Swift: Understanding // MARK

Question

What is the purpose of writing comments in Swift as:

// MARK: This is a comment

When you can also do:

// This is a comment

What does the // MARK achieve?

Answer 1

The // MARK: and // MARK: - syntax in Swift functions identically to the #pragma mark and #pragma mark - syntax in Objective-C.

When using this syntax (plus // TODO: and // FIXME:), you can get some extra information to show up in the quick jump bar.

Consider these few lines of source code:

// MARK: A mark comment lives here.

func isPrime(_ value: UInt) -> Bool { return true }

And for reference, the quick jump bar is at the top in Xcode:

It exists mostly to help with quick navigation within the file.

Note that the dash (// MARK: -) causes a nice separation line to show up. Consider this MARK comment:

// MARK: - A mark comment lives here.

The darker gray separator line just above the bolded option in that menu comes from the dash.

Additionally, we can achieve this separator line without a comment by simply not having any text after the dash:

// MARK: -

As mentioned, // TODO: and // FIXME: comments will also appear here.

// MARK: - Prime functions

func isPrime(_ value: UInt) -> Bool {
    // TODO: Actually implement the logic for this method
    return true
}

func nthPrime(_ value: UInt) -> Int {
    // FIXME: Returns incorrect values for some arguments
    return 2
}

• FIXMEs get a little band-aid icon that help them standout.
• MARK icon looks like a table of contents
• TODO icons look more like a checklist

Clicking on any line in the quick jump bar takes you directly to that line in the source code.