如何编写适配Array、Set和Dictionary的Swift Sequence扩展？

Hey there! Let's work through your two issues step by step to build an extension that plays nicely with all three collection types you mentioned.

Issue 1: Replacing the hardcoded variable with self #

Your original code references a specific numbers variable, but in a protocol extension, you need to use self to refer to the current instance of the sequence. We also need to add a type constraint—since isMultiple(of:) is a method for integer types, we’ll restrict the extension to sequences whose elements conform to BinaryInteger.

Here’s the fixed base extension that works for Array and Set:

extension Sequence where Element: BinaryInteger {
    var isAllEven: Bool {
        self.allSatisfy { $0.isMultiple(of: 2) }
    }
}

// Test with Set
let numberSet = Set([4, 8, 16])
print(numberSet.isAllEven) // Output: true

// Test with Array
let numberArray = [2, 4, 6]
print(numberArray.isAllEven) // Output: true

Issue 2: Adapting for Dictionary #

Dictionary’s Element type is a tuple (key: Key, value: Value), so we can’t reuse the exact same isAllEven property—we need to clarify whether we’re checking keys, values, or make the extension flexible enough for both.

Option 1: Add a dedicated property for Dictionary (check values) #

If your goal is to verify all values in a Dictionary are even, extend Dictionary with a constrained property:

extension Dictionary where Value: BinaryInteger {
    var isAllValuesEven: Bool {
        self.values.allSatisfy { $0.isMultiple(of: 2) }
    }
}

// Test with Dictionary
let numberDict = ["a": 2, "b": 4, "c": 6]
print(numberDict.isAllValuesEven) // Output: true

Option 2: Build a flexible generic method #

For a more versatile solution that works for all sequences (including Dictionary, where you can pick which part to check), use a generic method with key paths:

extension Sequence {
    func allElementsSatisfyEven<T: BinaryInteger>(_ keyPath: KeyPath<Element, T>) -> Bool {
        self.allSatisfy { $0[keyPath: keyPath].isMultiple(of: 2) }
    }
}

// Use with Array/Set
let intSet = Set([4, 8, 16])
print(intSet.allElementsSatisfyEven(\.self)) // Output: true

// Use with Dictionary (check values)
let valueDict = ["x": 2, "y": 4]
print(valueDict.allElementsSatisfyEven(\.value)) // Output: true

// Use with Dictionary (check integer keys)
let intKeyDict = [2: "apple", 4: "banana"]
print(intKeyDict.allElementsSatisfyEven(\.key)) // Output: true

This approach lets you target any integer property of the sequence’s elements, making it perfect for Dictionary’s key-value tuples.

内容的提问来源于stack exchange，提问作者Swantewit