• 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.

Only when you want to use a weak variable but are sure it will never be nil once it has been set during initialization.

Notes from Capturing Values In Swift Closures

Within a closure, referring to variables outside of the closure, strong references are created.

class MyClass {
    let a = 1
    let myClosure = {
        print(self.a)
    }
    myClosure() // 1
    a = 2
    myClosure() // 2
}

If we don’t want the strong ref, we can specify weak



class MyClass {
    let a = 1
    let myClosure = { [weak self] in
        guard let strongSelf = self else { return }
        print(strongSelf.a)
    }
    myClosure() // 1
    a = 2
    myClosure() // 1    <-- here's the difference
}

Nice article says it all.

class MySingleton {
    static let sharedInstance = MySingleton()
    private init() {}
}