBounds=true,outWidth将是原图的宽度;如果inJustDecodeBounds=false,outWidth将是经过缩放后的bitmap的宽度。如果解码过程中发生错误,将返回-1。
  • outHeight:bitmap的高度,如果inJustDecodeBounds=true,outHeight将是原图的高度;如果inJustDecodeBounds=false,outHeight将是经过缩放后的bitmap的高度。如果解码过程中发生错误,将返回-1。
  • outMimeType:获取图片的类型,如果获取不到图片类型或者解码发生错误,outMimeType将会被设置成null。
  • inMutable:设置Bitmap是否可更改,如在Bitmap上进行额外操作
  • inBitmap:解析Bitmap的时候重用其他Bitmap的内存,避免大块内存的申请和销毁,使用inBitmap时inMutable必须设置为true
ImageResizer .java:
public class ImageFetcher extends ImageResizer {
     ..............省略部分代码..............
  private static final int HTTP_CACHE_SIZE = 10 * 1024 * 1024; // 10MB
  private static final String HTTP_CACHE_DIR = "http";
  private static final int IO_BUFFER_SIZE = 8 * 1024;
  private DiskLruCache mHttpDiskCache;
  private File mHttpCacheDir;

  public ImageFetcher(Context context, int imageSize) {
      super(context, imageSize);
      init(context);
  }

  //初始化外存缓存
  private void init(Context context) {
      checkConnection(context);
      mHttpCacheDir = ImageCache.getDiskCacheDir(context, HTTP_CACHE_DIR);
  }

  //检查网络状态
  private void checkConnection(Context context) {
      final ConnectivityManager cm =
                (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
      final NetworkInfo networkInfo = cm.getActiveNetworkInfo();
      if (networkInfo == null || !networkInfo.isConnectedOrConnecting()) {
          Toast.makeText(context, "no_network_connection_toast", Toast.LENGTH_LONG).show();
          Log.e(TAG, "checkConnection - no connection found");
      }
  }

@Override
  protected Bitmap processBitmap(Object data) {
      return processBitmap(String.valueOf(data));
  }

  private Bitmap processBitmap(String data) {
      final String key = ImageCache.hashKeyForDisk(data);
      FileDescriptor fileDescriptor = null;
      FileInputStream fileInputStream = null;
      DiskLruCache.Snapshot snapshot;
      synchronized (mHttpDiskCacheLock) {
          // Wait for disk cache to initialize
          while (mHttpDiskCacheStarting) {
              try {
                  mHttpDiskCacheLock.wait();
              } catch (InterruptedException e) {
              }
          }

         if (mHttpDiskCache != null) {
             try {
                 //在外存中读取图片
                 snapshot = mHttpDiskCache.get(key);
                 if (snapshot == null) {
                      //外存中没有,则开始去服务器上下载
                     DiskLruCache.Editor editor = mHttpDiskCache.edit(key);
                     if (editor != null) {
                         if (downloadUrlToStream(data,
                             editor.newOutputStream(DISK_CACHE_INDEX))) {
                              //写入外存缓存,这里保存的是原始图
                             editor.commit();
                          } else {
                             editor.abort();
                         }
                     }
                     //从外存中读取出该图片
                     snapshot = mHttpDiskCache.get(key);
                 }
                 if (snapshot != null) {
                     fileInputStream =
                                (FileInputStream) snapshot.getInputStream(DISK_CACHE_INDEX);
                     fileDescriptor = fileInputStream.getFD();
                 }
             } catch (IOException e) {
                 Log.e(TAG, "processBitmap - " + e);
             } catch (IllegalStateException e) {
                 Log.e(TAG, "processBitmap - " + e);
             } finally {
                 if (fileDescriptor == null && fileInputStream != null) {
                     try {
                         fileInputStream.close();
                     } catch (IOException e) {
                     }
                 }
             }
          }
      }
     Bitmap bitmap = null;
     if (fileDescriptor != null) {
          //将取出来的图片压缩到指定大小并在后面的逻辑中保存,这里保存的是缩略图
         bitmap = decodeSampledBitmapFromDescriptor(fileDescriptor, mImageWidth,
                    mImageHeight, getImageCache());
     }
     if (fileInputStream != null) {
         try {
             fileInputStream.close();
         } catch (IOException e) {
         }
      }
     return bitmap;
  }

//根据URL去下载图片
 private boolean downloadUrlToStream(String urlString, OutputStream outputStream) {
     disableConnectionReuseIfNecessary();
     HttpURLConnection urlConnection = null;
     BufferedOutputStream out = null;
     BufferedInputStream in = null;
     try {
         final URL url = new URL(urlString);
      urlConnection = (HttpURLConnection) url.openConnection();
         in = new BufferedInputStream(urlConnection.getInputStream(), IO_BUFFER_SIZE);
         out = new BufferedOutputStream(outputStream, IO_BUFFER_SIZE);
         int b;
         while ((b = in.read()) != -1) {
             out.write(b);
         }
         return true;
     } catch (Exception e) {
         Log.e(TAG, "Error in downloadBitmap - " + e);
     } finally {
         if (urlConnection != null) {
             urlConnection.disconnect();
         }
         try {
             if (out != null) {
                 out.close();
             }
             if (in != null) {
                 in.close();
             }
         } catch (final IOException e) {
         }
     }
     return false;
 }
}

ImageFetcher继承自ImageResizer ,ImageFetcher根据URL从服务器中得到图片并分别把原始图和缩略图缓存到外存中,这里设置的两个缓存目录:
/storage/emulated/0/Android/data/package_name/http (用来存放原始图)
/storage/emulated/0/Android/data/package_name/thumbs (用来存放缩略图)
看下面两张图:


原始图大小基本都在100Kb以上,而缩略图基本都在10Kb以内~

最后再贴下源码地址:LruCache、DiskLruCache实现图片缓存

好了,本文暂时到这里,文章有点长了,后续会加上查看大图功能~

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