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<td >Qt 5.12</td><td ><a href="qtwidgets-index.html">Qt Widgets</a></td><td ><a href="examples-itemviews.html">Item Views Examples</a></td><td >Fetch More Example</td></tr></table><table class="buildversion"><tr>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#filelistmodel-class-definition">FileListModel Class Definition</a></li>
<li class="level1"><a href="#filelistmodel-class-implementation">FileListModel Class Implementation</a></li>
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<h1 class="title">Fetch More Example</h1>
<span class="subtitle"></span>
<!-- $$$itemviews/fetchmore-brief -->
<p>The Fetch More example shows how two add items to an item view model on demand.</p>
<!-- @@@itemviews/fetchmore -->
<!-- $$$itemviews/fetchmore-description -->
<div class="descr"> <a name="details"></a>
<p class="centerAlign"><img src="images/fetchmore-example.png" alt="" /></p><p>This example consists of a dialog where you can enter a directory name in the <b>Directory</b> edit field. The application loads and visualizes all files it finds as you are typing. It is not required to press [Enter] to launch the search.</p>
<p>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 are visible in the view).</p>
<p>In this example, we implement <code>FileListModel</code> - an item view model containing the entries of a directory. We also have <code>Window</code>, which sets up the GUI and feeds the model with directories.</p>
<p>Let's take a tour of <code>FileListModel</code>'s code.</p>
<a name="filelistmodel-class-definition"></a>
<h2 id="filelistmodel-class-definition">FileListModel Class Definition</h2>
<p>The <code>FileListModel</code> inherits <a href="../qtcore/qabstractlistmodel.html">QAbstractListModel</a> and contains the contents of a directory. It will add items to itself only when requested to do so by the view.</p>
<pre class="cpp">
<span class="keyword">class</span> FileListModel : <span class="keyword">public</span> <span class="type"><a href="../qtcore/qabstractlistmodel.html">QAbstractListModel</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
FileListModel(<span class="type"><a href="../qtcore/qobject.html">QObject</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="type">int</span> rowCount(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span>parent <span class="operator">=</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span>()) <span class="keyword">const</span> override;
<span class="type"><a href="../qtcore/qvariant.html">QVariant</a></span> data(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span>index<span class="operator">,</span> <span class="type">int</span> role <span class="operator">=</span> <span class="type"><a href="../qtcore/qt.html">Qt</a></span><span class="operator">::</span>DisplayRole) <span class="keyword">const</span> override;
<span class="keyword">signals</span>:
<span class="type">void</span> numberPopulated(<span class="type">int</span> number);
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> setDirPath(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span> <span class="operator">&</span>path);
<span class="keyword">protected</span>:
bool canFetchMore(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span>parent) <span class="keyword">const</span> override;
<span class="type">void</span> fetchMore(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span>parent) override;
<span class="keyword">private</span>:
<span class="type"><a href="../qtcore/qstringlist.html">QStringList</a></span> fileList;
<span class="type">int</span> fileCount;
};
</pre>
<p>The secret lies in the reimplementation of <a href="../qtcore/qabstractitemmodel.html#fetchMore">fetchMore()</a> and <a href="../qtcore/qabstractitemmodel.html#canFetchMore">canFetchMore()</a> from <a href="../qtcore/qabstractitemmodel.html">QAbstractItemModel</a>. These functions are called by the item view when it needs more items.</p>
<p>The <code>setDirPath()</code> function sets the directory the model will work on. We emit <code>numberPopulated()</code> each time we add a batch of items to the model.</p>
<p>We keep all directory entries in <code>fileList</code>. <code>fileCount</code> is the number of items that have been added to the model.</p>
<a name="filelistmodel-class-implementation"></a>
<h2 id="filelistmodel-class-implementation">FileListModel Class Implementation</h2>
<p>We start by checking out the <code>setDirPath()</code>.</p>
<pre class="cpp">
<span class="type">void</span> FileListModel<span class="operator">::</span>setDirPath(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span> <span class="operator">&</span>path)
{
<span class="type"><a href="../qtcore/qdir.html">QDir</a></span> dir(path);
beginResetModel();
fileList <span class="operator">=</span> dir<span class="operator">.</span>entryList();
fileCount <span class="operator">=</span> <span class="number">0</span>;
endResetModel();
}
</pre>
<p>We use a <a href="../qtcore/qdir.html">QDir</a> to get the contents of the directory. We need to inform <a href="../qtcore/qabstractitemmodel.html">QAbstractItemModel</a> that we want to remove all items - if any - from the model.</p>
<pre class="cpp">
bool FileListModel<span class="operator">::</span>canFetchMore(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span> <span class="comment">/* index */</span>) <span class="keyword">const</span>
{
<span class="keyword">if</span> (fileCount <span class="operator"><</span> fileList<span class="operator">.</span>size())
<span class="keyword">return</span> <span class="keyword">true</span>;
<span class="keyword">else</span>
<span class="keyword">return</span> <span class="keyword">false</span>;
}
</pre>
<p>The <code>canFetchMore()</code> 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.</p>
<p>And now, the <code>fetchMore()</code> function itself:</p>
<pre class="cpp">
<span class="type">void</span> FileListModel<span class="operator">::</span>fetchMore(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span> <span class="comment">/* index */</span>)
{
<span class="type">int</span> remainder <span class="operator">=</span> fileList<span class="operator">.</span>size() <span class="operator">-</span> fileCount;
<span class="type">int</span> itemsToFetch <span class="operator">=</span> <a href="../qtcore/qtglobal.html#qMin">qMin</a>(<span class="number">100</span><span class="operator">,</span> remainder);
<span class="keyword">if</span> (itemsToFetch <span class="operator"><</span><span class="operator">=</span> <span class="number">0</span>)
<span class="keyword">return</span>;
beginInsertRows(<span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span>()<span class="operator">,</span> fileCount<span class="operator">,</span> fileCount<span class="operator">+</span>itemsToFetch<span class="operator">-</span><span class="number">1</span>);
fileCount <span class="operator">+</span><span class="operator">=</span> itemsToFetch;
endInsertRows();
<span class="keyword">emit</span> numberPopulated(itemsToFetch);
}
</pre>
<p>We first calculate the number of items to fetch. <a href="../qtcore/qabstractitemmodel.html#beginInsertRows">beginInsertRows()</a> and <a href="../qtcore/qabstractitemmodel.html#endInsertRows">endInsertRows()</a> are mandatory for <a href="../qtcore/qabstractitemmodel.html">QAbstractItemModel</a> to keep up with the row insertions. Finally, we emit <code>numberPopulated()</code>, which is picked up by <code>Window</code>.</p>
<p>To complete the tour, we also look at <code>rowCount()</code> and <code>data()</code>.</p>
<pre class="cpp">
<span class="type">int</span> FileListModel<span class="operator">::</span>rowCount(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span> <span class="comment">/* parent */</span>) <span class="keyword">const</span>
{
<span class="keyword">return</span> fileCount;
}
<span class="type"><a href="../qtcore/qvariant.html">QVariant</a></span> FileListModel<span class="operator">::</span>data(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qmodelindex.html">QModelIndex</a></span> <span class="operator">&</span>index<span class="operator">,</span> <span class="type">int</span> role) <span class="keyword">const</span>
{
<span class="keyword">if</span> (<span class="operator">!</span>index<span class="operator">.</span>isValid())
<span class="keyword">return</span> <span class="type"><a href="../qtcore/qvariant.html">QVariant</a></span>();
<span class="keyword">if</span> (index<span class="operator">.</span>row() <span class="operator">></span><span class="operator">=</span> fileList<span class="operator">.</span>size() <span class="operator">|</span><span class="operator">|</span> index<span class="operator">.</span>row() <span class="operator"><</span> <span class="number">0</span>)
<span class="keyword">return</span> <span class="type"><a href="../qtcore/qvariant.html">QVariant</a></span>();
<span class="keyword">if</span> (role <span class="operator">=</span><span class="operator">=</span> <span class="type"><a href="../qtcore/qt.html">Qt</a></span><span class="operator">::</span>DisplayRole) {
<span class="keyword">return</span> fileList<span class="operator">.</span>at(index<span class="operator">.</span>row());
} <span class="keyword">else</span> <span class="keyword">if</span> (role <span class="operator">=</span><span class="operator">=</span> <span class="type"><a href="../qtcore/qt.html">Qt</a></span><span class="operator">::</span>BackgroundRole) {
<span class="type">int</span> batch <span class="operator">=</span> (index<span class="operator">.</span>row() <span class="operator">/</span> <span class="number">100</span>) <span class="operator">%</span> <span class="number">2</span>;
<span class="keyword">if</span> (batch <span class="operator">=</span><span class="operator">=</span> <span class="number">0</span>)
<span class="keyword">return</span> <a href="qapplication.html#qApp">qApp</a><span class="operator">-</span><span class="operator">></span>palette()<span class="operator">.</span>base();
<span class="keyword">else</span>
<span class="keyword">return</span> <a href="qapplication.html#qApp">qApp</a><span class="operator">-</span><span class="operator">></span>palette()<span class="operator">.</span>alternateBase();
}
<span class="keyword">return</span> <span class="type"><a href="../qtcore/qvariant.html">QVariant</a></span>();
}
</pre>
<p>Notice that the row count is only the items we have added so far, i.e., not the number of entries in the directory.</p>
<p>In <code>data()</code>, we return the appropriate entry from the <code>fileList</code>. We also separate the batches with a different background color.</p>
<p>Files:</p>
<ul>
<li><a href="qtwidgets-itemviews-fetchmore-fetchmore-pro.html">itemviews/fetchmore/fetchmore.pro</a></li>
<li><a href="qtwidgets-itemviews-fetchmore-filelistmodel-cpp.html">itemviews/fetchmore/filelistmodel.cpp</a></li>
<li><a href="qtwidgets-itemviews-fetchmore-filelistmodel-h.html">itemviews/fetchmore/filelistmodel.h</a></li>
<li><a href="qtwidgets-itemviews-fetchmore-main-cpp.html">itemviews/fetchmore/main.cpp</a></li>
<li><a href="qtwidgets-itemviews-fetchmore-window-cpp.html">itemviews/fetchmore/window.cpp</a></li>
<li><a href="qtwidgets-itemviews-fetchmore-window-h.html">itemviews/fetchmore/window.h</a></li>
</ul>
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<!-- @@@itemviews/fetchmore -->
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