Pass In Error

PassInError.swift

//  PassInError.swift
//  Created by Adam Kirk on 6/6/15.

import Foundation

typealias PassInErrorBlock = (error: NSError) -> Void

class PassInError: NSObject {

    dynamic var ref: NSError?

    let block: PassInErrorBlock?

    init(block: PassInErrorBlock? = nil) {
        self.block = block
        super.init()
        self.addObserver(self, forKeyPath: "ref", options: nil, context: nil)
    }

    deinit {
        self.removeObserver(self, forKeyPath: "ref")
    }


    // MARK: - KVO

    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        if let smartError = object as? PassInError {
            if keyPath == "ref" {
                if let error = self.ref {
                    if let block = self.block {
                        block(error: error)
                    }
                    if let underlyingError = error.userInfo?[NSUnderlyingErrorKey] as? NSError {
                        println("ERROR =================================\n" +
                                "\(underlyingError.localizedDescription)\n" +
                                "=======================================\n")
                    }
                }
            }
        }
    }

}

Usage

Now if anything is assigned to error, it'll be printed to the console:

NSFileManager.defaultManager().moveItemAtPath("blah", toPath: "blah2", error: &PassInError().ref)

This will print:

ERROR =================================
The operation couldn’t be completed. No such file or directory
=======================================

Or if you want to handle the error:

let error = PassInError { error in
    println("Couldn't move your dumb file: \(error)")
}
NSFileManager.defaultManager().moveItemAtPath("blah", toPath: "blah2", error: &error.ref)