Android高效安全加载图片的方法详解

Creates Bitmap objects from various sources, including files, streams,and byte-arrays.

 /**
 * Synonym for opening the given resource and calling
 * {@link #decodeResourceStream}.
 *
 * @param res The resources object containing the image data
 * @param id The resource id of the image data
 * @param opts null-ok; Options that control downsampling and whether the
 *  image should be completely decoded, or just is size returned.
 * @return The decoded bitmap, or null if the image data could not be
 *  decoded, or, if opts is non-null, if opts requested only the
 *  size be returned (in opts.outWidth and opts.outHeight)
 * @throws IllegalArgumentException if {@link BitmapFactory.Options#inPreferredConfig}
 *  is {@link android.graphics.Bitmap.Config#HARDWARE}
 *  and {@link BitmapFactory.Options#inMutable} is set, if the specified color space
 *  is not {@link ColorSpace.Model#RGB RGB}, or if the specified color space's transfer
 *  function is not an {@link ColorSpace.Rgb.TransferParameters ICC parametric curve}
 */
 public static Bitmap decodeResource(Resources res, int id, Options opts) {

 /**
 * If set to true, the decoder will return null (no bitmap), but
 * the <code>out...</code> fields will still be set, allowing the caller to
 * query the bitmap without having to allocate the memory for its pixels.
 */
 public boolean inJustDecodeBounds;

 BitmapFactory.Options options = new BitmapFactory.Options();
 // 指定在解析图片文件时，仅仅解析边缘信息而不创建 bitmap 对象。
 options.inJustDecodeBounds = true;
 // R.drawable.test 是使用的 2560x1920 的测试图片资源文件。
 BitmapFactory.decodeResource(getResources(), R.drawable.test, options);
 int width = options.outWidth;
 int height = options.outHeight;
 Log.i(TAG, "width: " + width + ", height: " + height);

 /**
 * If set to a value > 1, requests the decoder to subsample the original
 * image, returning a smaller image to save memory. The sample size is
 * the number of pixels in either dimension that correspond to a single
 * pixel in the decoded bitmap. For example, inSampleSize == 4 returns
 * an image that is 1/4 the width/height of the original, and 1/16 the
 * number of pixels. Any value <= 1 is treated the same as 1. Note: the
 * decoder uses a final value based on powers of 2, any other value will
 * be rounded down to the nearest power of 2.
 */
 public int inSampleSize;

 /**
 * @param originWidth the width of the origin bitmap
 * @param originHeight the height of the origin bitmap
 * @param desWidth the max width of the desired bitmap
 * @param desHeight the max height of the desired bitmap
 * @return the optimal sample size to make sure the size of bitmap is not more than the desired.
 */
 public static int calculateSampleSize(int originWidth, int originHeight, int desWidth, int desHeight) {
 int sampleSize = 1;
 int width = originWidth;
 int height = originHeight;
 while((width / sampleSize) > desWidth && (height / sampleSize) > desHeight) {
  sampleSize *= 2;
 }
 return sampleSize;
 }

 int sampleSize = BitmapCompressor.calculateSampleSize(2560, 1920, 100, 100);
 Log.i(TAG, "sampleSize: " + sampleSize);

 public static Bitmap compressBitmapResource(Resources res, int resId, int inSampleSize) {
 BitmapFactory.Options options = new BitmapFactory.Options();
 options.inJustDecodeBounds = false;
 options.inSampleSize = inSampleSize;
 return BitmapFactory.decodeResource(res, resId, options);
 }

 /**
 * BitmapRegionDecoder can be used to decode a rectangle region from an image.
 * BitmapRegionDecoder is particularly useful when an original image is large and
 * you only need parts of the image.
 *
 * <p>To create a BitmapRegionDecoder, call newInstance(...).
 * Given a BitmapRegionDecoder, users can call decodeRegion() repeatedly
 * to get a decoded Bitmap of the specified region.
 *
 */
 public final class BitmapRegionDecoder { ... }

 /**
 * Create a BitmapRegionDecoder from an input stream.
 * The stream's position will be where ever it was after the encoded data
 * was read.
 * Currently only the JPEG and PNG formats are supported.
 *
 * @param is The input stream that holds the raw data to be decoded into a
 *  BitmapRegionDecoder.
 * @param isShareable If this is true, then the BitmapRegionDecoder may keep a
 *   shallow reference to the input. If this is false,
 *   then the BitmapRegionDecoder will explicitly make a copy of the
 *   input data, and keep that. Even if sharing is allowed,
 *   the implementation may still decide to make a deep
 *   copy of the input data. If an image is progressively encoded,
 *   allowing sharing may degrade the decoding speed.
 * @return BitmapRegionDecoder, or null if the image data could not be decoded.
 * @throws IOException if the image format is not supported or can not be decoded.
 *
 * <p class="note">Prior to {@link android.os.Build.VERSION_CODES#KITKAT},
 * if {@link InputStream#markSupported is.markSupported()} returns true,
 * <code>is.mark(1024)</code> would be called. As of
 * {@link android.os.Build.VERSION_CODES#KITKAT}, this is no longer the case.</p>
 */
 public static BitmapRegionDecoder newInstance(InputStream is,
  boolean isShareable) throws IOException { ... }

 /**
 * Decodes a rectangle region in the image specified by rect.
 *
 * @param rect The rectangle that specified the region to be decode.
 * @param options null-ok; Options that control downsampling.
 *  inPurgeable is not supported.
 * @return The decoded bitmap, or null if the image data could not be
 *  decoded.
 * @throws IllegalArgumentException if {@link BitmapFactory.Options#inPreferredConfig}
 *  is {@link android.graphics.Bitmap.Config#HARDWARE}
 *  and {@link BitmapFactory.Options#inMutable} is set, if the specified color space
 *  is not {@link ColorSpace.Model#RGB RGB}, or if the specified color space's transfer
 *  function is not an {@link ColorSpace.Rgb.TransferParameters ICC parametric curve}
 */
 public Bitmap decodeRegion(Rect rect, BitmapFactory.Options options) { ... }

 // 解析得到原图的长宽值，方便后面进行局部显示时指定需要显示的区域。
 BitmapFactory.Options options = new BitmapFactory.Options();
 options.inJustDecodeBounds = true;
 BitmapFactory.decodeResource(getResources(), R.drawable.test, options);
 int width = options.outWidth;
 int height = options.outHeight;

 try {
 // 创建局部解析器 
 InputStream inputStream = getResources().openRawResource(R.drawable.test);
 BitmapRegionDecoder decoder = BitmapRegionDecoder.newInstance(inputStream,false);
 
 // 指定需要显示的矩形区域，这里要显示的原图的左上 1/4 区域。
 Rect rect = new Rect(0, 0, width / 2, height / 2);

 // 创建位图配置，这里使用 RGB_565，每个像素占 2 字节。
 BitmapFactory.Options regionOptions = new BitmapFactory.Options();
 regionOptions.inPreferredConfig = Bitmap.Config.RGB_565;
 
 // 创建得到指定区域的 Bitmap 对象并进行显示。
 Bitmap regionBitmap = decoder.decodeRegion(rect,regionOptions);
 ImageView imageView = (ImageView) findViewById(R.id.main_image);
 imageView.setImageBitmap(regionBitmap);
 } catch (Exception e) {
 e.printStackTrace();
 }

/**
 * A cache that holds strong references to a limited number of values. Each time
 * a value is accessed, it is moved to the head of a queue. When a value is
 * added to a full cache, the value at the end of that queue is evicted and may
 * become eligible for garbage collection.
 */
public class LruCache<K, V> { ... }

 // 获得进程可以使用的最大内存量
 int maxMemory = (int) Runtime.getRuntime().maxMemory();
 
 mCache = new LruCache<String, Bitmap>(maxMemory / 4) {
  @Override
  protected int sizeOf(String key, Bitmap value) {
   return value.getByteCount();
  }
 };

 public Bitmap get(String key) {
  Bitmap bitmap = mCache.get(key);
  if (bitmap != null) {
   return bitmap;
  } else {
   new BitmapAsyncTask().execute(key);
   return null;
  }
 }

 private class BitmapAsyncTask extends AsyncTask<String, Void, Bitmap> {
  @Override
  protected Bitmap doInBackground(String... url) {
   Bitmap bitmap = getBitmapFromUrl(url[0]);
   if (bitmap != null) {
    mCache.put(url[0],bitmap);
   }
   return bitmap;
  }

  private Bitmap getBitmapFromUrl(String url) {
   Bitmap bitmap = null;
   // 在这里要利用给定的 url 信息从网络获取 bitmap 信息.
   return bitmap;
  }
 }