ASImageNode backing store sharing for memory and CPU reduction (#1974)

AsyncDisplayKit/Private/ASWeakMap.h

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+//
+//  ASWeakMap.h
+//  AsyncDisplayKit
+//
+//  Created by Chris Danford on 7/11/16.
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+
+/**
+ * This class is used in conjunction with ASWeakMap.  Instances of this type are returned by an ASWeakMap,
+ * must retain this value for as long as they want the entry to exist in the map.
+ */
+@interface ASWeakMapEntry<Value> : NSObject
+
+@property (nonatomic, retain, readonly) Value value;
+
+@end
+
+
+/**
+ * This is not a full-featured map.  It does not support features like `count` and FastEnumeration because there
+ * is not currently a need.
+ *
+ * This is a map that does not retain keys or values added to it.  When both getting and setting, the caller is
+ * returned a ASWeakMapEntry and must retain it for as long as it wishes the key/value to remain in the map.
+ * We return a single Entry value to the caller to avoid two potential problems:
+ *
+ * 1) It's easier for callers to retain one value (the Entry) and not two (a key and a value).
+ * 2) Key values are tested for `isEqual` equality.  If if a caller asks for a key "A" that is equal to a key "B"
+ *    already in the map, then we need the caller to retain key "B" and not key "A".  Returning an Entry simplifies
+ *    the semantics.
+ *
+ * The underlying storage is a hash table and the Key type should implement `hash` and `isEqual:`.
+ */
+@interface ASWeakMap<__covariant Key : NSObject *, Value> : NSObject
+
+/**
+ * Read from the cache.  The Value object is accessible from the returned ASWeakMapEntry.
+ */
+- (nullable ASWeakMapEntry<Value> *)entryForKey:(Key)key;
+
+/**
+ * Put a value into the cache.  If an entry with an equal key already exists, then the value is updated on the existing entry.
+ */
+- (ASWeakMapEntry<Value> *)setObject:(Value)value forKey:(Key)key;
+
+@end
+
+
+NS_ASSUME_NONNULL_END