CODEWITHSUNDEEP
iosobjective-cswiftuiview
Kevvv
Kevvv

Reputation: 4023

Converting points between view coordinate systems

I'm having a difficult time understanding the difference between convert(_:to:) and convert(_:from:) for some reason.

Let's say I have two views (superView and subview) and a root view:

let superView = UIView(frame: CGRect(origin: .init(x: 25, y: 25), size: CGSize(width: 100, height: 100)))
rootView.addSubview(view)

let subview = UIView(frame: CGRect(origin: .init(x: 25, y: 25), size: .init(width: 50, height: 50)))
superView.addSubview(subview)

convert(_:to:) converts a point from the receiver’s coordinate system to that of the specified view, according to the documentation:

let convertToWindow = subview.convert(subview.bounds, to: window)
// {x 50 y 50 w 50 h 50 }

convert(_:from:) converts a point from the coordinate system of a given view to that of the receiver, according to the documentation:

let convertFromWindow = subview.convert(subview.bounds, from: window)
// {x -50 y -50 w 50 h 50 }

Is the "receiver" subview in this case? If so, I think I sort of understand convert(_:from:) to be converting subview's coordinates with respect to is own coordinates (and not its super view) to the position within window, which is why it's going from {x 0 y 0 w 50 h 50 } to {x 50 y 50 w 50 h 50 }.

However, what is convert(_:from:) doing when it's converting subviews.bound that already uses the coordinate system of subview? Why doesn't subview.convert(subview.bounds, from: window) return {x 0 y 0 w 50 h 50 }?

Finally, if the convert method merely converts a point or a bound to and from a view, why does the instance the method belongs to make a difference in the result of the conversion? For example, subview.convert(subview.bounds, to: window) and superView.convert(subview.bounds, to: window) yield different results.

Upvotes: 3

Views: 1757

Answers (2)

meaning-matters
meaning-matters

Reputation: 22936

The "receiver" is indeed the object on which you make the call.

The name of the method should be understood looking from the perspective of this "receiver":

  • 'to'-method: Coming from the receiver, what would the rectangle be when going to the view given as parameter.

  • 'from'-method: Going towards the receiver, what would the rectangle be when coming from the view given as parameter.

In other words:

  • With the 'to'-method you go to the other view.
  • With the 'from'-method you come back from the other view.

Try to visualise it as traveling between the receiver view and the parameter view. You (i.e. the receiver) can go to, and come back from the parameter view.

I hope this helps.

Upvotes: 1

matt
matt

Reputation: 534893

Yes, in an expression like subview.convert..., we call subview the "receiver" — it is the object to which the convert message is sent, so it receives that message.

Why doesn't subview.convert(subview.bounds, from: window) return {x 0 y 0 w 50 h 50 }?

The mental mistake you're making here is that you think subview.bounds somehow pins this value to subview. It doesn't. The first parameter is merely some numbers. The convert method doesn't know where you got those numbers from. That is why you have to tell it where you got them — with the second parameter:

  • If you say from:, you are saying, "I got these numbers from the second parameter's coordinate system".

  • If you say to:, you are saying, "I got these numbers from the receiver's coordinate system".

why does the instance the method belongs to make a difference in the result of the conversion

Because that is the other view in the story:

  • If you say to:, you are saying, "I got these numbers from the receiver's coordinate system, and I want you to convert them to the second parameter's coordinate system."

  • If you say from:, you are saying, "I got these numbers from the second parameter's coordinate system, and I want you to convert them to the receiver's coordinate system."

Remember, the first parameter is just some numbers. You can get them from anywhere you want, including the inside of your head, but you won't get the "right" answer if you lie about this in the rest of the expression — as some of your examples do.

Upvotes: 7

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