Move section index to property

Summary:
This pull request removes the `sectionForSectionController:` method from the `IGListCollectionContext` protocol so that the protocol is exclusively for presentation methods.

This should not add new functionality, but rather makes the index directly accessible on the section controllers themselves. This change makes sense because at no time will there be an update to the list that the list adapter is unaware of and so it will always be able to set and update any indexes for a section controller that has changed.

Issue fixed: #609

- [X] All tests pass. Demo project builds and runs.
- [X] I added tests, an experiment, or detailed why my change isn't tested.
- [X] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [X] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes #671

Reviewed By: jessesquires

Differential Revision: D4942159

Pulled By: amonshiz

fbshipit-source-id: d648cfdd381cbf1d9ee7ff549ae27d2972a84622

CHANGELOG.md

@@ -60,6 +60,8 @@ This release closes the [3.0.0 milestone](https://github.com/Instagram/IGListKit
 
 - `IGListUpdatingDelegate` now requires method `-reloadItemInCollectionView:fromIndexPath:toIndexPath:` to handle reloading cells between index paths. [Ryan Nystrom](https://github.com/rnystrom) [(#657)](https://github.com/Instagram/IGListKit/pull/657)
 
+- `-[IGListCollectionContext sectionForSectionController:]` has been removed and replaced with the `NSInteger sectionIndex` property on `IGListSectionController`. [Andrew Monshizadeh](https://github.com/amonshiz) [#671](http://github.com/Instagram/IGListKit/pull/671)
+
 ### Enhancements
 
 - Added an initializer on `IGListAdapter` that does not take a `workingRangeSize` and defaults it to 0. [BasThomas](https://github.com/BasThomas) [(#414)](https://github.com/Instagram/IGListKit/issues/414)

Examples/Examples-iOS/IGListKitExamples/SectionControllers/DisplaySectionController.swift

@@ -33,31 +33,26 @@ final class DisplaySectionController: IGListSectionController, IGListDisplayDele
 
     override func cellForItem(at index: Int) -> UICollectionViewCell {
         let cell = collectionContext!.dequeueReusableCell(of: LabelCell.self, for: self, at: index) as! LabelCell
-        let section = collectionContext!.section(for: self)
-        cell.text = "Section \(section), cell \(index)"
+        cell.text = "Section \(self.sectionIndex), cell \(index)"
         return cell
     }
 
     // MARK: IGListDisplayDelegate
 
     func listAdapter(_ listAdapter: IGListAdapter, willDisplay sectionController: IGListSectionController) {
-        let section = collectionContext!.section(for: self)
-        print("Will display section \(section)")
+        print("Will display section \(self.sectionIndex)")
     }
 
     func listAdapter(_ listAdapter: IGListAdapter, willDisplay sectionController: IGListSectionController, cell: UICollectionViewCell, at index: Int) {
-        let section = collectionContext!.section(for: self)
-        print("Did will display cell \(index) in section \(section)")
+        print("Did will display cell \(index) in section \(self.sectionIndex)")
     }
 
     func listAdapter(_ listAdapter: IGListAdapter, didEndDisplaying sectionController: IGListSectionController) {
-        let section = collectionContext!.section(for: self)
-        print("Did end displaying section \(section)")
+        print("Did end displaying section \(self.sectionIndex)")
     }
 
     func listAdapter(_ listAdapter: IGListAdapter, didEndDisplaying sectionController: IGListSectionController, cell: UICollectionViewCell, at index: Int) {
-        let section = collectionContext!.section(for: self)
-        print("Did end displaying cell \(index) in section \(section)")
+        print("Did end displaying cell \(index) in section \(self.sectionIndex)")
     }
 
 }

Examples/Examples-iOS/IGListKitExamples/SectionControllers/ListeningSectionController.swift

@@ -24,8 +24,7 @@ final class ListeningSectionController: IGListSectionController, IncrementListen
     }
 
     func configureCell(cell: LabelCell) {
-        let section = collectionContext!.section(for: self)
-        cell.text = "Section: \(section), value: \(value)"
+        cell.text = "Section: \(self.sectionIndex), value: \(value)"
     }
 
     // MARK: IGListSectionController Overrides

Examples/Examples-iOS/IGListKitExamples/SectionControllers/WorkingRangeSectionController.swift

@@ -71,8 +71,7 @@ final class WorkingRangeSectionController: IGListSectionController, IGListWorkin
             let url = URL(string: urlString)
             else { return }
 
-        let section = collectionContext?.section(for: self) ?? 0
-        print("Downloading image \(urlString) for section \(section)")
+        print("Downloading image \(urlString) for section \(self.sectionIndex)")
 
         task = URLSession.shared.dataTask(with: url) { data, response, err in
             guard let data = data, let image = UIImage(data: data) else {