Where is supplementaryViewOfKind used? - ios

Question

I'm trying to figure out where supplementaryViewOfKind is used. I looked into the documentation and some other questions on SO, still I wasn't able to understand what it does. From my understanding it's to be used as an auxiliary Headerview.
collectionView.registerClass(imageCell.self, forCellWithReuseIdentifier: "imageCell")
collectionView.registerClass(imageCell2.self, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "imageCell2"
Can't I alternatively have imageCell2 registered this way:
collectionView.registerClass(imageCell2.self, forCellWithReuseIdentifier: "imageCell2")
and then when I need, at cellForItemAtIndexPath I switch cases for the which cell to present? or again am I not understand the existensial purpose of supplementaryViewOfKind

Answer 1

You'd better check this article:
Collection View Programming Guide for iOS
In the section Custom Layouts: A Worked Example, you can find an example of custom layout using Supplementary View.
In the example above, connecting lines are implemented using Supplementary Views.
You can use Supplementary Views in various ways with your custom layouts.

Answer 2

The shortest answer
Watch this video:
https://developer.apple.com/videos/play/wwdc2012/205/
The short answer
The supplementary view, like its name, provides supplementary material that needs some data to be displayed properly. Cells and supplementary views can be closely related to each other or have little to do with each other.
The concept is a little different than UITableView so you might want to read below how it is different. In short, UICollectionView is intended so people can have a group of/groups of views laid out in a non-linear way, according to their own rules.
Example
Let's say I made a map using UICollectionView. I could use UICollectionViewCells to display buildings.
Here, I can use the supplementary view in infinitely different ways.
I can choose to use it as a label to display the name of town. In this case I would be using one supplementary view for one section.
OR I can use supplementary view to display the name of each building. In this case I would have the same number of supplementary views and cells.
OR I could actually do both, registering different classes to the same collection view.
UICollectionView vs UITableView
It's easy to get confused because the terminology used for UICollectionView is similar to what is used in UITableView, but it's crucial to know the differences between the two in order to understand what things do in collection view.
a) It has a layout object whose main concern is the display of cells registered to the collection view. As far as I remember the stock UICollectionViewLayout does nothing, or provides minimal layout functionality whereas the UITableView class has a built-in way of laying out its cells--i.e. a linear vertical row.
b) You define the rules through the layout object. This rule governs how things--cells, supplementary views--are displayed in the collection view.
c) Because the collection view is intended so you define a custom layout object, so as to display a set of objects(i.e. cells, supplementary views) following a certain rule (i.e. the rules defined by the layout object) that aren't laid out in a linear manner, there's no real sense of "section header" in a collection view. A section header in a table view is a view object that provides the title or summary of what is in the section. A supplementary view doesn't have this "one per section" limitation. You can use it like a section header if you want, but you don't have to.
d) As mentioned in the example, you can register many different kinds of supplementary views. It's not like the table view where you only get a header and a footer.
e) The section and item in the IndexPath are just logical groups. (Notice UICollectionView doesn't use the term row, since it doesn't assume cells will be laid out in a row) Cells in a section don't even have to be laid out close to each other if that's how the layout rules are defined in the layout object.
f) The index path of cells and that of supplementary views have no connection with each other unless you decide to logically group them together. You can have cells but no supplementary views, the same number of both, or more supplementary views than the cells. They are two different discrete entities unless you choose to logically group them and use them together.
g) In case people want a somewhat linear but different from UITableView look (e.g. a horizontally scrolled table view or 4x4 layout of pictures), Apple provides UICollectionViewFlowLayout, a subclass of UICollectionViewLayout. Using this you can have the table view like behavior, as well as the header and footer as supplementary views.

Answer 3

I'm trying to figure out where supplementaryViewOfKind is used
A layout can define supplementary view kinds however it likes, to give you a way to specify things that are not ordinary cells (items).
A good example is the way UICollectionViewFlowLayout lets you ask for section headers and footers. They are supplementary views. In particular, they are supplementary views of kind UICollectionElementKindSectionHeader and UICollectionElementKindSectionFooter. See the documentation.
In my Latin app's list of chapters and sections, the chapters (black bars with white numbers at left) are supplementary views:

Answer 4

The supplementaryViews are the Paris and Chamonix in the image Below.
They are neither the header, nor the footer nor the cell. They are something else. Similar to cells you would have register them and then also dequeue them, or that they also do take data from the dataSource so in that sense they are data driven, but they just don't contain data for the cells, they contain meta data...
Just jump to minute 5:47 of this WWDDC video. Though I suggest you to see the entire video from beginning.