Add ExternalCache that falls back to InternalCache if not available.

library/src/main/java/com/bumptech/glide/load/engine/cache/ExternalCacheDiskCacheFactory.java

@@ -8,7 +8,10 @@
  * disk cache directory.
  *
  * <p><b>Images can be read by everyone when using external disk cache.</b>
+ *
+ * @deprecated use {@link ExternalPreferredCacheDiskCacheFactory} instead.
  */
+@Deprecated
 public final class ExternalCacheDiskCacheFactory extends DiskLruCacheFactory {
 
   public ExternalCacheDiskCacheFactory(Context context) {

library/src/main/java/com/bumptech/glide/load/engine/cache/ExternalPreferredCacheDiskCacheFactory.java

@@ -0,0 +1,64 @@
+package com.bumptech.glide.load.engine.cache;
+
+import android.content.Context;
+
+import android.support.annotation.Nullable;
+import java.io.File;
+
+/**
+ * Creates an {@link com.bumptech.glide.disklrucache.DiskLruCache} based disk cache in the external
+ * disk cache directory, which falls back to the internal disk cache if no external storage is
+ * available. If ever fell back to the internal disk cache, will use that one from that moment on.
+ *
+ * <p><b>Images can be read by everyone when using external disk cache.</b>
+ */
+public final class ExternalPreferredCacheDiskCacheFactory extends DiskLruCacheFactory {
+
+  public ExternalPreferredCacheDiskCacheFactory(Context context) {
+    this(context, DiskCache.Factory.DEFAULT_DISK_CACHE_DIR,
+        DiskCache.Factory.DEFAULT_DISK_CACHE_SIZE);
+  }
+
+  public ExternalPreferredCacheDiskCacheFactory(Context context, int diskCacheSize) {
+    this(context, DiskCache.Factory.DEFAULT_DISK_CACHE_DIR, diskCacheSize);
+  }
+
+  public ExternalPreferredCacheDiskCacheFactory(final Context context, final String diskCacheName,
+                                                final int diskCacheSize) {
+    super(new CacheDirectoryGetter() {
+      @Nullable
+      private File getInternalCacheDirectory() {
+        File cacheDirectory = context.getCacheDir();
+        if (cacheDirectory == null) {
+          return null;
+        }
+        if (diskCacheName != null) {
+          return new File(cacheDirectory, diskCacheName);
+        }
+        return cacheDirectory;
+      }
+
+      @Override
+      public File getCacheDirectory() {
+        File internalCacheDirectory = getInternalCacheDirectory();
+
+        // Already used internal cache, so keep using that one,
+        // thus avoiding using both external and internal with transient errors.
+        if ((null != internalCacheDirectory) && internalCacheDirectory.exists()) {
+          return internalCacheDirectory;
+        }
+
+        File cacheDirectory = context.getExternalCacheDir();
+
+        // Shared storage is not available.
+        if (cacheDirectory == null) {
+          return internalCacheDirectory;
+        }
+        if (diskCacheName != null) {
+          return new File(cacheDirectory, diskCacheName);
+        }
+        return cacheDirectory;
+      }
+    }, diskCacheSize);
+  }
+}