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An important point to consider when designing Cascades applications is the way your C++ code will interact with the QML UI layer. Typically, graphical user interface frameworks promote the model-view-controller (MVC) pattern, which separates your application's logic in three distinct responsibilities (see Figure 3-2).

n Models are responsible for managing your application's data and provide an abstraction layer for accessing and updating it. Typically, they represent the domain objects in your application. Models don't know how to display themselves. However, they can notify controllers and views when their state changes.

n Views are the visual representation of your application data. The same data can be represented by multiple views in different ways, such as a chart or a list of values. Views are displayed to the user.

n A controller effectively plays the role of a mediator between the model and the view. It handles user input and updates the model and view accordingly. In simple applications, you will usually have a single controller; but in more complex scenarios, nothing stops you from having multiple task-oriented controllers.

Figure 3-2. MVC interactions

When a model's state changes, it notifies its associated controllers and views so that they can handle the new state. Depending on the degree of separation you may want to achieve, you can also enforce that all model interactions go strictly through the controller. The most fundamental idea is that controllers and views depend on the model, but the opposite is not true. Models are therefore truly independent elements of your applications.

The Cascades framework does not enforce the MVC pattern. For example, there is no controller class to extend. However, Cascades is sufficiently flexible so that you design your application using the MVC pattern, should you choose so. Figure 3-3 illustrates the fundamental elements of a standard Cascades application.

Figure 3-3. Cascades application elements

The QML layer shows a typical scene graph consisting of a root Page control and a Container with multiple children. Signals and slots are represented using dashed arrows (the signal is the start of the arrow and the corresponding slot is the end). Property bindings are the links shown with a full dot

on both sides. Direct references to an element are shown as arrows with an empty diamond at their start. As illustrated in Figure 3-3, you can break up your application in a C++ layer that contains your application business logic and a QML layer that contains your application's views (typically, user interactions (such as a clicked button) is handled in the QML layer using JavaScript).

As mentioned previously, it is always a good idea to expose C++ logic to the QML layer using coarse-grained services. This is the reason why the application delegate is your central entry point to the C++ application logic (the cloud symbol represents the QML document context from which you can access the application delegate). Interactions between the application delegate and QML controls should be essentially done using signals and slots and property bindings (as shown in Figure 3-3, you can also directly access a UI control from C++ in your application delegate, but this is strongly discouraged). Mapping this to the MVC pattern, you can see that you have lots

of flexibility in defining where your controllers reside. For example, you could decide that the application delegate is your sole controller that handles all interactions between UI controls and the domain model.

Alternatively, you could also split the controllers between JavaScript and the application delegate. Finally, you could also use property bindings between your application delegate and UI controls exclusively. In this case, the only interactions between the UI layer and the application delegate would happen through property updates (in other words, this is would be a form of reactive programming where the data flow between C++ and QML governs the application's state).

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