Print the type signature of an objc method found in the runtime

dumpMethodSignature.swift

import ObjectiveC.runtime

private func dumpMethodSignature(_ clsName: String, _ selName: String) {
    let cls: AnyClass = NSClassFromString(clsName)!
    let sel = NSSelectorFromString(selName)
    let m = class_getInstanceMethod(cls, sel)!

    let nargs = method_getNumberOfArguments(m)
    let ret = String(cString: method_copyReturnType(m))
    let enc = method_getTypeEncoding(m).map { String(cString: $0) } ?? "?"
    print("== \(clsName) \(selName)")
    print("  encoding: \(enc)")
    print("  return:   \(ret)")
    print("  args (\(nargs)):")
    for i in 0..<nargs {
        if let t = method_copyArgumentType(m, i) {
            print("    [\(i)] \(String(cString: t))")
            free(t)
        }
    }
}