With

With is a Swift micro-library that provides a with statement, akin to 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)
lethitTestOptions=with([SCNHitTestOption : Any]()){$0[.boundingBoxOnly]=true$0[.rootNode]= _myRootNode
if #available(iOS 11.0, tvOS 11.0, macOS 10.13,*){$0[.searchMode]=SCNHitTestSearchMode.all.rawValue
}else{$0[.sortResults]=true$0[.firstFoundOnly]=false}}

Or like this:

// initializes the object-type `newButton` with title, sizing, styling, etc.
// and adds the view to a superview
letnewButton=with(UIButton(type:.system)){
$0.titleLabel!.font =.systemFont(ofSize:13)
$0.setTitle("My Button", for:.normal)
$0.autoresizingMask =[.flexibleLeftMargin,.flexibleBottomMargin ]
$0.contentEdgeInsets =UIEdgeInsets(top:6.5, left:7, bottom:6.5, right:7)with($0.layer){ layer in
layer.borderWidth =1.0
layer.borderColor =UIColor.white.cgColor
layer.cornerRadius =5}
$0.sizeToFit()
rootViewController.view.addSubview($0)}

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
letdateString=withMap(DateFormatter()){
$0.dateStyle =.medium
$0.timeStyle =.none
$0.locale =Locale(identifier:"en_US")letcurrentDate=Date()return $0.string(from: currentDate)}

Alternatives

Swift natively supports immediately-called closures, so it's possible to write, for instance, the DateFormatter example above as this without the need for with/withMap:

letdateString={
$0.dateStyle =.medium
$0.timeStyle =.none
$0.locale =Locale(identifier:"en_US")letcurrentDate=Date()return $0.string(from: currentDate)}((DateFormatter()))

However, this puts the operations out of order of how they are executed— the DateFormatter() construction is the last line but happens first, then the closure above it is executed. I prefer With's top-to-bottom ordering stylistically.