let fetchRequest = NSFetchRequest(entityName: "Foo")
var thePredicate: NSPredicate?
thePredicate = NSPredicate(format: "message == 'example'")

The entity Foo has a message string attribute

Rather than passing a static string as a predicate's criteria. It is possible to substitute values by using format specifiers. There are five format specifiers:

• %K is a var arg substitution for a key path.
• %@ is a var arg substitution for an object value-often a string, number, date, or an array.
• %ld is a var arg substitution for an int value.
• %la is a var arg substitution for a double.
• %a is a var arg substitution for a float.

In the following example, the %K format specifier serves as the left-hand argument which passes in the "message" property dynamically. The %@ format specifier serves as the right-hand argument to dynamically pass in a string containing the word "example".

let predicate = NSPredicate(format:"%K == %@", "message", "example")