I have prototyped multilayer timeline with custom cells, where:
a. Each cell has possibly different size. Some cell sizes can be more than visible rect of ScrollView,
b. The gap between cells may be different (even though it appears same in the picture below), except the first(base) layer where the cell gap is fixed to 2 points,
c. Each cell can be selected and trimmed/expanded from each end using UIPanGestureRecognizer. Trimming/Expansion have custom rules. For the base layer, cell simply pushes other cells as it expands or contracts. For other layers however, the trimming or expansion have to respect boundaries of neighbouring cells.
d. Timeline can be zoomed horizontally which has the effect of scaling cells
e. Cells can be dragged and dropped to other rows subject to custom rules.
I have implemented all this using UIScrollView. By default all cells are initialized and added to UIScrollView, whether they are visible or not. But now I am hitting limits as I draw more content on each cell. Which means I need to reuse cells and draw only visible content. I discussed this with Apple Engineers in WWDC labs and one of the engineer suggested I use UICollectionView with custom layout where I can get lot of functionality for free (such as cell reuse, drag and drop). He suggested me looking into WWDC 2018 video (session 225) on UICollectionView. But as I look at custom layout of UICollectionView, it's not clear to me:
Q1. How to manually trim/expand select cells in UICollectionView with custom layout using UIPanGesture? In case of UIScrollView, I just have a UIPanGestureRecognizer on cell and do the trimming and expansion of it's frame (respecting given boundary conditions).
Q2. How to scale all the cells with a given zoom factor? With UIScrollView, I simply scale the frames of each cell and then calculate contentOffset to reposition UIScrollView around the point of zoom.
Even with UICollectionView with just one cell which has width say 10x of UICollectionView frame width, I will need further optimization to draw content on only visible portion rather than the whole cell. How is that possible with UICollectionViewCell to draw only part of the cell that's visible on screen?