Adding 'extensions to conform to protocols' rule.

README.md

@@ -6,7 +6,7 @@
  * [Immutable controllers](./rules/immutable-controller.md)
  * [Use lazy actions](./rules/use-lazy-actions.md)
  * Extract presentation logic into view models
- * Use extensions when conforming to protocols
+ * [Use extensions when conforming to protocols](./rules/use-extensions-when-conforming-to-protocols.md)
  * Use private extensions for private methods
  * [Use property hooks to configure outlets](./rules/property-hooks-for-outlets.md)
  * [Avoid structs that use closure that capture self](./rules/avoid-struct-closure-self.md)

rules/use-extensions-when-conforming-to-protocols.md

@@ -0,0 +1,62 @@
+## Use extensions when conforming to protocols
+
+### Description
+
+When a class must conform to a protocol, for example with UI classes' delegates, we declare all the properties and functions needed in a separate extension which implements that protocol.
+
+
+
+### Rationale
+Even if the class is following all other rules and the Single Responsibility Principle, having all its functions declared inside the class make this confusing and difficult to find. The class can have many private and public functions, but if the class must conform to a protocol, this sure is a particular task. When those protocol conforming functions are extracted into a separate extension, whenever someone wants to review how the class manages that task, the only search needed is looking for the extension that implements the protocol. This prevents the person from having to first, search which are the functions the protocol requires and second, search through all the set of functions the class has for those wanted ones. Moreover, it helps break the class's responsabilities into smaller "tasks", making it easier to follow and understand.
+
+### Example
+
+```swift
+public final class SignUpController: UIViewController {
+
+    private lazy var signUpView: SignUpView = .loadFromNib()
+
+    private let _viewModel: SignUpViewModel
+
+    public init(viewModel: SignUpViewModel) {
+        _viewModel = viewModel
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    public override func viewDidLoad() {
+        super.viewDidLoad()
+        bindViewModel()
+    }
+
+    public bindViewModel() {
+        signUpView.emailTextField.delegate = self
+        _viewModel.email <~ signUpView.emailTextField.rex_textSignal
+        signUpView.passwordTextField.delegate = self
+        _viewModel.password <~ signUpView.passwordTextField.rex_textSignal
+
+        bindButtons()
+        ...
+    }
+
+    ...
+
+}
+
+extension SignUpController: UITextFieldDelegate {
+
+    public func textFieldShouldReturn(textField: UITextField) -> Bool {
+        guard validCredetentials() else {
+            textField.resignFirstResponder()
+            return true
+        }
+        if textField == signUpView.emailTextField {
+            signUpView.passwordTextField.becomeFirstResponder()
+        } else {
+            signUpView.emailTextField.becomeFirstResponder()
+        }
+        return true
+    }
+
+}
+
+```