Auto diffing section controller

Summary:
Started work on the plane to get this moving since #418  is up and ready to land. We'll likely need to spend some time fleshing out the API of this, and I think I'll split it up into a couple different PRs once ready for review. Putting this up now to get early feedback.

This adds an auto-diffing section controller as outlined in #38. There are several key parts:

- Subclass a new section controller `IGListAutoSectionController` (naming wip)
- Connect a data source
- Implement the data source methods that do 3 things:
  - Given a top-level object, transform it into an array of **diffable** view models
  - Given a view model, return a cell
  - Given a view model, return a size for a cell
- A new protocol for the cell `IGListBindable` so that we can control when the cell is updated w/ the view model.
  - The most important part of this is that it unlocks moving and reloading a cell, which you can't do w/ `UICollectionView`

- [ ] Unit test `reloadObjects:`
- [x] Add
Closes https://github.com/Instagram/IGListKit/pull/494

Reviewed By: amonshiz

Differential Revision: D4696966

Pulled By: rnystrom

fbshipit-source-id: f21b8341b3ed4389f2a4a106d0d316f481ba6943

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

@@ -0,0 +1,93 @@
+/**
+ Copyright (c) 2016-present, Facebook, Inc. All rights reserved.
+ 
+ The examples provided by Facebook are for non-commercial testing and evaluation
+ purposes only. Facebook reserves all rights not expressly granted.
+ 
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+import UIKit
+import IGListKit
+
+final class MonthSectionController: IGListBindingSectionController, IGListBindingSectionControllerDataSource, IGListBindingSectionControllerSelectionDelegate {
+    
+    var selectedDay: Int = -1
+    
+    override init() {
+        super.init()
+        dataSource = self
+        selectionDelegate = self
+    }
+    
+    // MARK: IGListBindingSectionControllerDataSource
+    
+    func sectionController(_ sectionController: IGListBindingSectionController, viewModelsFor object: Any) -> [IGListDiffable] {
+        guard let month = object as? Month else { return [] }
+        
+        let date = Date()
+        let today = Calendar.current.component(.day, from: date)
+        
+        var viewModels = [IGListDiffable]()
+        
+        viewModels.append(MonthTitleViewModel(name: month.name))
+        
+        for day in 1..<(month.days + 1) {
+            let viewModel = DayViewModel(
+                day: day,
+                today: day == today,
+                selected: day == selectedDay,
+                appointments: month.appointments[day]?.count ?? 0
+            )
+            viewModels.append(viewModel)
+        }
+        
+        for appointment in month.appointments[selectedDay] ?? [] {
+            viewModels.append(appointment)
+        }
+        
+        return viewModels
+    }
+    
+    func sectionController(_ sectionController: IGListBindingSectionController, cellForViewModel viewModel: Any, at index: Int) -> UICollectionViewCell {
+        let cellClass: AnyClass
+        if viewModel is DayViewModel {
+            cellClass = CalendarDayCell.self
+        } else if viewModel is MonthTitleViewModel {
+            cellClass = MonthTitleCell.self
+        } else {
+            cellClass = LabelCell.self
+        }
+        return collectionContext?.dequeueReusableCell(of: cellClass, for: self, at: index) ?? UICollectionViewCell()
+    }
+    
+    func sectionController(_ sectionController: IGListBindingSectionController, sizeForViewModel viewModel: Any, at index: Int) -> CGSize {
+        guard let width = collectionContext?.containerSize.width else { return .zero }
+        if viewModel is DayViewModel {
+            let square = width / 7.0
+            return CGSize(width: square, height: square)
+        } else if viewModel is MonthTitleViewModel {
+            return CGSize(width: width, height: 30.0)
+        } else {
+            return CGSize(width: width, height: 55.0)
+        }
+    }
+    
+    // MARK: IGListBindingSectionControllerSelectionDelegate
+    
+    func sectionController(_ sectionController: IGListBindingSectionController, didSelectItemAt index: Int, viewModel: Any) {
+        guard let dayViewModel = viewModel as? DayViewModel else { return }
+        if dayViewModel.day == selectedDay {
+            selectedDay = -1
+        } else {
+            selectedDay = dayViewModel.day
+        }
+        update(animated: true)
+    }
+
+}