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Separate an application's domain state and rules into a model, its presentation into a view, and interpretation of user actions into a controller. The controller requests model operations; the view renders coherent model state. A playlist model, for example, owns song order independently of whether a menu, button or keyboard action requests a move. The Observer pattern often carries model-change notifications to views, but model-view-controller defines responsibilities rather than requiring a particular notification mechanism. This supports separation of concerns and testing domain logic independently of a graphical interface.

In model-view-controller, the model owns application data and its rules: here it is the ordered DataList with its uniqueness condition on horizontal coordinates. A view renders those data, for example as a line plot or a bar plot. View-specific decisions such as whether to show axes need not change the model. A controller interprets actions such as entering a new sample or selecting a plot style, and requests the appropriate model or view operation.

This separation of concerns permits several views of the same data and allows model operations to be tested without drawing a window. When the model changes, affected views need a notification or another refresh mechanism. The Observer pattern in part (d) supplies one such mechanism, without making the model depend on every concrete view software class. The division is

In the Observer pattern, an observable subject keeps registered observers behind a shared object interface. After a relevant update, the subject calls that object interface on each observer. Observers can register and unregister independently, and the subject needs no knowledge of their concrete software classes.

For this model-view-controller application, DataList is the subject, while graph views are observers. After a successful insertion, a notification tells each registered view to redraw from the updated model. An invalid insertion leaves the model unchanged and needs no change notification. Calling observers only after restoring the representation invariant prevents views from seeing a half-completed insertion. Hence

Model-view-controller divides an interactive application into a model containing application state and domain rules, a view rendering that state, and a controller interpreting user actions and requesting model operations. In the converter, the numeric amounts and exchange relation belong to the model, the text fields and labels form the view, and the edit handler parses input and chooses which quantity to preserve. A playlist model similarly owns song order independently of the controls used to change it.

A user acts through the view, the controller interprets that input, the model validates and commits a change, and the view then renders the resulting state. The exact packaging varies: a widget can combine view and controller responsibilities while still separating domain logic. Keeping the model independent of particular controls supports separation of concerns, encapsulation, reuse of the same model in several views, and unit testing of domain behavior without a graphical interface.

The Observer pattern often supplies the model-to-view notification path: views subscribe to model changes, so the model need not directly know every concrete view. It can also carry view input events to a controller, as in Scala Swing. Model-view-controller is an architectural separation of responsibilities; the Observer pattern is one mechanism for their communication. They are related without being identical, and small applications can implement the separation with direct calls where event subscriptions add no benefit.

Assign distinct responsibilities to components with explicit interfaces, so one responsibility can change without requiring unrelated changes elsewhere. For example, a model-view-controller application separates domain state, presentation and input interpretation. The Observer pattern can communicate a coherent state change to several views while keeping concrete display details out of the model. The separation should follow actual responsibilities; multiplying classes without a clear boundary does not establish it.

The organization of a software system's major components, their responsibilities and their interactions. A software architecture constrains which components own state and which may request or observe changes. Model-view-controller is one example for interactive applications.