I do not know how to apply this information. I am eager to learn, but I do not know how

Thank you!

As I said in my question I already have done that. I am asking how to "pause" or stop the animation not how to "remove". When I use removeAllAnimations, view1 goes to alphaValue = 1 (in your example) I want to click stop/pause and the animation stop at that point, at some % between 0 and 1 alphaValue. Perhaps at 0.3 or 0.5 or whatever it is when the user click to stop/pause

Clever solution!

Thank you for reformulating my question more clearly. Your assumptions are right. Your solution works well. I have some doubts about the var controller. I understand what you say that I must put the var to nil when I close the window. But if the close button is in another view controller, that is in another class, should I make var controller global in the first place?. But if I understand global vars are not recommended? I am not sure if I explain well my problem.

It works very well. Thank you. I want to learn. I have been google this and there are some things I do not understand: Why the () at the end? Why do you make the var private?

Ok. I understand. Thank you

I get: When I change the text of a label, that label, and all the stack change abruptly. I expect: When I change the text of a label, that label and all the stack changes the width slowly (in my example, in 4 seconds). (If I knew what text the user put I could change the width of the elements? But I cannot know that). On the other side, I am in macOS. I tried transform to see what happens but it says: Value of type NSTextField does not have transform. In any case, I do not know the size of the label. So, I do not think transform is the way to go?