Selections are applied to a collection of model indexes held by a selection model. The most recent selection of items applied is known as the current selection. The effects of this selection can be modified even after its application through the use of certain types of selection commands. Delegates are expected to be able to render their contents themselves by implementing the paint() and sizeHint() functions. However, simple widget-based delegates can subclass QStyledItemDelegate instead of QAbstractItemDelegate, and take advantage of the default implementations of these functions.
List and table models only provide a flat structure in which items of data are stored. As a result, they may insert new rows (and columns) when data is dropped on an existing item in a view, or they may overwrite the item’s contents in the model using some of the data supplied. Tree models are often able to add child items containing new data to their underlying data stores, and will therefore behave more predictably as far as the user is concerned. The convenience views (QListWidget, QTableWidget, and QTreeWidget) override the default drag and drop functionality to provide less flexible, but more natural behavior that is appropriate for many applications. For more information on drag and drop in convenience views, you can see Using drag and drop with item views. In the model/view architecture, the model provides a standard interface that views and delegates use to access data.
The request reaches the controller…
All of the standard views construct their own selection models by default, and interact with them in the normal way. The selection model being used by a view can be obtained through the selectionModel() function, and a replacement selection model can be specified with setSelectionModel(). The ability to control the selection model used by a view is useful when we want to provide multiple consistent views onto the same model data. As well as presenting data, views handle navigation between items, and some aspects of item selection. The views also implement basic user interface features, such as context menus and drag and drop.
They may render individual items of data themselves, or use delegates to handle both rendering and editing features. This class defines an interface that is used by views and delegates to access data. The data itself does not have to be stored in the model; it can be held in a data structure or repository provided by a separate class, a file, a database, or some other application component. To solve this problem, the model/view framework uses proxy models to manage the information supplied between individual models and views. Proxy models are components that behave like ordinary models from the perspective of a view, and access data from source models on behalf of that view.
The final product is known as the view…
We install the delegate’s default event filter on the editor to ensure that it provides the standard editing shortcuts that users expect. Additional shortcuts can be added to the editor to allow more sophisticated behavior; these are discussed in the section on Editing Hints. The setItemDelegate() function allows you to install a custom delegate for a standard view, and it is necessary to use this function when setting the delegate for a custom view. You have all different sizes and shapes, and you grab the ones you need to build the spaceship. In a web app, models help the controller retrieve all of the information it needs from the database. The controller contains logic that updates the model and/or view in response to input from the users of the app.
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