How to modify view.alphaValue at runtime and take effect - swift

Question

I modified the alphaValue at runtime but has not any effect , what is the problem. how to modify the alphaValue at runntime ?
I want to adjust the alphaValue of view at runtime
Xcode 10.2.1 Swift 4.2
//
// ViewController.swift
// macosappalphaValuetest
//
// Created by miller on 2019/7/25.
// Copyright © 2019 miller. All rights reserved.
//
import Cocoa
class ViewController: NSViewController {
#IBAction func btnclick1(_ sender: Any) {
self.view.alphaValue = 0.8
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
override var representedObject: Any? {
didSet {
// Update the view, if already loaded.
}
}
override func viewWillAppear() {
super.viewWillAppear()
view.window?.isOpaque = false
view.window?.backgroundColor = NSColor(red: 0.5, green: 0.5, blue: 0.5, alpha: 0.5)
self.view.alphaValue = 0.4
}
}

Answer 1

In NSViewController's viewWillAppear method your view's window is probably nil. You can check it by printing print(view.window). So you need to get the correct instance of your window. See below method:
func validWindow() -> NSWindow? {
var window = NSApplication.shared.mainWindow
if (window == nil) {
window = NSApplication.shared.windows.first
}
return window
}
And just write below code to make your window transparent:
guard let window = self.validWindow() else { return }
window.alphaValue = 0.5
It works well for me.
Output:
Increasing and Decreasing alpha value recursively:
func decreaseAlpha(_ window: NSWindow) {
let alpha = window.alphaValue - 0.1
if alpha < 0.0 {
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {
self.increaseAlpha(window)
}
return
}
window.alphaValue = alpha
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {
self.decreaseAlpha(window)
}
}
func increaseAlpha(_ window: NSWindow) {
let alpha = window.alphaValue + 0.1
if alpha > 1.0 {
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {
self.decreaseAlpha(window)
}
return
}
window.alphaValue = alpha
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {
self.increaseAlpha(window)
}
}
And write this in viewDidLoad: method:
guard let window = self.validWindow() else { return }
decreaseAlpha(window)
Output:

Answer 2

alphaValue is the Objective-C version of alpha. Applying alpha on the viewController view will make it slightly transparent and possible show the viewController that is beneath it if there is one. If there are no
view controllers beneath it, it will lean towards a darker background as the default background is black.