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glide、picasso 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：
GIF.gif
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：
main_class.png
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() 和 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.jpg
缩略图.jpg
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~