With

With is a Swift micro-library that provides a with statement— which is modeled after the with functionality found in Python, JavaScript, Visual Basic, Object Pascal, Delphi; using found in C#; and also/let found in Kotlin.

With provides a set of overloaded generic free functions that are useful for:

• Object or value initialization and setup in a declarative style (think Swift UI’s hierarchical style, but for anything, anywhere).
• Performing multiple operations on an object/value fetched via a method/property, while staying D.R.Y and without needing to create a local var (while still avoiding invoking the method/getter repeatedly).
• Performing calculations with an object/value that only needs to live long enough to be configured and do some calc (and you’re only interested in a result object/value).

With provides a free function func with(_ subject: SubjectT, operations: (inout SubjectT) -> Void) -> SubjectT that can be used on any object or value type to do stuff like this:

// initializes a value-type `hitTestOptions` dictionary  for use with
//   `SCNView`'s `hitTest(…)` with the desired options some of which have
//   changed in newer OS versions (which the standard dictionary literal syntax
//   can't cleanly do)
let hitTestOptions = with([SCNHitTestOption : Any]()) { o in
    o[.boundingBoxOnly] = true
    o[.rootNode] = _myRootNode
    if #available(iOS 11.0, tvOS 11.0, macOS 10.13, *) {
        o[.searchMode] = SCNHitTestSearchMode.all.rawValue
    } else {
        o[.sortResults] = true
        o[.firstFoundOnly] = false
    }
}

Or like this:

// initializes the object-type `newButton` with title, sizing, styling, etc.
//   and adds the view to a superview
let newButton = with(UIButton(type: .system)) { b in
    b.titleLabel!.font = .systemFont(ofSize: 13)
    b.setTitle("My Button", for: .normal)
    b.autoresizingMask = [ .flexibleLeftMargin, .flexibleBottomMargin ]
    b.contentEdgeInsets = UIEdgeInsets(top: 6.5, left: 7, bottom: 6.5, right: 7)
    with(b.layer) { l in
        l.borderWidth = 1.0
        l.borderColor = UIColor.white.cgColor
        l.cornerRadius = 5
    }
    b.sizeToFit()
    
    rootViewController.view.addSubview(b)
}

With also has an alternate func withMap(_ subject: SubjectT, operations: (inout SubjectT) -> ReturnT) -> SubjectT form that can return an arbitrary value from the closure (instead of the value passed in):

// initializes a `DateFormatter`, configures it, and uses it to calculate a
//   `String` which is the only thing we want to hang onto
let dateString = withMap(DateFormatter()) { f in
    f.dateStyle = .medium
    f.timeStyle = .none
    f.locale = Locale(identifier: "en_US")
    
    let currentDate = Date()
    return f.string(from: currentDate)
}