背景颜色的 alpha 在 iOS 中不起作用

Question

Do you have any idea why alpha parameter does not work on my view ?

  self.view.backgroundColor = UIColor.blackColor().colorWithAlphaComponent(0.4)

please also check this video

Video

thanks

Answer 1

If you are using storyboard and present view controller using segue, the presentation attribute of segue that show you view controller with transparent background should be over full screen as shown in attachment.

Then in your view controller set the background color by using this :

self.view.backgroundColor = UIColor(white: 0, alpha: 0.4)

Answer 2

It's not strange, it's behaving exactly as it should. Although many of UIColor's methods are class methods, there are still a few instance methods, and this is one of them. From the UIColor documentation .

colorWithAlphaComponent:

Creates and returns a color object that has the same color space and component values as the receiver, but has the specified alpha component. So, colorWithAlphaComponent: just changes the alpha value of its receiver. Example:

let black = UIColor.blackColor() // 1.0 alpha
self.view.backgroundColor = black.colorWithAlphaComponent(0.5) // 0.5 alpha

Answer 3

您正在呈现ViewController，因此它将无法工作，因为您必须在其他 ViewController 上添加UIViewController 的UIView ，然后才能工作。

   [[self view] addSubview:myViewController.view];

Answer 4

I found an issue on iOS 12 which is related to the title (but not the answers though) where withAlphaComponent does not work.

1. In a storyboard, set a view background colour to an opaque named colour
2. In the code (viewDidLoad of the viewController for instance), change the background colour of this view to the same named colour but with a different alpha (using withAlphaComponent)

Expected result: the view background colour is the the named one with some alpha. Actual result: the view background colour is he named colour without alpha.

The issue does not occur if the colour set in the storyboard is any colour but the named one. It does not occur on iOS 14.1 either.