A type conforming to ExpressibleByStringInterpolation uses a helper type called StringInterpolation to perform its interpolation. Many types can use DefaultStringInterpolation, which implements String's interpolation behavior. Types can also implement custom behavior by providing their own type conforming to StringInterpolationProtocol.
In addition to its formal requirements, init(literalCapacity:interpolationCount:) and appendLiteral(_:), StringInterpolationProtocol has an additional, informal requirement, appendInterpolation. String interpolations using \() syntax are translated into calls to matching appendInterpolation methods.
StringInterpolationProtocol conformers must provide at least one appendInterpolation method which:
- Is an instance method, as opposed to a
staticorclassmethod - Does not specify a return type, explicitly returns
Void, or is marked with the@discardableResultattribute - Is at least as accessible as its containing type
There are no restrictions on an appendInterpolation method's argument list, generic parameters, availability, or error-throwing behavior.
If appendInterpolation is overloaded, the Swift compiler will choose an appropriate overload using the labels and argument types of each interpolation. When choosing an overload, any accessible appendInterpolation instance method may be used, even if it does not meet all of the requirements above. However, if a StringInterpolationProtocol conformer doesn't have any appendInterpolation methods which meet all of the requirements, an error will be reported at compile time.
To learn more about customizing string interpolation behavior, see the standard library documentation of the ExpressibleByStringInterpolation and StringInterpolationProtocol protocols.