+
+
+ +

IGListDiffable and Equality

+ +

This guide details how to write a good isEqual: method. IGListKit requires that models implement the method isEqualToDiffableObject: which should perform the same type of check, but without impacting behavior in Objective-C containers like NSDictionary and NSSet.

+

IGListDiffable bare minimum

+ +

The quickest way to get started with diffable models is use the object itself as the identifier, and use the super -[NSObject isEqual:] implementation for equality:

+
- (id<NSObject>)diffIdentifier {
+  return self;
+}
+
+- (BOOL)isEqualToDiffableObject:(id<IGListDiffable>)object {
+  return [self isEqual:object];
+}
+
+

Writing better Equality methods

+ +

Even though IGListKit uses the method isEqualToDiffableObject:, the concepts of writing a good equality check apply in general.

+ +

Here are the basics to writing good -isEqual: and -hash functions. Note this is all ObjC but translates well to Swift.

+ +
    +
  • If you override -isEqual: you must override -hash + +
    • +
  • Always compare the pointer first + +
      +
    • Saves a lot of wasteful objc_msgSend(...) and value comparisons if checking the same instance
      • +
    • +
  • When comparing object values, always check for nil before -isEqual: + +
      +
    • e.g. [nil isEqual:nil] returns NO
      • +
    • Instead do left == right || [left isEqual:right]
      • +
    • +
  • Always compare the cheapest values first + +
      +
    • Doing [self.array isEqual:other.array] && self.intVal == other.array is hella wasteful if the intVals are different. Use lazy eval!
      • +
    • +
+ +

As an example, if I had a User model with the following interface:

+
@interface User : NSObject
+@property NSInteger pk;
+@property NSString *name;
+@property NSArray *posts;
+@end
+
+ +

You would implement its equality methods like so:

+
@implementation User
+
+- (NSUInteger)hash {
+  return self.pk;
+}
+
+- (BOOL)isEqual:(id)object {
+  if (self == object) return YES;
+  if (![object isKindOfClass:[User class]]) return NO;
+
+  User *right = object;
+  return self.pk == right.pk 
+  && (self.name == right.name || [self.name isEqual:right.name])
+  && (self.posts == right.posts || [self.posts isEqualToArray:right.posts]);
+}
+
+@end
+
+

Using both IGListDiffable and isEqual

+ +

Making your objects work universally with Objective-C containers and IGListKit is really easy once you’ve implemented isEqual: and hash.

+
@interface User (IGListDiffable) <IGListDiffable>
+@end
+
+@implementation User (IGListDiffable)
+
+- (id<NSObject>)diffIdentifier {
+  return @(self.pk);
+}
+
+- (BOOL)isEqualToDiffableObject:(id<IGListDiffable>)object {
+  return [self isEqual:object];
+}
+
+@end
+
+ +
+
+ +