KVC Collection Operators

KVC Collection Operators allows actions to be performed on a collection using key path notation in valueForKeyPath:. Any time you see @ in a key path, it denotes a particular aggregate function whose result can be returned or chained, just like any other key path.

→ nshipster.com/kvc-collection-operators/

The article discusses Key-Value Coding (KVC) Collection Operators in Objective-C, which provide a concise, Ruby-like syntax for performing aggregate operations on collections using valueForKeyPath:.

These operators, denoted by @ in key paths, fall into three categories:

• Simple Collection Operators (@count, @sum, @avg, @max, @min) return scalars like numbers or dates (e.g., [products valueForKeyPath:@"@avg.price"] yields 881.50);
• Object Operators (@unionOfObjects, @distinctUnionOfObjects) return arrays of property values, with the latter removing duplicates;
• Array and Set Operators (@distinctUnionOfArrays, @unionOfArrays, @distinctUnionOfSets) combine values from nested collections

Using a Product class example, the article demonstrates how operators automatically handle scalar boxing into NSNumber or NSValue. It also highlights a custom DSL extension by swizzling valueForKeyPath: to support complex queries, though advises using NSPredicate for clarity.

Written before Swift’s dominance, the article showcases Objective-C’s expressive power, with Swift’s modern syntax enhancing similar KVC patterns for efficient collection manipulation.

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