1.4.2 The View
We introduced the View as the component of our application whose role is to interact with the User, by accepting its input and showing the contents of the Model, and operates on it in read-only. Being the face of the application, it is also the one that is more likely to change and adapt, often under pressure of non-programming factors such as visual design and usability needs.
A View holds the following responsibilities:
- handle “purely GUI” intelligence, like behavior on resizing, repainting (e.g. on showing the window), data displaying and visual formatting.
- handle and dispatch primary events such as mouse clicks and keyboard key presses
- listens for Model notifications and responds by fetching and repainting the new state from the Model (reactivity).
A View should not:
- perform directly any modifying action on the Model as a consequence to UI events. Instead, they should delegate this task to Controllers.
- perform any operation that is competence of the Model, nor store Model data, except for caching to improve rendering performance. Cached data are never authoritative, and should never be pushed back into the Model, or handed out to other objects.
A View is generally composed out of reusable, generic visual building blocks called Widgets. Examples of widgets are buttons, checkboxes, and menus. Widgets, differently from Views, are not reactive: they have no concept of Model nor of notification listening. Their visual content is modified by means of method calls. Complex UI interfaces are assembled from a collection of Widgets, hierarchically contained in dialogs, windows and other visual containers. The resulting UI becomes a View when Model observing capabilities are added.
MVC is not only limited to GUI representations, and Views are not necessarily graphical objects. In fact, anything that can report information to the User in some form can be classified as a View. For example, a musical notation Model can be observed by two Views: one that shows the musical notation on screen and another that plays it on the speakers. —