Dead simple calculator in Swift 3

CalculatorViewController.swift

//
//  Created by Soemarko Ridwan on 3/23/17.
//  Copyright © 2017 Soemarko Ridwan. All rights reserved.
//
//  Dependencies: (to simplify the code)
//  - ActionKit: https://github.com/ActionKit/ActionKit/tree/swift3
//    For closure style buttons
//  - AutoLayout Helper: https://github.com/ustwo/autolayout-helper-swift
//    I was never a fan of interface builder, a lot more across devices.
//
//  Photo: https://tmblr.co/ZBVAFy2Js5KA6
//
import UIKit

class aViewController: UIViewController, UITextFieldDelegate {

	var field: UITextField? = nil
	var operand: String? = ""

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
		let wrapper = UIView()
		self.view.addSubview(wrapper)

		wrapper.addTopConstraint(toView: self.view, constant:80.0)
		wrapper.addLeftConstraint(toView: self.view, constant:10.0)
		wrapper.addRightConstraint(toView: self.view, constant:-10.0)
		wrapper.addHeightConstraint(toView: nil, constant: 40.0)
		wrapper.backgroundColor = UIColor(white: 0.85, alpha: 0.9)

		field = UITextField()

		wrapper.addSubview(field!)
		field?.fillSuperView(UIEdgeInsetsMake(5, 10, 5, 10))

		field?.keyboardType = .decimalPad

		let toolBar = UIToolbar()
		toolBar.barStyle = .default
		toolBar.isTranslucent = true

		let clearButton = UIBarButtonItem(title: "clear") {
			// clear
			self.field?.text = ""
			self.operand = ""
		}
		let addButton = UIBarButtonItem(title: "+") {
			// add
			if self.operand != "" {
				let result = self.compute(string: (self.field?.text)!, operand: self.operand!)
				self.field?.text = String(result)
			}
			self.operand = "+"
			self.field?.text?.append("+")
		}
		let subtractButton = UIBarButtonItem(title: "-") {
			// sub
			if self.operand != "" {
				let result = self.compute(string: (self.field?.text)!, operand: self.operand!)
				self.field?.text = String(result)
			}
			self.operand = "-"
			self.field?.text?.append("-")
		}
		let multiplyButton = UIBarButtonItem(title: "*") {
			// mul
			if self.operand != "" {
				let result = self.compute(string: (self.field?.text)!, operand: self.operand!)
				self.field?.text = String(result)
			}
			self.operand = "*"
			self.field?.text?.append("*")
		}
		let divideButton = UIBarButtonItem(title: "/") {
			// div
			if self.operand != "" {
				let result = self.compute(string: (self.field?.text)!, operand: self.operand!)
				self.field?.text = String(result)
			}
			self.operand = "/"
			self.field?.text?.append("/")
		}
		let equalButton = UIBarButtonItem(title: "=") {
			// equal
			if self.operand != "" {
				let result = self.compute(string: (self.field?.text)!, operand: self.operand!)
				self.field?.text = String(result)
			}
			self.operand = ""
		}

		let flexSpace = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
		let fixedSpace = UIBarButtonItem(barButtonSystemItem: .fixedSpace, target: nil, action: nil)
		fixedSpace.width = 20.0

		toolBar.setItems([clearButton, flexSpace, addButton, fixedSpace, subtractButton, fixedSpace, multiplyButton, fixedSpace, divideButton, fixedSpace, equalButton, fixedSpace], animated: false)


		toolBar.isUserInteractionEnabled = true
		toolBar.sizeToFit()

		field?.inputAccessoryView = toolBar
		field?.delegate = self
    }

	override func viewDidAppear(_ animated: Bool) {
		super.viewDidAppear(animated)

		field?.becomeFirstResponder()
	}

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

	func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
		let char = string.cString(using: String.Encoding.utf8)!
		let isBackSpace = strcmp(char, "\\b")

		if isBackSpace == -92 {
			if  (textField.text?.hasSuffix("+"))! ||
				(textField.text?.hasSuffix("-"))! ||
				(textField.text?.hasSuffix("*"))! ||
				(textField.text?.hasSuffix("/"))! {
				self.operand = ""
				print("remove operand")
			}
		}

		return true
	}

	func compute(string: String, operand: String) -> Double {
		let comps = string.components(separatedBy: operand)

		if comps.count == 0 {
			return 0
		}
		else if comps.count == 1 {
			return Double(comps[0])!
		}

		let left = Double(comps[0]) ?? 0.0
		let right = Double(comps[1]) ?? 0.0

		switch operand {
		case "+":
			return left + right
		case "-":
			return left - right
		case "*":
			return left * right
		case "/":
			return left / right
		default:
			return 0
		}
	}

}