关于android 图片加载优化

一时失言乱红尘 2022-07-26 01:45 171阅读 0赞

原文地址：[http://blog.csdn.net/yudajun/article/details/9323941][http_blog.csdn.net_yudajun_article_details_9323941]

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);  
            }  
        }

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][http_blog.csdn.net_yudajun_article_details_9323941]

[http_blog.csdn.net_yudajun_article_details_9323941]: http://blog.csdn.net/yudajun/article/details/9323941