Why? Scaling images on the fly impact performance.
How? If you have no control over the incoming image size, then scale the image outside of the main thread before displaying.

What? A queue that runs tasks FIFO.

Types? 

• Serial – one at a time
• system-provided: main queue
• you can create your own
• Concurrent – 1+ tasks at a time
• system-provided: 4 queues (high, default, low, and background)
• you can create your own, but 4 system ones should be good enough

For:

• UITableViewCells
• UICollectionViewCells
• UITableViewHeaderFooterViews

If not reuse, it will create one each time a cell is displayed which impacts scrolling performance.
Remember to reuse

• headers and footers
• supplementary views

Opaque means no transparency defined. If not set, it will impact performance, especially the animated UIs.
Why? iOS needs to go deeper into the view hierarchy to figure out what color to render.
How? Use Debug\Color Blended Layers option to locate non-opaque views.

Notes from NSError

• domain: the subsystem
• code: error code within that subsystem
• userInfo: a dictionary, contains all other details

Also available: localizedDescription

Run the block of code before leaving the current scope. Similar to “finally” in other language’s try/catch block.

Notify objects when changes to properties of other objects. Very similar to willSet and didSet, but they can be defined outside of the type defination.

observation = observe(
            \.objectToObserve.myDate,
            options: [.old, .new]
        ) { object, change in
            print("myDate changed from: \(change.oldValue!), updated to: \(change.newValue!)")
        }

Why? UI rendering (UIKit) works in the main thread.
How? All heavy lifting (time-consuming) should be moved out of main thread, including:

• Disk I/O
• Networking, e.g., API calls
• Large computations
• Use GCD, and/or Operation

UIControl actions will send events to a chain of responders, if the 1st one doesn’t implement the action, then it goes deeper to the 2nd one, till it’s handled or no more responders in the chain.

What? Since override is support by Swift, it has to be determined at runtime what methods/properties to call, so an indirect call cannot be avoid.
When performance is important, to minimize and help compiler optimization, use private, final access levels to declare methods/properties.