Untested port of RNTimer to Swift

RepeatingTimer.swift

// Totally untested version of https://github.com/rnapier/RNTimer

import Foundation

final class RepeatingTimer {
    var block: () -> Void
    let source: dispatch_source_t

    init(timeInterval: NSTimeInterval, block: () -> Void) {
        assert(timeInterval != 0)

        self.block = block;

        source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,
            0, 0,
            dispatch_get_main_queue())

        let nsec = UInt64(timeInterval) * NSEC_PER_SEC

        dispatch_source_set_timer(source,
            dispatch_time(DISPATCH_TIME_NOW, Int64(nsec)),
            nsec, 0)

        dispatch_source_set_event_handler(source, block);
        dispatch_resume(source);
    }

    deinit {
        invalidate()
    }

    func invalidate() {
        dispatch_source_cancel(source)
        block = {}
    }
    
    func fire() {
        block()
    }
}