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

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.