DispatchQueue+Helper.swift

extension DispatchQueue {

    /// Helper method to use when reading from a resource
    /// that is being isolated by this queue.
    ///
    /// Using this method retriving a value like this:
    /// ```
    /// var result: T?
    /// queue.sync {
    ///     result = resource["key"]
    /// }
    /// ```
    /// Can instead be done like this:
    /// ```
    /// let value = queue.access { 
    ///     resource["key"]
    /// }
    /// ```
    /// - parameter access: A closure to be synchronously dispatched onto the queue.
    /// - returns: Forwards the return value of the passed in `access` closure.
    @discardableResult func access<T>(_ access: @escaping () throws -> T) rethrows -> T {
        var result: T? = nil
        try sync { result = try access() }
        return result!
    }

}