Android中的Bitmap集锦

Bitmap第一印象就是内存"土豪"，这样就造就了它天生的任性。在2.2以及之前的Android是单线程垃圾回收器，在2.3以后才是并发的回收，且在3.0以前Bitmap的存储方式也是放开的Bitmap对象存储在Dalvik heap中，而Bitmap对象的像素数据则存储在Native Memory（本地内存）中或者说Derict Memory（直接内存）中，这我们在回收bitmap的时候很容报"Canvas: trying to use a recycled bitmap"错误而导致了程序奔溃，在3.0开始并发回收但是任然会出现内存不足的情况，列出一些在图片中我们常使用的一些方法。

1、LruCache缓存

mMemoryCache = new LruCache(mCacheParams.memCacheSize) {

                /**
                 * Notify the removed entry that is no longer being cached
                 */
                @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 SoftRefrence set for possible use with inBitmap later
                            mReusableBitmaps.add(new SoftReference(oldValue.getBitmap()));
                        }
                    }
                }

                /**
                 * Measure item size in kilobytes rather than units which is more practical
                 * for a bitmap cache
                 */
                @Override
                protected int sizeOf(String key, BitmapDrawable value) {
                    final int bitmapSize = getBitmapSize(value) / 1024;
                    return bitmapSize == 0 ? 1 : bitmapSize;
                }
            };

mReusableBitmaps这个用于被寄出去的bitmap，用于后面Options.inBitmap中使用。那么是怎么使用的呢？

2、Options进行缩放

Options.inSampleSize  我们通过改变该值来起到缩放当前的bitmap作用，通过设置options.inMutable =true然后按从mReusableBitmaps中找符合要求且被回收的bitmap从而设置到options.inBitmap中，这果这个字段被设置了，则解码时会把重用这个字段所引用的那张Bitmap，避免重新分配内存。

private static boolean canUseForInBitmap(
            Bitmap candidate, BitmapFactory.Options targetOptions) {
        int width = targetOptions.outWidth / targetOptions.inSampleSize;
        int height = targetOptions.outHeight / targetOptions.inSampleSize;

        return candidate.getWidth() == width && candidate.getHeight() == height;
    }

 mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,mCacheParams.diskCacheSize);

从diskLruCache中读取数据

 final DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                    if (snapshot != null) {
                        if (BuildConfig.DEBUG) {
                            Log.d(TAG, "Disk cache hit");
                        }
                        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);
                        }
                    }