了解 Glide 的内存缓存与加载 gif 动态图片，可以从 DiskCacheStrategy 作为突破口，DiskCacheStrategy 的结果就是将大小变换后的结果保存为图像。
在这个过程中，GIF 有以下过程。
下载 GIF -> 提取帧 -> 优化帧大小 -> 保存帧

磁盘缓存策略（Disk Cache Strategy）

DiskCacheStrategy 方法应用到每一个单独的请求。 目前支持的策略允许你阻止加载过程使用或写入磁盘缓存，选择性地仅缓存无修改的原生数据，或仅缓存变换过的缩略图，或是兼而有之。

默认的策略叫做 AUTOMATIC，它会尝试对本地和远程图片使用最佳的策略。当你加载远程数据（比如，从URL下载）时，AUTOMATIC 策略仅会存储未被你的加载过程修改过(比如：变换，裁剪)的原始数据，因为下载远程数据相比调整磁盘上已经存在的数据要昂贵得多。对于本地数据，AUTOMATIC 策略则会仅存储变换过的缩略图，因为即使你需要再次生成另一个尺寸或类型的图片，取回原始数据也很容易。

为此，GIF 将 DiskCacheStrategy 指定为 RESOURCE，并引导它在从原始 GIF 文件中提取帧后立即使用。对于 GIF，应该使用 RESOURCE。 对于其他格式，如果你重复查看图像或检索（本地或远程）大图像并在小视图中显示它们，就可以指定为其他的 DiskCacheStrategy 方式，如果你倾向于查看一次图像，或者只应用一个小的转换，会倾向于 RESOURCE。

默认情况下，Glide 会在开始一个新的图片请求之前检查以下多级的缓存：

• 活动资源 (Active Resources) - 现在是否有另一个 View 正在展示这张图片？
• 内存缓存 (Memory cache) - 该图片是否最近被加载过并仍存在于内存中？
• 资源类型（Resource） - 该图片是否之前曾被解码、转换并写入过磁盘缓存？
• 数据来源 (Data) - 构建这个图片的资源是否之前曾被写入过文件缓存？
  前两步检查图片是否在内存中，如果是则直接返回图片。后两步则检查图片是否在磁盘上，以便快速但异步地返回图片。

如果四个步骤都未能找到图片，则Glide会返回到原始资源以取回数据（原始文件，Uri, Url等）。

Glide 的 GifDecoder 总是按设计将完整的 GIF 数据加载到内存中。它一次动态地解码每一个，但实际的 GIF 数据总是加载到内存中。

//Shared interface for GIF decoders.
public interface GifDecoder {

  /** File read status: No errors. */
  int STATUS_OK = 0;
  /** File read status: Error decoding file (may be partially decoded). */
  int STATUS_FORMAT_ERROR = 1;
  /** File read status: Unable to open source. */
  int STATUS_OPEN_ERROR = 2;
  /** Unable to fully decode the current frame. */
  int STATUS_PARTIAL_DECODE = 3;
  /** The total iteration count which means repeat forever. */
  int TOTAL_ITERATION_COUNT_FOREVER = 0;

  /** Android Lint annotation for status codes that can be used with a GIF decoder. */
  @Retention(RetentionPolicy.SOURCE)
  @IntDef(value = {STATUS_OK, STATUS_FORMAT_ERROR, STATUS_OPEN_ERROR, STATUS_PARTIAL_DECODE})
  @interface GifDecodeStatus {
  }

  /**
   * An interface that can be used to provide reused {@link android.graphics.Bitmap}s to avoid GCs
   * from constantly allocating {@link android.graphics.Bitmap}s for every frame.
   */
  interface BitmapProvider {
    /**
     * Returns an {@link Bitmap} with exactly the given dimensions and config.
     *
     * @param width  The width in pixels of the desired {@link android.graphics.Bitmap}.
     * @param height The height in pixels of the desired {@link android.graphics.Bitmap}.
     * @param config The {@link android.graphics.Bitmap.Config} of the desired {@link
     *               android.graphics.Bitmap}.
     */
    @NonNull
    Bitmap obtain(int width, int height, @NonNull Bitmap.Config config);

    /**
     * Releases the given Bitmap back to the pool.
     */
    void release(@NonNull Bitmap bitmap);

    /**
     * Returns a byte array used for decoding and generating the frame bitmap.
     *
     * @param size the size of the byte array to obtain
     */
    @NonNull
    byte[] obtainByteArray(int size);

    /**
     * Releases the given byte array back to the pool.
     */
    void release(@NonNull byte[] bytes);

    /**
     * Returns an int array used for decoding/generating the frame bitmaps.
     */
    @NonNull
    int[] obtainIntArray(int size);

    /**
     * Release the given array back to the pool.
     */
    void release(@NonNull int[] array);
  }

  int getWidth();

  int getHeight();

  @NonNull
  ByteBuffer getData();

  /**
   * Returns the current status of the decoder.
   *
   * <p> Status will update per frame to allow the caller to tell whether or not the current frame
   * was decoded successfully and/or completely. Format and open failures persist across frames.
   * </p>
   */
  @GifDecodeStatus
  int getStatus();

  /**
   * Move the animation frame counter forward.
   */
  void advance();

  /**
   * Gets display duration for specified frame.
   *
   * @param n int index of frame.
   * @return delay in milliseconds.
   */
  int getDelay(int n);

  /**
   * Gets display duration for the upcoming frame in ms.
   */
  int getNextDelay();

  /**
   * Gets the number of frames read from file.
   *
   * @return frame count.
   */
  int getFrameCount();

  /**
   * Gets the current index of the animation frame, or -1 if animation hasn't not yet started.
   *
   * @return frame index.
   */
  int getCurrentFrameIndex();

  /**
   * Resets the frame pointer to before the 0th frame, as if we'd never used this decoder to
   * decode any frames.
   */
  void resetFrameIndex();

  /**
   * Gets the "Netscape" loop count, if any. A count of 0 means repeat indefinitely.
   *
   * @deprecated Use {@link #getNetscapeLoopCount()} instead.
   *             This method cannot distinguish whether the loop count is 1 or doesn't exist.
   * @return loop count if one was specified, else 1.
   */
  @Deprecated
  int getLoopCount();

  /**
   * Gets the "Netscape" loop count, if any.
   * A count of 0 ({@link GifHeader#NETSCAPE_LOOP_COUNT_FOREVER}) means repeat indefinitely.
   * It must not be a negative value.
   * <br>
   * Use {@link #getTotalIterationCount()}
   * to know how many times the animation sequence should be displayed.
   *
   * @return loop count if one was specified,
   *         else -1 ({@link GifHeader#NETSCAPE_LOOP_COUNT_DOES_NOT_EXIST}).
   */
  int getNetscapeLoopCount();

  /**
   * Gets the total count
   * which represents how many times the animation sequence should be displayed.
   * A count of 0 ({@link #TOTAL_ITERATION_COUNT_FOREVER}) means repeat indefinitely.
   * It must not be a negative value.
   * <p>
   *     The total count is calculated as follows by using {@link #getNetscapeLoopCount()}.
   *     This behavior is the same as most web browsers.
   *     <table border='1'>
   *         <tr class='tableSubHeadingColor'><th>{@code getNetscapeLoopCount()}</th>
   *             <th>The total count</th></tr>
   *         <tr><td>{@link GifHeader#NETSCAPE_LOOP_COUNT_FOREVER}</td>
   *             <td>{@link #TOTAL_ITERATION_COUNT_FOREVER}</td></tr>
   *         <tr><td>{@link GifHeader#NETSCAPE_LOOP_COUNT_DOES_NOT_EXIST}</td>
   *             <td>{@code 1}</td></tr>
   *         <tr><td>{@code n (n > 0)}</td>
   *             <td>{@code n + 1}</td></tr>
   *     </table>
   * </p>
   *
   * @see <a href="https://bugs.chromium.org/p/chromium/issues/detail?id=592735#c5">Discussion about
   *      the iteration count of animated GIFs (Chromium Issue 592735)</a>
   *
   * @return total iteration count calculated from "Netscape" loop count.
   */
  int getTotalIterationCount();

  /**
   * Returns an estimated byte size for this decoder based on the data provided to {@link
   * #setData(GifHeader, byte[])}, as well as internal buffers.
   */
  int getByteSize();

  /**
   * Get the next frame in the animation sequence.
   *
   * @return Bitmap representation of frame.
   */
  @Nullable
  Bitmap getNextFrame();

  /**
   * Reads GIF image from stream.
   *
   * @param is containing GIF file.
   * @return read status code (0 = no errors).
   */
  @GifDecodeStatus
  int read(@Nullable InputStream is, int contentLength);

  void clear();

  void setData(@NonNull GifHeader header, @NonNull byte[] data);

  void setData(@NonNull GifHeader header, @NonNull ByteBuffer buffer);

  void setData(@NonNull GifHeader header, @NonNull ByteBuffer buffer, int sampleSize);

  /**
   * Reads GIF image from byte array.
   *
   * @param data containing GIF file.
   * @return read status code (0 = no errors).
   */
  @GifDecodeStatus
  int read(@Nullable byte[] data);


  /**
   * Sets the default {@link android.graphics.Bitmap.Config} to use when decoding frames of a GIF.
   *
   * <p>Valid options are {@link android.graphics.Bitmap.Config#ARGB_8888} and
   * {@link android.graphics.Bitmap.Config#RGB_565}.
   * {@link android.graphics.Bitmap.Config#ARGB_8888} will produce higher quality frames, but will
   * also use 2x the memory of {@link android.graphics.Bitmap.Config#RGB_565}.
   *
   * <p>Defaults to {@link android.graphics.Bitmap.Config#ARGB_8888}
   *
   * <p>This value is not a guarantee. For example if set to
   * {@link android.graphics.Bitmap.Config#RGB_565} and the GIF contains transparent pixels,
   * {@link android.graphics.Bitmap.Config#ARGB_8888} will be used anyway to support the
   * transparency.
   */
  void setDefaultBitmapConfig(@NonNull Bitmap.Config format);
}

image.png

//Set of available caching strategies for media.
public abstract class DiskCacheStrategy {
  ...省略
  //Writes resources to disk after they've been decoded.
  public static final DiskCacheStrategy RESOURCE =
      new DiskCacheStrategy() {
        @Override
        public boolean isDataCacheable(DataSource dataSource) {
          return false;
        }

        @Override
        public boolean isResourceCacheable(
            boolean isFromAlternateCacheKey, DataSource dataSource, EncodeStrategy encodeStrategy) {
          return dataSource != DataSource.RESOURCE_DISK_CACHE
              && dataSource != DataSource.MEMORY_CACHE;
        }

        @Override
        public boolean decodeCachedResource() {
          return true;
        }

        @Override
        public boolean decodeCachedData() {
          return false;
        }
      };
}

如果加载一个很大的 gif 文件，可以先 zip 该文件。GifHeaderParser.parseHeader() 就是 gif 的解析器.

/**
 * A class responsible for creating {@link com.bumptech.glide.gifdecoder.GifHeader}s from data
 * representing animated GIFs.
 *
 * @see <a href="https://www.w3.org/Graphics/GIF/spec-gif89a.txt">GIF 89a Specification</a>
 */
public class GifHeaderParser {
  ...省略
  public GifHeader parseHeader() {
    if (rawData == null) {
      throw new IllegalStateException("You must call setData() before parseHeader()");
    }
    if (err()) {
      return header;
    }

    readHeader();
    if (!err()) {
      readContents();
      if (header.frameCount < 0) {
        header.status = STATUS_FORMAT_ERROR;
      }
    }

    return header;
  }
}

//An animated Drawable that plays the frames of an animated GIF.
class GifDrawable extends Drawable implements GifFrameLoader.FrameCallback, Animatable
, Animatable2Compat

缓存的刷新

因为磁盘缓存使用的是哈希键，所以并没有一个比较好的方式来简单地删除某个特定url或文件路径对应的所有缓存文件。如果你只允许加载或缓存原始图片的话，问题可能会变得更简单，但因为Glide还会缓存缩略图和提供多种变换(transformation)，它们中的任何一个都会导致在缓存中创建一个新的文件，而要跟踪和删除一个图片的所有版本无疑是困难的。

在实践中，使缓存文件无效的最佳方式是在内容发生变化时（url，uri，文件路径等）更改你的标识符。