決策模式: DiskCacheStrategy of Glide

Glide is a fast and efficient open source media management and image loading framework for Android that wraps media decoding, memory and disk caching, and resource pooling into a simple and easy to use interface.

這次要從 Glide 的硬體快取決策去看決策模式的應用。

決策模式提供了當情況的變化在於算法細節時，我們可以進行的封裝方式。

下面是抽象類別 DiskCacheStrategy 的介面
跟他提供了五種不同的決策 (為了方便閱讀我把實作細節都拿掉了)

• ALL : 全部圖像包含尺寸變換都儲存快取

• NONE : 不作儲存快取

• DATA : 在解碼前儲存（只存遠端資料）

• RESOURCE : 在解碼後儲存 （包含本地端跟遠端）

• AUTOMATIC : 自動判斷是否該儲存快取 (預設值)

下面的五種決策類宣告成 static final，讓編譯器在編譯時會創建他們的實體，
在整個使用上變得更加容 易。

public abstract class DiskCacheStrategy {

 

  public static final DiskCacheStrategy ALL = new DiskCacheStrategy() {

    public boolean isDataCacheable(DataSource dataSource) {}

    public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,EncodeStrategy encodeStrategy) {}

    public boolean decodeCachedResource() {}

    public boolean decodeCachedData() {}

  };



  public static final DiskCacheStrategy NONE = new DiskCacheStrategy() {

    public boolean isDataCacheable(DataSource dataSource) {}

    public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,EncodeStrategy encodeStrategy) {}

    public boolean decodeCachedResource() {}

    public boolean decodeCachedData() {}

  };



  public static final DiskCacheStrategy DATA = new DiskCacheStrategy() {

    public boolean isDataCacheable(DataSource dataSource) {}

    public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,EncodeStrategy encodeStrategy) {}

    public boolean decodeCachedResource() {}

    public boolean decodeCachedData() {}

  };



  public static final DiskCacheStrategy RESOURCE = new DiskCacheStrategy() {

    public boolean isDataCacheable(DataSource dataSource) {}

    public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,EncodeStrategy encodeStrategy) {}

    public boolean decodeCachedResource() {}

    public boolean decodeCachedData() {}

  };



  public static final DiskCacheStrategy AUTOMATIC = new DiskCacheStrategy() {

    public boolean isDataCacheable(DataSource dataSource) {}

    public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,EncodeStrategy encodeStrategy) {}

    public boolean decodeCachedResource() {}

    public boolean decodeCachedData() {}

  };

 // the interface 

  public abstract boolean isDataCacheable(DataSource dataSource);

  public abstract boolean isResourceCacheable(boolean isFromAlternateCacheKey,

      DataSource dataSource, EncodeStrategy encodeStrategy);

  public abstract boolean decodeCachedResource();

  public abstract boolean decodeCachedData();

}

藉由封裝進 RequestOptions 讓使用者可以自由改變想要的策略

在資源解碼時使用
下方是類 DecodeJob 部分源碼

// Decide the stage

private Stage getNextStage(Stage current) {

    switch (current) {

      case INITIALIZE:

        return diskCacheStrategy.decodeCachedResource()

            ? Stage.RESOURCE_CACHE : getNextStage(Stage.RESOURCE_CACHE);

      case RESOURCE_CACHE:

        return diskCacheStrategy.decodeCachedData()

            ? Stage.DATA_CACHE : getNextStage(Stage.DATA_CACHE);

      case DATA_CACHE:

        // Skip loading from source if the user opted to only retrieve the resource from cache.

        return onlyRetrieveFromCache ? Stage.FINISHED : Stage.SOURCE;

      case SOURCE:

      case FINISHED:

        return Stage.FINISHED;

      default:

        throw new IllegalArgumentException("Unrecognized stage: " + current);

    }

  }



@Synthetic

@NonNull

Resource onResourceDecoded(DataSource dataSource, @NonNull Resource decoded) {

    ...



    Resource result = transformed;

    boolean isFromAlternateCacheKey = !decodeHelper.isSourceKey(currentSourceKey);

    if (diskCacheStrategy.isResourceCacheable(isFromAlternateCacheKey, dataSource,

        encodeStrategy)) {

         ...

      }

      LockedResource lockedResult = LockedResource.obtain(transformed);

      deferredEncodeManager.init(key, encoder, lockedResult);

      result = lockedResult;

    }

    return result;

}

簡易架構圖

Glide.with(context)

         .load(imageUrl)

         .apply(RequestOptions()

            .diskCacheStrategy(DiskCacheStrategy.RESOURCE))

         .into(imageView)

基本使用 Glide 流程圖（Create）


針對創建 Target 部分的流程圖（Start）


更加詳細版
*下方類別圖只畫了部分相關的類別