with and then for UIKit and SwiftUI

with.swift

/// Perform an immediate mutation of `subject`. The `transform` function may
/// just mutate the given `subject` or replace it entirely.
///
/// - Parameters:
///     - subject: A value to be transformed.
///     - transform: A closure that mutates or replaces the `subject`.
@inlinable func with<T>(_ subject: T, _ transform: (_ subject: inout T) throws -> Void) rethrows -> T {
    var subject = subject
    try transform(&subject)
    return subject
}

// We use a global with, so the object-based one is called `then`.
protocol Thenable { /* no requirements */ }

extension Thenable {
    /// Create a copy (if a struct) or use same object (if class), improving chainability e.g. after init method.
    /// Example:
    /// struct Foo: Thenable {
    ///   var bar: Int = 0
    ///   var baz: Bool = false
    /// }
    ///
    /// Make a copy of foo, overriding an arbitrary subset of properties
    /// let foo2 = foo.then { $0.bar = 7; $0.baz = true }
    ///
    /// Example then classes. This is especially useful if you want to use the auto return:
    ///
    /// func getSettingsController() -> DocumentSettingsViewController {
    ///     DocumentSettingsViewController(style: UITableView.insetGroupStyle).then {
    ///        $0.title = localizedString(.Document_Presentation)
    ///        $0.pdfViewController = getPDFViewController()
    ///         $0.showsAllSettingsOptions = true
    ///     }
    /// }
    @discardableResult @inlinable func then(_ config: (inout Self) throws -> Void) rethrows -> Self {
        return try with(self, config)
    }
}

// Adopt `then` for any NSObject.
extension NSObject: Thenable { }

@available(iOS 13.0, *)
extension View {
    /// Helper to simplify the copy, mutate and return pattern.
    func then(_ body: (inout Self) -> Void) -> Self {
        with(self, body)
    }
}