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# Model/View Programming

- Introduction
- The Model/View Architecture

- Models
- Views
- Delegates
- Convenience Classes

- The Model/View Components
- Related Examples

## Introduction

Qt 4 introduces a new set of item view classes that use a model/view architecture to manage the relationship between data and the way it is presented to the user. The separation of functionality introduced by this architecture gives developers greater flexibility and access to more features than was previously possible. In this document, we give a brief introduction to the model/view paradigm, outline the concepts involved, and describe the architecture of the item view system. Each of the components in the architecture is explained, and examples are given that show how to use the classes provided.

## The Model/View Architecture

Model-View-Controller (MVC) is a design pattern originating from Smalltalk that is often used when building user interfaces. In Design Patterns, Gamma et al. write:

> MVC consists of three kinds of objects. The Model is the application object, the View is its screen presentation, and the Controller defines the way the user interface reacts to user input. Before MVC, user interface designs tended to lump these objects together. MVC decouples them to increase flexibility and reuse.

If the view and the controller objects are combined, the result is the model/view architecture. This still separates the way that data is stored from the way that it is presented to the user, but provides a simpler framework based on the same principles. This separation makes it possible to display the same data in several different views, and to implement new types of views, without changing the underlying data structures. To allow flexible handling of user input, we introduce the concept of the delegate. The advantage of having a delegate in this framework is that it allows the way items of data are rendered and edited to be customized.

| The model/view architecture The model communicates with a source of data, providing an interface for the other components in the architecture. The nature of the communication depends on the type of data source, and the way the model is implemented. The view obtains model indexes from the model; these are references to items of data. By supplying model indexes to the model, the view can retrieve items of data from the data source. In standard views, a delegate renders the items of data. When an item is edited, the delegate communicates with the model directly using model indexes.

Generally, the model/view classes can be separated into the three groups described above: models, views, and delegates. Each of these components are defined by abstract classes that provide common interfaces and, in some cases, default implementations of features. Abstract classes are meant to be subclassed in order to provide the full set of functionality expected by other components; this also allows specialized components to be written.

Models, views, and delegates communicate with each other using signals and slots:

- Signals from the model inform the view about changes to the data held by the data source.
- Signals from the view provide information about the user's interaction with the items being displayed.
- Signals from the delegate are used during editing to tell the other model and view about the state of the editor.

### Models

All item models are based on the QAbstractItemModel class. 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 seperate class, a file, a database, or some other application component. QAbstractItemModel provides an interface to data that is flexible enough to handle views that represent data in the form of tables, lists, and trees. However, when implementing new models for list and table-like data structures, the QAbstractListModel and QAbstractTableModel classes are better starting points because they provide appropriate default implementations of common functions. Each of these classes can be subclassed to provide models that support specialized kinds of lists and tables.

The QStandardItemModel class provides a ready-made model that can be used to handle arbitrary pieces of data which it stores internally.

### Views

Complete implementations are provided for different kinds of views: QListView displays a list of items, QTableView displays data from a model in a table, and QTreeView shows model items of data in a hierarchical list. Each of these classes is based on the QAbstractItemView abstract base class. Although these classes are ready-to-use implementations, they can also be subclassed to provide customized views.

### Delegates

QAbstractItemDelegate is the abstract base class for delegates in the model/view framework. A default delegate implementation is provided by the QItemDelegate class, and this is used as the default delegate by Qt's standard views.

### Convenience Classes

A number of convenience classes are derived from the standard view classes for the benefit of applications that rely on Qt's item-based item view and table classes. They are not intended to be subclassed, but simply exist to provide a familiar interface to the equivalent classes in Qt 3. Examples of such classes include QListWidget, QTreeWidget, and QTableWidget; these provide similar behavior to the `QListBox`, `QListView`, and `QTable` classes in Qt 3.

These classes are less flexible than the view classes, and cannot be used with arbitrary models. We recommend that you use a model/view approach to handling data in item views unless you strongly need a item-based set of classes.

## The Model/View Components

The following sections describe the way in which the model/view pattern is used in Qt. Each section provides an example of use, and is followed by a section showing how you can create new components.

- Using Models and Views
  - Introduction
  - Using Views with an Existing Model
- Model Classes
  - Basic Concepts
  - Model Indexes
  - Rows and Columns
  - Parents of Items
  - Item Roles
  - Summary of Concepts
  - Using Model Indexes
  - Further Reading
- Creating New Models
  - Introduction
  - A Read-Only Example Model
  - Dimensions of The Model
  - Model Headers and Data
  - An Editable Model
  - Making the Model Editable
  - Inserting and Removing Rows
  - Next Steps
- View Classes
  - Concepts
  - Using an Existing View
  - Using a Model
  - Using Multiple Views onto the Same Model
  - Handling Selections of Items
  - Sharing Selections Between Views
- Handling Selections in Item Views
  - Concepts
  - Using a Selection Model
  - Selecting Items
  - Reading the Selection State
  - Updating a Selection
  - Selecting All Items in a Model
- Delegate Classes
  - Concepts
  - Using an Existing Delegate
  - A Simple Delegate
  - Providing an Editor
  - Submitting Data to the Model
  - Updating the Editor's Geometry
  - Editing Hints
- Item View Convenience Classes
  - Overview
  - List Widgets
  - Tree Widgets
  - Table Widgets
  - Common Features
  - Hidden Items
  - Selections
  - Searching

See also the list of Model/View Classes.

## Related Examples

- Dir View
- Spin Box Delegate
- Pixelator
- Simple Tree Model
- Chart

[Next: Using Models and Views]