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<td >Qt 5.9</td><td ><a href="qtwidgets-index.html">Qt Widgets</a></td><td >Part 3 - Navigating between Entries</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#defining-the-addressbook-class">Defining the AddressBook Class</a></li>
<li class="level1"><a href="#implementing-the-addressbook-class">Implementing the AddressBook Class</a></li>
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<h1 class="title">Part 3 - Navigating between Entries</h1>
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<!-- $$$tutorials/addressbook/part3-description -->
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<p>The address book is now about half complete. We should add the capability to navigate among the contacts, but first we must decide what sort of a data structure we need for containing these contacts.</p>
<p>In the previous section, we used a <a href="../qtcore/qmap.html">QMap</a> of key-value pairs with the contact's name as the <i>key</i>, and the contact's address as the <i>value</i>. This works well for our case. However, in order to navigate and display each entry, a little bit of enhancement is needed.</p>
<p>We enhance the <a href="../qtcore/qmap.html">QMap</a> by making it replicate a data structure similar to a circularly-linked list, where all elements are connected, including the first element and the last element. The figure below illustrates this data structure.</p>
<p class="centerAlign"><img src="images/addressbook-tutorial-part3-linkedlist.png" alt="" /></p><a name="defining-the-addressbook-class"></a>
<h2 id="defining-the-addressbook-class">Defining the AddressBook Class</h2>
<p>To add navigation functions to the address book, we must add two more slots to the <code>AddressBook</code> class: <code>next()</code> and <code>previous()</code> to the <code>addressbook.h</code> file:</p>
<pre class="cpp">
<span class="type">void</span> next();
<span class="type">void</span> previous();
</pre>
<p>We also require another two <a href="qpushbutton.html">QPushButton</a> objects, so we declare <code>nextButton</code> and <code>previousButton</code> as private variables:</p>
<pre class="cpp">
<span class="type"><a href="qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>nextButton;
<span class="type"><a href="qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>previousButton;
</pre>
<a name="implementing-the-addressbook-class"></a>
<h2 id="implementing-the-addressbook-class">Implementing the AddressBook Class</h2>
<p>In the <code>AddressBook</code> constructor in <code>addressbook.cpp</code>, we instantiate <code>nextButton</code> and <code>previousButton</code> and disable them by default. This is because navigation is only enabled when there is more than one contact in the address book.</p>
<pre class="cpp">
nextButton <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qpushbutton.html">QPushButton</a></span>(tr(<span class="string">"&Next"</span>));
nextButton<span class="operator">-</span><span class="operator">></span>setEnabled(<span class="keyword">false</span>);
previousButton <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qpushbutton.html">QPushButton</a></span>(tr(<span class="string">"&Previous"</span>));
previousButton<span class="operator">-</span><span class="operator">></span>setEnabled(<span class="keyword">false</span>);
</pre>
<p>We then connect these push buttons to their respective slots:</p>
<pre class="cpp">
connect(nextButton<span class="operator">,</span> SIGNAL(clicked())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(next()));
connect(previousButton<span class="operator">,</span> SIGNAL(clicked())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(previous()));
</pre>
<p>The image below is the expected graphical user interface.</p>
<p class="centerAlign"><img src="images/addressbook-tutorial-part3-screenshot.png" alt="" /></p><p>We follow basic conventions for <code>next()</code> and <code>previous()</code> functions by placing the <code>nextButton</code> on the right and the <code>previousButton</code> on the left. In order to achieve this intuitive layout, we use <a href="qhboxlayout.html">QHBoxLayout</a> to place the widgets side-by-side:</p>
<pre class="cpp">
<span class="type"><a href="qhboxlayout.html">QHBoxLayout</a></span> <span class="operator">*</span>buttonLayout2 <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qhboxlayout.html">QHBoxLayout</a></span>;
buttonLayout2<span class="operator">-</span><span class="operator">></span>addWidget(previousButton);
buttonLayout2<span class="operator">-</span><span class="operator">></span>addWidget(nextButton);
</pre>
<p>The <a href="qhboxlayout.html">QHBoxLayout</a> object, <code>buttonLayout2</code>, is then added to <code>mainLayout</code>.</p>
<pre class="cpp">
mainLayout<span class="operator">-</span><span class="operator">></span>addLayout(buttonLayout2<span class="operator">,</span> <span class="number">2</span><span class="operator">,</span> <span class="number">1</span>);
</pre>
<p>The figure below shows the coordinates of the widgets in <code>mainLayout</code>.</p>
<p class="centerAlign"><img src="images/addressbook-tutorial-part3-labeled-layout.png" alt="" /></p><p>Within our <code>addContact()</code> function, we have to disable these buttons so that the user does not attempt to navigate while adding a contact.</p>
<pre class="cpp">
nextButton<span class="operator">-</span><span class="operator">></span>setEnabled(<span class="keyword">false</span>);
previousButton<span class="operator">-</span><span class="operator">></span>setEnabled(<span class="keyword">false</span>);
</pre>
<p>Also, in our <code>submitContact()</code> function, we enable the navigation buttons, <code>nextButton</code> and <code>previousButton</code>, depending on the size of <code>contacts</code>. As mentioned earlier, navigation is only enabled when there is more than one contact in the address book. The following lines of code demonstrates how to do this:</p>
<pre class="cpp">
<span class="type">int</span> number <span class="operator">=</span> contacts<span class="operator">.</span>size();
nextButton<span class="operator">-</span><span class="operator">></span>setEnabled(number <span class="operator">></span> <span class="number">1</span>);
previousButton<span class="operator">-</span><span class="operator">></span>setEnabled(number <span class="operator">></span> <span class="number">1</span>);
</pre>
<p>We also include these lines of code in the <code>cancel()</code> function.</p>
<p>Recall that we intend to emulate a circularly-linked list with our <a href="../qtcore/qmap.html">QMap</a> object, <code>contacts</code>. So, in the <code>next()</code> function, we obtain an iterator for <code>contacts</code> and then:</p>
<ul>
<li>If the iterator is not at the end of <code>contacts</code>, we increment it by one.</li>
<li>If the iterator is at the end of <code>contacts</code>, we move it to the beginning of <code>contacts</code>. This gives us the illusion that our <a href="../qtcore/qmap.html">QMap</a> is working like a circularly-linked list.</li>
</ul>
<pre class="cpp">
<span class="type">void</span> AddressBook<span class="operator">::</span>next()
{
<span class="type"><a href="../qtcore/qstring.html">QString</a></span> name <span class="operator">=</span> nameLine<span class="operator">-</span><span class="operator">></span>text();
<span class="type"><a href="../qtcore/qmap.html">QMap</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">,</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">></span><span class="operator">::</span>iterator i <span class="operator">=</span> contacts<span class="operator">.</span>find(name);
<span class="keyword">if</span> (i <span class="operator">!</span><span class="operator">=</span> contacts<span class="operator">.</span>end())
i<span class="operator">+</span><span class="operator">+</span>;
<span class="keyword">if</span> (i <span class="operator">=</span><span class="operator">=</span> contacts<span class="operator">.</span>end())
i <span class="operator">=</span> contacts<span class="operator">.</span>begin();
nameLine<span class="operator">-</span><span class="operator">></span>setText(i<span class="operator">.</span>key());
addressText<span class="operator">-</span><span class="operator">></span>setText(i<span class="operator">.</span>value());
}
</pre>
<p>Once we have iterated to the correct object in <code>contacts</code>, we display its contents on <code>nameLine</code> and <code>addressText</code>.</p>
<p>Similarly, for the <code>previous()</code> function, we obtain an iterator for <code>contacts</code> and then:</p>
<ul>
<li>If the iterator is at the end of <code>contacts</code>, we clear the display and return.</li>
<li>If the iterator is at the beginning of <code>contacts</code>, we move it to the end.</li>
<li>We then decrement the iterator by one.</li>
</ul>
<pre class="cpp">
<span class="type">void</span> AddressBook<span class="operator">::</span>previous()
{
<span class="type"><a href="../qtcore/qstring.html">QString</a></span> name <span class="operator">=</span> nameLine<span class="operator">-</span><span class="operator">></span>text();
<span class="type"><a href="../qtcore/qmap.html">QMap</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">,</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">></span><span class="operator">::</span>iterator i <span class="operator">=</span> contacts<span class="operator">.</span>find(name);
<span class="keyword">if</span> (i <span class="operator">=</span><span class="operator">=</span> contacts<span class="operator">.</span>end()){
nameLine<span class="operator">-</span><span class="operator">></span>clear();
addressText<span class="operator">-</span><span class="operator">></span>clear();
<span class="keyword">return</span>;
}
<span class="keyword">if</span> (i <span class="operator">=</span><span class="operator">=</span> contacts<span class="operator">.</span>begin())
i <span class="operator">=</span> contacts<span class="operator">.</span>end();
i<span class="operator">-</span><span class="operator">-</span>;
nameLine<span class="operator">-</span><span class="operator">></span>setText(i<span class="operator">.</span>key());
addressText<span class="operator">-</span><span class="operator">></span>setText(i<span class="operator">.</span>value());
}
</pre>
<p>Again, we display the contents of the current object in <code>contacts</code>.</p>
<p>Files:</p>
<ul>
<li><a href="qtwidgets-tutorials-addressbook-part3-addressbook-cpp.html">tutorials/addressbook/part3/addressbook.cpp</a></li>
<li><a href="qtwidgets-tutorials-addressbook-part3-addressbook-h.html">tutorials/addressbook/part3/addressbook.h</a></li>
<li><a href="qtwidgets-tutorials-addressbook-part3-main-cpp.html">tutorials/addressbook/part3/main.cpp</a></li>
<li><a href="qtwidgets-tutorials-addressbook-part3-part3-pro.html">tutorials/addressbook/part3/part3.pro</a></li>
</ul>
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<!-- @@@tutorials/addressbook/part3 -->
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