Ios – Cannot override mutable property ‘refreshControl’ of type ‘UIRefreshControl?’ with covariant type ‘UIRefreshControl’

Firefox uses the error console to present the errors found on a web page.

To see it, press ctrl + shift + j.

Other browsers like Microsoft Internet Explorer have opted to alert the errors found by default, that is why you see them on an popup.

As can be read on the MDN Error Console page:

The Error Console is a tool available in most Mozilla-based applications that is used for reporting errors in the application chrome and in web pages user opens. It reports JavaScript-related errors and warnings, CSS errors and arbitrary messages from chrome code. In Firefox, the Error Console can be opened from the tools menu or by Ctrl-Shift-J.

For an immediately presentation of errors on Firefox, for anyone willing to use add-ons:

Console² has an option to gain focus automatically when errors are found.

Web Developer contains three icons, one for DocType, other for CSS and other for JS errors, allowing you to know whenever an error occurs.

When you override a variable, you can't change it's type. Why? Suppose that you are able to do that, then the following scenario would be possible:

var A: PostVC = PostVC() // some initialization 
var B: ArcaneCardVC = A // this is a valid state since `PostVC` is a subclass of `ArcaneCardVC`

What should be the type of B.currentCard? Hmm, this is a complicated question. You can answer that its type should be PostCard. Ok, lets add other classes to the party:

class OtherCard: ArcaneCardView {

}

class OtherVC: ArcaneCardVC {    
    override var currentCard: OtherCard?      
}

Considerer now the following code:

var A: ArcaneCardVC = PostVC()
var B: ArcaneCardVC = OtherVC()
A.currentCard = B.currentCard // something will crash here!!!

For avoiding this kind of behavior, you can't change the type of a property when you are subclassing.