• 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() {}
}

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