<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <!-- fetchmore.qdoc --> <title>Fetch More Example | Qt Widgets 5.12.2</title> <link rel="stylesheet" type="text/css" href="style/offline-simple.css" /> <script type="text/javascript"> document.getElementsByTagName("link").item(0).setAttribute("href", "style/offline.css"); // loading style sheet breaks anchors that were jumped to before // so force jumping to anchor again setTimeout(function() { var anchor = location.hash; // need to jump to different anchor first (e.g. none) location.hash = "#"; setTimeout(function() { location.hash = anchor; }, 0); }, 0); </script> </head> <body> <div class="header" id="qtdocheader"> <div class="main"> <div class="main-rounded"> <div class="navigationbar"> <table><tr> <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> <td id="buildversion" width="100%" align="right"><a href="qtwidgets-index.html">Qt 5.12.2 Reference Documentation</a></td> </tr></table> </div> </div> <div class="content"> <div class="line"> <div class="content mainContent"> <div class="sidebar"> <div class="toc"> <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> </ul> </div> <div class="sidebar-content" id="sidebar-content"></div></div> <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> </div> <!-- @@@itemviews/fetchmore --> </div> </div> </div> </div> </div> <div class="footer"> <p> <acronym title="Copyright">©</acronym> 2019 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners.<br/> The documentation provided herein is licensed under the terms of the <a href="http://www.gnu.org/licenses/fdl.html">GNU Free Documentation License version 1.3</a> as published by the Free Software Foundation.<br/> Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners. </p> </div> </body> </html>