See Apple release note

Notes from IBInspectable / IBDesignable

user-defined runtime attributes: allows updating attribute value in Xcode

IBInspectable: easlier editable in Xcode

Can add inspectable property to existing classes via extension.

IBDesignable: WYSIWYG.

Use prepareForInterfaceBuilder() to provide dummy data at design time. It’s not run in shipping code.

A clusure that is

• passed in to a function as parameter
• called after the function has returned

Closures are non-escaping by default in Swift 3+.

Use @escaping to mark it as escaping.

• api calls
• disk I/O
• data preparation

• let means constant, and cannot be set after initialized.
• read-only also means cannot be set but since it is computed so it could return different value at different times.

Extension and Delegation

• Visits location service (CLVisit): most power-efficient way. Not for navigation or real-time. Good for doing async background processing related to location. Or to identify location pattern. Requires Always authorization. 
• Region monitoring (Geofencing): Define a circular region and your app gets a notification when crossing the boundary. Apple’s built-in Reminder app uses that too. Very little battery consumption. It is built upon Core Location’s significant-change location service. Requires Always authorization. 
• Significant-change location service: power-friendly way. Requires Always authorization. 
• Standard location service: Real-time. Most power consumption. 

When? Read a lot, but no/min updates.
What? Images, API call results, computational results (e.g., row heights).
How? Use NSCache, networking frameworks that supports caching, set cache policy (e.g., NSURLRequest.CachePolicy.returnCacheDataElseLoad).

Use it when you need to pass the reference to a value variable (vs. just the value) into a function.