source view controller vs. presenting view controller

Question

I'm reading a book that states that source view controller is not necessarily a presenting view controller. The source VC is the one that calls presentViewController:... method, and the presenting VC(it's view) is the one that gets obscured by a presented VC view. I can't think of a single example in which the presenting VC is not the same as source VC. Please provide some. Thanks

Text from book:

“Original presenter: The view controller to which presentViewController:animated:completion: was sent. Apple sometimes refers to this view controller as the source; “original presenter” is my own term. The presented view controller is set as the original presenter’s presentedViewController.

Presenting view controller: The presented view controller’s presentingViewController. This is the view controller whose view is replaced or covered by the presented view controller’s view. By default, it is the view controller whose view is the entire interface — namely, either the root view controller or an already existing presented view controller. It might not be the same as the original presenter. The presented view controller is set as the presenting view controller’s presentedViewController. Thus, the presented view controller might be the presentedViewController of two different view controllers.”

Answer 1

I was asking myself the same question when learning about view controller transitions, specifically when trying to understand the animationControllerForPresentedController:presentingController:sourceController: method from the UIViewControllerTransitioningDelegate protocol. The struggle was about the difference between the presentingController and sourceController arguments.

I found the answer in the View Controller Programming Guide. You can find the following in the Presenting View Controllers Modally section:

The view controller that calls the presentViewController:animated:completion: method may not be the one that actually performs the modal presentation. The presentation style determines how that view controller is to be presented, including the characteristics required of the presenting view controller. For example, a full-screen presentation must be initiated by a full-screen view controller. If the current presenting view controller is not suitable, UIKit walks the view controller hierarchy until it finds one that is. Upon completion of a modal presentation, UIKit updates the presentingViewController and presentedViewController properties of the affected view controllers.

This means that the 'source' view controller concept is not something randomly created by the author of the book you mentioned.

Answer 2

I'm afraid the book you're reading may have made a complex subject even more complex by adding the so-called Source View Controller to the whole mix of names.

For starters, there is no such concept of a "Source" in View Controllers. You have a parentViewController and childViewControllers only when talking about Container View Controllers. And you have a presentingViewController and a presentedViewController only when talking about presenting View Controllers modally.

You also have View Controllers whose main purpose is to manage other View Controllers, namely the Navigation Controller, the Tab Bar Controller, the Split View Controller and the Popover Presentation Controller. So any given View Controller may query itself to know if it is "attached" to a Navigation Controller, for instance.

What I'm guessing your book is implying is that a Source VC would be the one that makes another VC appear on screen. In this sense, it is right. The "source" may be different than the presenting VC. Take this snippet:

[self presentViewController:aViewController animated:YES completion:nil];

In the example above, the presenting VC is also the "source" VC. But here:

[someViewController presentViewController:anotherViewController animated:YES completion:nil];

We are calling the method on some VC lying somewhere passing yet another VC as argument. So in this case the presenting VC is someViewController and the presented VC is anotherViewController. Should this line of code be inside a third VC, then said third VC would be the "source".

But that's the thing! This last snippet of code may not even be inside a View Controller in the first place, so it's a bit odd to think about "source VCs".