SyncedOp.swift

/// Synched operations will only run one at a time.
/// The `operation` method is responsible for calling `complete` to notify the object when it is done.
final class SyncedOp {
    // Use a GCD Semaphore to synchronize flag access.
    private let semaphore = DispatchSemaphore(value: 1)
    
    private var isBusy = false
    private var isRetriggerQueued = false
    
    internal var operation: () -> () = {}
    internal var retriggerOperation: () -> () = {}
    
    internal func run() {
        semaphore.wait()
        if isBusy {
            isRetriggerQueued = true
            semaphore.signal()
        } else {
            isBusy = true
            isRetriggerQueued = false
            semaphore.signal()
            operation()
        }
    }
    
    internal func complete() {
        semaphore.wait()
        isBusy = false
        if isRetriggerQueued {
            isRetriggerQueued = false
            semaphore.signal()
            retriggerOperation()
        } else {
            semaphore.signal()
        }
    }
}