关于android 图片加载优化
android应用对图片处理算是比较频繁的了，尤其是在程序加载大量图片和高分辨率图片时，最容易产生oom异常，下面是个人平时一些省内存加载方法
方法一：
public Bitmap decodeFile(String filePath) { Bitmap bitmap = null; BitmapFactory.Options options = new BitmapFactory.Options(); options.inPurgeable = true; try { BitmapFactory.Options.class.getField("inNativeAlloc").setBoolean( options, true); } catch (IllegalArgumentException e) { e.printStackTrace(); } catch (SecurityException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (NoSuchFieldException e) { e.printStackTrace(); } if (mFilePath != null) { bitmap = BitmapFactory.decodeFile(mFilePath, options); } return bitmap; }
方法二：
public Bitmap ReadBitMap(Context context, int resId){ BitmapFactory.Options opt = new BitmapFactory.Options(); opt.inPreferredConfig = Bitmap.Config.RGB_565; opt.inPurgeable = true; opt.inInputShareable = true; //获取资源图片 InputStream is = context.getResources().openRawResource(resId); return BitmapFactory.decodeStream(is,null,opt); }
如果你的控件大小小于原始图片大小，那么就需要对图片进行压缩处理，来减少内存使用。
现在知道了原图片的尺寸，根据实际情况决定你要加载它缩小多少倍后的图片。例如你用一个128x96的ImageView显示一张1024x768的原图，根本没有必要把原图读加载到内存。
加载一张缩小后的图片到内存，只需要把BitmapFactory.Options对象的inSampleSize设为true，
然后给inSampleSize设一个值就行了（可以理解inSampleSize为n，图片就缩小到1/n大小）。
public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) { // 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); // Decode bitmap with inSampleSize set options.inJustDecodeBounds = false; return BitmapFactory.decodeResource(res, resId, options); } 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; int inSampleSize = 1; if (height > reqHeight || width > reqWidth) { if (width > height) { inSampleSize = Math.round((float)height / (float)reqHeight); } else { inSampleSize = Math.round((float)width / (float)reqWidth); } } return inSampleSize; }
方法三：使用内存缓存
对于缓存，没有大小或者规则适用于所有应用，它依赖于你分析自己应用的内存使用确定自己的方案。 缓存太小可能只会增加额外的内存使用，缓存太大可能会导致内存溢出或者应用其它模块可使用内存太小
private LruCache<String, Bitmap> mMemoryCache; @Override protected void onCreate(Bundle savedInstanceState) { ... // Get max available VM memory, exceeding this amount will throw an // OutOfMemory exception. Stored in kilobytes as LruCache takes an // int in its constructor. final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024); // Use 1/8th of the available memory for this memory cache. final int cacheSize = maxMemory / 8; mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap bitmap) { // The cache size will be measured in kilobytes rather than // number of items. return bitmap.getByteCount() / 1024; } }; ... } public void addBitmapToMemoryCache(String key, Bitmap bitmap) { if (getBitmapFromMemCache(key) == null) { mMemoryCache.put(key, bitmap); } } public Bitmap getBitmapFromMemCache(String key) { return mMemoryCache.get(key); }
加载压缩后的图片到ImageView显示
public void loadBitmap(int resId, ImageView imageView) { final String imageKey = String.valueOf(resId); final Bitmap bitmap = getBitmapFromMemCache(imageKey); if (bitmap != null) { mImageView.setImageBitmap(bitmap); } else { mImageView.setImageResource(R.drawable.image_placeholder); BitmapWorkerTask task = new BitmapWorkerTask(mImageView); task.execute(resId); } }
BitmapWorkerTask加载图片后，也要把图片缓存到内存中：
class BitmapWorkerTask extends AsyncTask { ... // Decode image in background. @Override protected Bitmap doInBackground(Integer... params) { final Bitmap bitmap = decodeSampledBitmapFromResource( getResources(), params[0], 100, 100)); addBitmapToMemoryCache(String.valueOf(params[0]), bitmap); return bitmap; } ... }
方法四：使用磁盘缓存
你的应用也有可能被其他任务打断，如电话呼入，应用在后台有可能会被结束，这样缓存的数据也会丢失。 当用户回到应用时，所有的图片还需要重新获取一遍。 磁盘缓存可应用到这种场景中，它可以减少你获取图片的次数，当然，从磁盘获取图片比从内存中获取要慢的多，所以它需要在非UI线程中完成。 示例代码中是磁盘缓存的一个实现，在Android4.0源码中（libcore/luni/src/main/java/libcore/io/DiskLruCache.java）， 有更加强大和推荐的一个实现，它的向后兼容使在已发布过的库中很方便使用它
private DiskLruCache mDiskCache; private static final int DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB private static final String DISK_CACHE_SUBDIR = "thumbnails"; @Override protected void onCreate(Bundle savedInstanceState) { ... // Initialize memory cache ... File cacheDir = getCacheDir(this, DISK_CACHE_SUBDIR); mDiskCache = DiskLruCache.openCache(this, cacheDir, DISK_CACHE_SIZE); ... } class BitmapWorkerTask extends AsyncTask { ... // Decode image in background. @Override protected Bitmap doInBackground(Integer... params) { final String imageKey = String.valueOf(params[0]); // Check disk cache in background thread Bitmap bitmap = getBitmapFromDiskCache(imageKey); if (bitmap == null) { // Not found in disk cache // Process as normal final Bitmap bitmap = decodeSampledBitmapFromResource( getResources(), params[0], 100, 100)); } // Add final bitmap to caches addBitmapToCache(String.valueOf(imageKey, bitmap); return bitmap; } ... } public void addBitmapToCache(String key, Bitmap bitmap) { // Add to memory cache as before if (getBitmapFromMemCache(key) == null) { mMemoryCache.put(key, bitmap); } // Also add to disk cache if (!mDiskCache.containsKey(key)) { mDiskCache.put(key, bitmap); } } public Bitmap getBitmapFromDiskCache(String key) { return mDiskCache.get(key); } // Creates a unique subdirectory of the designated app cache directory. Tries to use external // but if not mounted, falls back on internal storage. public static File getCacheDir(Context context, String uniqueName) { // Check if media is mounted or storage is built-in, if so, try and use external cache dir // otherwise use internal cache dir final String cachePath = Environment.getExternalStorageState() == Environment.MEDIA_MOUNTED || !Environment.isExternalStorageRemovable() ? context.getExternalCacheDir().getPath() : context.getCacheDir().getPath(); return new File(cachePath + File.separator + uniqueName); }
运行时的配置改变，例如屏幕横竖屏切换了有好的用户体验，你可能不想在这种情况下，重新获取一遍图片。
幸好你可以使用上面讲的内存缓存。缓存可以通过使用一个Fragment（调用setRetainInstance(true)被传到新的Activity，
当新的Activity被创建后，只需要重新附加Fragment，你就可以得到这个Fragment并访问到存在的缓存，把里面的图片快速的显示出来
private LruCache mMemoryCache; @Override protected void onCreate(Bundle savedInstanceState) { ... RetainFragment mRetainFragment = RetainFragment.findOrCreateRetainFragment(getFragmentManager()); mMemoryCache = RetainFragment.mRetainedCache; if (mMemoryCache == null) { mMemoryCache = new LruCache(cacheSize) { ... // Initialize cache here as usual } mRetainFragment.mRetainedCache = mMemoryCache; } ... } class RetainFragment extends Fragment { private static final String TAG = "RetainFragment"; public LruCache mRetainedCache; public RetainFragment() {} public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) { RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG); if (fragment == null) { fragment = new RetainFragment(); } return fragment; } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setRetainInstance(true); } }
