What? Classes like NSDateFormatter, NSCalendar are heavy to instantiate.
How? Create singleton (and it should be thread-safe if the singleton is created properly.)
Tips: If possible, use UNIX epoch (an Int) to represent a date. That speeds up the date object creation (vs. creation by parsing date formatted string.)

• bounds: uses its own coordinate system, used to place views in itself.
• frame: uses parent view’s coordinate system, used to place it in the parent view.

• NSUserDefaults – a tiny piece of info
• JSON files, NSCoding – large one time data, when used, need to first load from disk to memory. And entire payload has to be loaded before being used.
• SQLite, Core Data – large queryable data

Object A holds a reference to Object B while Object B also holds a reference to Object A, then we have a retain cycle here. To break it, one of the objects should be defined as weak. So the ARC can eventually count down to zero then release both objects.

Load the subviews in the scroll view only when they need to be displayed. Similar to how UITableView works.

For small image repeated/tiled, use colorWithPatternImage

For large full-size image, use UIImageView

What? Functions you can pass as arguments to other functions. Such as map, filter, reduce, flatMap.

• map – operate on each element in the collection (then return a new collection).
• filter – return a subset of the collection that satisfies some condition. 
• reduce – combine and return a single value. 
• flatMap – flatten a collection of collections into a collection. 

• cell creation
• transparency
• gradient
• shadow
• off-screen drawing
• image scaling
• row height calculation
• api call
• data transformation
• deep subviews

What? The vars that are marked lazy won’t be initialized until it’s used the first time.
When to use, if the var is a view? 

• When the view has a good chance of never needed based on the condition. e.g., views in a collapsible panel. 
• When the view isn’t needed before user interaction starts. Such as off-screen views, or views can be shown with delay.