(From Hacking With Swift)
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[label1]|", options: [], metrics: nil, views: viewsDictionary))
The H: parts means that we're defining a horizontal layout; we'll do a vertical layout soon. The pipe symbol, |, means "the edge of the view." We're adding these constraints to the main view inside our view controller, so this effectively means "the edge of the view controller." Finally, we have [label1], which is a visual way of saying "put label1 here". Imagine the brackets, [ and ], are the edges of the view.
So, "H:|[label1]|" means "horizontally, I want my label1 to go edge to edge in my view." But there's a hiccup: what is "label1"? Sure, we know what it is because it's the name of our variable, but variable names are just things for humans to read and write – the variable names aren't actually saved and used when the program runs.
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[label1]-[label2]-[label3]-[label4]-[label5]|", options: [], metrics: nil, views: viewsDictionary))
That's identical to the previous five, except for the VFL part. This time we're specifying V:, meaning that these constraints are vertical. And we have multiple views inside the VFL, so lots of constraints will be generated. The new thing in the VFL this time is the - symbol, which means "space". It's 10 points by default, but you can customise it.
