protocolClasses

protocolClasses is a configuration setting used by URL loading systems to determine which custom protocol handlers should be considered when processing network requests. In modern networking APIs, it is typically an optional array of classes that subclass a general URL protocol handler. Each class in the array is expected to be able to decide whether it can handle a given request and, if so, to load the data for that request.

How it works: When a request is issued, the system iterates the classes listed in protocolClasses in order. For each class, it calls a method that indicates whether the class can handle the request. The first class that returns true becomes responsible for loading the request. That class then creates an instance and carries out the loading, relaying data or errors back to the client. If no class can handle the request, the system falls back to the standard loading mechanisms.

Usage: To use protocolClasses, developers create a subclass of the base URL protocol handler and implement the required methods, including a test for applicability, the canonical form of the request, and the loading lifecycle (startLoading and stopLoading). The custom class is then added to the protocolClasses array of a URLSessionConfiguration (or analogous configuration object) before creating the URLSession. Existing protocol handlers can be preserved by appending them to the array rather than replacing it entirely.

Considerations: Care must be taken to avoid infinite loops or recursive handling, to ensure thread safety, and to respect privacy and security policies. Common use cases include request interception for debugging, conditional caching, mock responses for testing, or implementing custom transport behavior for specialized backends.