Definition: The model holds all the data, state, and application logic. The controller takes user input and changes the model appropriately. The view provides a visual presentation of the model.

This is actually a compound pattern of several patterns. The controller is a Strategy pattern that configures the view. The model is an Observer that observes the controller to see how it changes. Everything is decoupled.

Usage: This class is typically used to split gui widgets into 3 parts so that you can change one part without changing the others.

Example

class TextField {
	TextField() {
		this(new TextFieldController(this), new TextFieldModel())
	}

	TextField(TextFieldController controller, TextFieldModel model) {
		setController(controller)
		setModel(model)
	}

	draw(Graphics g) {
		output getModel().getText()
	}

	TextFieldController getController() {return controller}

	TextFieldModel getModel() {return model}

	setController(TextFieldController controller) {this.controller = controller}

	setModel(TextFieldModel model) {this.model = model}

	TextFieldController controller;
	TextFieldModel model;
}

class TextFieldController {
	TextFieldController(TextField field) {
		this.field = field
	}

	respondToKeyEvent(KeyEvent event) {
		TextFieldModel model = field.getModel();
		String text = model.getText();
		update text to match the event
		model.setText(text)
	}

	TextField field
}

class TextFieldModel {
	TextFieldModel() {
		setText("")
	}

	getText() {return text}
	setText(String text) {this.text = text}

	String text
}

The point of all this is so that the TextFieldController and TextFieldModel classes can be subclassed without having to subclass the TextField itself.