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# Using Models and Views

- Introduction
- Using Views with an Existing Model

## Introduction

Two of the standard models provided by Qt are QStandardItemModel and QFileSystemModel. QStandardItemModel is a multi-purpose model that can be used to represent various different data structures needed by list, table, and tree views. This model also holds the items of data. QFileSystemModel is a model that maintains information about the contents of a directory. As a result, it does not hold any items of data itself, but simply represents files and directories on the local filing system.

QFileSystemModel provides a ready-to-use model to experiment with, and can be easily configured to use existing data. Using this model, we can show how to set up a model for use with ready-made views, and explore how to manipulate data using model indexes.

## Using Views with an Existing Model

The QListView and QTreeView classes are the most suitable views to use with QFileSystemModel. The example presented below displays the contents of a directory in a tree view next to the same information in a list view. The views share the user's selection so that the selected items are highlighted in both views.

We set up a QFileSystemModel so that it is ready for use, and create some views to display the contents of a directory. This shows the simplest way to use a model. The construction and use of the model is performed from within a single `main()` function:

     int main(int argc, char *argv[])
     {
         QApplication app(argc, argv);
         QSplitter *splitter = new QSplitter;

         QFileSystemModel *model = new QFileSystemModel;
         model->setRootPath(QDir::currentPath());

The model is set up to use data from a certain file system. The call to setRootPath() tell the model which drive on the file system to expose to the views.

We create two views so that we can examine the items held in the model in two different ways:

         QTreeView *tree = new QTreeView(splitter);
         tree->setModel(model);
         tree->setRootIndex(model->index(QDir::currentPath()));

         QListView *list = new QListView(splitter);
         list->setModel(model);
         list->setRootIndex(model->index(QDir::currentPath()));

The views are constructed in the same way as other widgets. Setting up a view to display the items in the model is simply a matter of calling its setModel() function with the directory model as the argument. We filter the data supplied by the model by calling the setRootIndex() function on each view, passing a suitable model index from the file system model for the current directory.

The `index()` function used in this case is unique to QFileSystemModel; we supply it with a directory and it returns a model index. Model indexes are discussed in the Model Classes chapter.

The rest of the function just displays the views within a splitter widget, and runs the application's event loop:

         splitter->setWindowTitle("Two views onto the same file system model");
         splitter->show();
         return app.exec();
     }

In the above example, we neglected to mention how to handle selections of items. This subject is covered in more detail in the chapter on Handling Selections in Item Views. Before examining how selections are handled, you may find it useful to read the Model Classes chapter which describes the concepts used in the model/view framework.

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