Using .allSatisfy in Swift

You'll often find yourself in a position where you need to check if a particular condition applies across a collection of objects. Luckily, Swift provides the allSatisfy() method to handle just that.

Let's say we want to write a function to verify that a student has completed all of the requirements for graduation.

Initially, you might write something like this:

struct Prerequisite {
    var isCompleted: Bool
    var courseName: String
}

struct Student {
    let degreeRequirements = [
        Prerequisite(isCompleted: true, 
        	courseName: "Introduction to Programming"),
        Prerequisite(isCompleted: true, 
        	courseName: "Discrete Mathematics"),
        Prerequisite(isCompleted: true, 
        	courseName: "Mathematics for Algorithms and Systems"),
        Prerequisite(isCompleted: true, 
        	courseName: "Advanced Data Structures"),
        Prerequisite(isCompleted: true, 
        	courseName: "Theory of Computability"),
        Prerequisite(isCompleted: true, 
        	courseName: "Ubiquitous Computing")
    ]

    func canGraduate() -> Bool {
        for requirement in degreeRequirements {
            if !requirement.isCompleted {
                return false
            }
        }

        return true
    }
}

Or, you might get fancy and use filter instead:

func canGraduate() -> Bool {
    // Filters by failed prerequisites
    // If the list is empty, the user can graduate
    degreeRequirements.filter { $0.isCompleted == false }.isEmpty
}

While these approaches all certainly work, we can simplify this expression even further using the underutilized allSatisfy function:

func canGraduate() -> Bool {
    // Only returns true if all items in degreeRequirements
    // satisfy the requirement
    degreeRequirements.allSatisfy { $0.isCompleted }
}

This approach is far more readable than the previous option, and anyone reading this code can immediately see the exact requirements that will cause this function to return true.

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