A simple expression watcher for Swift apps

gistfile1.txt

// Sometimes you want to watch a particular expression over time. Continually dumping
// the value in a loop can be painful because of the sheer volume of values. This snippet
// loops on a background thread checking the result of the passed expression - if the 
// value changes (and only then) does it output the new value. Note that this is NOT meant
// as a replacement for a KVO-observable property... it is better suited to expressions
// that do not post KVO events.
//
// Usage:  watchExpression(every: 0.1) { someCodeThatReturnsAValue() }
//
// You can control how often the value will be checked using the every parameter (the example 
// above checks every 100ms). The return value can be any object that conforms to the 
// Equatable protocol.
//
// Note that this is NOT meant to be left in production code. It is purely meant to be 
// a debugging tool.

var timer: DispatchSourceTimer?
func watchExpression<T: Equatable>(every: TimeInterval, expression: @escaping () -> T?) {
    let formatter = DateFormatter()
    formatter.dateFormat = "HH:mm:ss.SSS"
    formatter.timeZone = TimeZone.current
    func dump(value: T?) {
        let output: Any = value ?? "nil"
        let time = formatter.string(from: Date())
        print(">>> \(time) \(output)")
    }
    
    var previousValue = expression()
    dump(value: previousValue)
    
    let queue = DispatchQueue(label: "expression.watcher", attributes: .concurrent)
    timer?.cancel()
    timer = DispatchSource.makeTimerSource(flags: [], queue: queue)
    timer?.scheduleRepeating(deadline: .now(), interval: every)
    timer?.setEventHandler {
        let currentValue = expression()
        if currentValue != previousValue {
            dump(value: currentValue)
            previousValue = currentValue
        }
    }
    timer?.resume()
}