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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\example itemviews/fetchmore
\title Fetch More Example
\examplecategory {User Interface Components}
\ingroup examples-itemviews
\brief The Fetch More example shows how to add items to an item view
model on demand.
\image fetchmore-example.png
When you have large - or perhaps even infinite - data sets, you
will need to add items to the model in batches, and preferably only
when the items are needed by the view (i.e., when they become visible
in the view).
In this example, we implement \c FileListModel - an item view
model containing the entries of a directory. We also have \c
Window, which sets up the GUI and feeds the model with
directories.
The UI consists of a dialog with a list showing the contents
of the root directory. Directories can be navigated by double-clicking.
At the bottom, there is a log window displaying messages when the view
asks the model for more data.
To exercise it, navigate to a large directory (say \c /bin), and scroll
to the bottom. Log messages appear showing the data being retrieved.
Let's take a tour of \c {FileListModel}'s code.
\section1 FileListModel Class Definition
The \c FileListModel inherits QAbstractListModel and contains the
contents of a directory. It will add items to itself only when
requested to do so by the view.
\snippet itemviews/fetchmore/filelistmodel.h 0
The secret lies in the reimplementation of
\l{QAbstractItemModel::}{fetchMore()} and
\l{QAbstractItemModel::}{canFetchMore()} from QAbstractItemModel.
These functions are called by the item view when it needs more
items.
The \c setDirPath() function sets the directory the model will
work on. We emit \c numberPopulated() each time we add a batch of
items to the model.
We keep all directory entries in \c fileList. \c fileCount is the
number of items that have been added to the model.
\section1 FileListModel Class Implementation
We start by checking out the \c setDirPath().
\snippet itemviews/fetchmore/filelistmodel.cpp 0
We use a QDir to get the contents of the directory. We need to
inform QAbstractItemModel that we want to remove all items - if
any - from the model.
\snippet itemviews/fetchmore/filelistmodel.cpp 1
The \c canFetchMore() function is called by the view when it needs
more items. We return true if there still are entries that we have
not added to the model; otherwise, we return false.
And now, the \c fetchMore() function itself:
\snippet itemviews/fetchmore/filelistmodel.cpp 2
We first calculate the number of items to fetch.
\l{QAbstractItemModel::}{beginInsertRows()} and
\l{QAbstractItemModel::}{endInsertRows()} are mandatory for
QAbstractItemModel to keep up with the row insertions. Finally, we
emit \c numberPopulated(), which is picked up by \c Window.
To complete the tour, we also look at \c rowCount() and \c data().
\snippet itemviews/fetchmore/filelistmodel.cpp 4
Notice that the row count is only the items we have added so far,
i.e., not the number of entries in the directory.
In \c data(), we return the appropriate entry from the \c
fileList. We also separate the batches with a different background
color.
*/
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