UICollectionView performBatchUpdates crash demo

UICollectionViewPlayground.swift

//: Playground - noun: a place where people can play

import UIKit

class MockDataSource: NSObject, UICollectionViewDataSource {
	let collectionView: UICollectionView
	
	var numItems: Int
	
	init(numItems: Int, collectionViewFrameSize: CGSize) {
		self.numItems = numItems
		self.collectionView = UICollectionView(frame: CGRect(origin: .zero, size: collectionViewFrameSize),
		                                       collectionViewLayout: UICollectionViewFlowLayout())
			super.init()
			
			collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
			collectionView.dataSource = self
	}
	
	func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
		return numItems
	}
	
	func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
		return collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
	}
}

let dataSource = MockDataSource(numItems: 6, collectionViewFrameSize: .zero)
// At this point, because the dataSource's collection view has a size of zero, it will
// not initially check its data source for the number of items or sections.
// **Setting a non-zero size will cause it to check its data source immediately,
// avoiding the crash bug.**

// This triggers the collection view to ask its data source for the initial number of items.
// **If this line is commented out, and the initialFrameSize is zero, the performBatchUpdates()
// call below will crash.**
dataSource.collectionView.layoutIfNeeded()

// Increase the item count...
dataSource.numItems += 1

// ...and tell the collection view about the items that were added.
dataSource.collectionView.performBatchUpdates({
	dataSource.collectionView.insertItems(at: [IndexPath(item: 0, section: 0)])
})

// If the collection view has properly initialized, the batch update will succeed:
// The item count that the collection view stored during layoutIfNeeded() was 6,
// the batch update adds 1 row, and the count after the update will be 7 to match
// the number reported by collectionView(_:numberOfItemsInSection:).

// If the collection view *hadn't* retrieved the initial item count yet (e.g. because
// we gave it a non-zero size or caused layoutIfNeeded() to be called), then
// performBatchUpdates() will call collectionView(_:numberOfItemsInSection:) itself
// after the increment already happened: so the stored item count at the start of the
// batch update will be 7 instead of 6. As a result it will die with an
// NSInternalInconsistencyException.