--- /dev/null
+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+// TODO: Remove this once Auth moves over to Core's instance registration system.
+/** @class FIRAppAssociationRegistration
+ @brief Manages object associations as a singleton-dependent: At most one object is
+ registered for any given host/key pair, and the object shall be created on-the-fly when
+ asked for.
+ */
+@interface FIRAppAssociationRegistration<ObjectType> : NSObject
+
+/** @fn registeredObjectWithHost:key:creationBlock:
+ @brief Retrieves the registered object with a particular host and key.
+ @param host The host object.
+ @param key The key to specify the registered object on the host.
+ @param creationBlock The block to return the object to be registered if not already.
+ The block is executed immediately before this method returns if it is executed at all.
+ It can also be executed multiple times across different method invocations if previous
+ execution of the block returns @c nil.
+ @return The registered object for the host/key pair, or @c nil if no object is registered
+ and @c creationBlock returns @c nil.
+ @remarks The method is thread-safe but non-reentrant in the sense that attempting to call this
+ method again within the @c creationBlock with the same host/key pair raises an exception.
+ The registered object is retained by the host.
+ */
++ (nullable ObjectType)registeredObjectWithHost:(id)host
+ key:(NSString *)key
+ creationBlock:(ObjectType _Nullable (^)(void))creationBlock;
+
+@end
+
+NS_ASSUME_NONNULL_END