UITapGestureRecognizer called immediately

问题

I am trying to add a UITapGestureRecognizer to a imageview as shown below

let gest=UITapGestureRecognizer(target: self, action: Selector(imgPressed()));
gest.delegate=self
thalaImg.addGestureRecognizer(gest)

And here is the imgPressed function :

func imgPressed()
{ 
    let alert=UIAlertController(title: "Img", message: "img pressed", preferredStyle: .Alert) 
    let okButton=UIAlertAction(title: "Ok", style: .Default, handler: nil) 
    alert.addAction(okButton) 
    presentViewController(alert, animated: true, completion: nil) 
}

I added the 3 lines of code in viewdidload, added a breakpoint and ran the application.What i observed is that once the compiler comes to the first line i.e let gest..., it is calling the action method immediately even before the application starts running.Since obviously the window is yet to load, it throws the below warning

Warning: Attempt to present on whose view is not in the window hierarchy!

I don't understand why this is happening .Can someone please help me with this issue?

Thank you

回答1:

You should use following syntax:

let gest = UITapGestureRecognizer(target: self, action: #selector(imgPressed))

Note, that Selector can accept initial type Void as well as String. Properly old-style Selector initialization looks like this:

let action: Selector = Selector("imgPressed")

Now you may check, that if you will try combine new style with old style you will get compilation error:

let action = Selector(imgPressed) // Error!

Funny thing is that you probably will try to resolve this error by appending bracers, and you will not have any errors after that! But this is completely useless. Look at following 3 lines of code, they are equivalent

let action = Selector(imgPressed())
let action = Selector(Void())
let action = Selector()

exept one thing - in first line you called imgPressed() function, returned Void, by your hands. Thats it =)

回答2:

Change your gesture line like following if swift 2.0 or less

let gest=UITapGestureRecognizer(target: self, action: Selector("imgPressed"));

if you are using swift 2.2 or greater than that

let gest=UITapGestureRecognizer(target: self, action: #selector(imgPressed));

Hope this will help you.

来源：https://stackoverflow.com/questions/37943477/uitapgesturerecognizer-called-immediately