How to initialize properties that depend on each other

Question

I want a picture to move to the bottom. If I press a button the pic should move down by 1.

I added the picture and a button:

var corX = 0
var corY =         


        

Answer 1

//
//  ViewController.swift
//
//  Created by Shivank Agarwal on 19/05/18.
//  Copyright © 2018 Shivank Agarwal. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    var corX = 0
    var corY = 0
    var runter: UIButton = UIButton()
    var image = UIImage(named: "panzerBlau.jpg")
    var panzer = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()
        panzer.image = image;
        self.view.addSubview(panzer);
        panzer.frame = CGRect(x: CGFloat(corX), y: CGFloat(corY), width: 30, height: 40)
        runter.backgroundColor = UIColor.red
        view.addSubview(runter)
        view.addSubview(panzer)
        runter.addTarget(self, action: Selector(("fahren")), for:UIControlEvents.touchUpInside)
    }

    private func fahren(){
        corY += 100
    }

    private func updatePanzerFrame(){
        panzer.frame = CGRect(x: CGFloat(corX), y: CGFloat(corY), width: 30, height: 40)
    }
}

Note: Do not add panzer imageView every time when user tap only add it on viewDidLoad()

Answer 2

I'm addressing the title of the question:

Both lazy and computed properties help you deal with when the initial value for a property is not known until after the object is initialized. But there are some differences. I've highlighted the differences with bold.

If you simply need to initialize a variable after some other variable(s) is initialized then you should use lazy ie if the point is to simply add a delay (so all required properties get initialized before) then using lazy is the right way to go for it.

But if you need to constantly change a variable based on another, then you need a computed property that would work both ways:

• if the computed property set then it sets the variables its related stored properties
• if the stored properties are set (or are reset again) then it will trigger a change in then computed property.

_{if you change the lazy property's value it won't affect the storied properties that it was based on. see here}

---

A good example for using a lazy property would be that once you have firstName & lastName then you would lazily instantiate a fullName and likely you would never change the firstName lastName of your object your fullName is a onetime only... Or perhaps something that can only be done by lazy properties is that up until you don't access the property it won't ever get initialized, therefore this would decrease the initialization load of your class. Loading a heavy calculation.

Additionally using the lazy will signal to other developers: "Hey first go read about the other properties and understand what they are...then come to this lazy property...since the value of this is based on them + this is likely a heavy computation that shouldn't be accessed too early..."

As for computed property a good example would be if you set the temperature to Fahrenheit then you also want your celsius temperature to change its value...and if you set the celsius temperature then again you want to change your Fahrenheit value.

As a result computed property would add extra computation...and if your computation is very simple and isn't called too frequently then it's nothing to worry about but if it get's called too often or is very CPU-consuming then it might be better to think of other options...

Answer 3

@MartinR has pointed out the major issue here:

var corX = 0
var corY = 0
var panzer = UIImageView(frame: CGRectMake(corX, corY, 30, 40))

The problem is that a Swift default initializer cannot refer to the value of another property, because at the time of initialization, the property doesn't exist yet (because the instance itself doesn't exist yet). Basically, in panzer's default initializer you are implicitly referring to self.corX and self.corY - but there is no self because self is exactly what we are in the middle of creating.

One workaround is to make the initializer lazy:

class ViewController: UIViewController {
    var corX : CGFloat = 0
    var corY : CGFloat = 0
    lazy var panzer : UIImageView = UIImageView(frame: CGRectMake(self.corX, self.corY, 30, 40))
    // ...
}

That's legal because panzer doesn't get initialized until later, when it is first referred to by your actual code. By that time, self and its properties exist.

Answer 4

Your dependent property needs to be:

1. lazy
2. Have an explicit : Type
3. Use self. to access other properties

Example:

let original = "foo"

// Good:
lazy var depend: String = self.original

// Error:
     var noLazy: String = self.original // Error: Value of type '(NSObject) -> () -> URLData' has no member 'original'
lazy var noType         = self.original // Error: Value of type '(NSObject) -> () -> URLData' has no member 'original'
lazy var noSelf: String =      original // Error: Instance member 'original' cannot be used on type 'YourClass'