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<ul>
<li class="level1"><a href="#walkthrough">Walkthrough</a></li>
<li class="level2"><a href="#the-cell-component">The Cell Component</a></li>
<li class="level2"><a href="#the-main-qml-file">The Main QML File</a></li>
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<h1 class="title">QML Tutorial 2 - QML Components</h1>
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<p>This chapter adds a color picker to change the color of the text.</p>
<p class="centerAlign"><img src="images/declarative-tutorial2.png" alt="" /></p><p>Our color picker is made of six cells with different colors. To avoid writing the same code multiple times for each cell, we create a new <code>Cell</code> component. A component provides a way of defining a new type that we can re-use in other QML files. A QML component is like a black-box and interacts with the outside world through properties, signals and functions and is generally defined in its own QML file. (For more details, see the <a href="../qtqml/qml-qtqml-component.html">Component</a> documentation). The component's filename must always start with a capital letter.</p>
<p>Here is the QML code for <code>Cell.qml</code>:</p>
<pre class="qml">

  import QtQuick 2.0

  <span class="type"><a href="qml-qtquick-item.html">Item</a></span> {
      <span class="name">id</span>: <span class="name">container</span>
      property <span class="type">alias</span> <span class="name">cellColor</span>: <span class="name">rectangle</span>.<span class="name">color</span>
      signal <span class="type">clicked</span>(color cellColor)

      <span class="name">width</span>: <span class="number">40</span>; <span class="name">height</span>: <span class="number">25</span>

      <span class="type"><a href="qml-qtquick-rectangle.html">Rectangle</a></span> {
          <span class="name">id</span>: <span class="name">rectangle</span>
          <span class="name">border</span>.color: <span class="string">&quot;white&quot;</span>
          <span class="name">anchors</span>.fill: <span class="name">parent</span>
      }

      <span class="type"><a href="qml-qtquick-mousearea.html">MouseArea</a></span> {
          <span class="name">anchors</span>.fill: <span class="name">parent</span>
          <span class="name">onClicked</span>: <span class="name">container</span>.<span class="name">clicked</span>(<span class="name">container</span>.<span class="name">cellColor</span>)
      }
  }

</pre>
<a name="walkthrough"></a>
<h2 id="walkthrough">Walkthrough</h2>
<a name="the-cell-component"></a>
<h3 >The Cell Component</h3>
<pre class="qml">

  <span class="type"><a href="qml-qtquick-item.html">Item</a></span> {
      <span class="name">id</span>: <span class="name">container</span>
      property <span class="type">alias</span> <span class="name">cellColor</span>: <span class="name">rectangle</span>.<span class="name">color</span>
      signal <span class="type">clicked</span>(color cellColor)

      <span class="name">width</span>: <span class="number">40</span>; <span class="name">height</span>: <span class="number">25</span>

</pre>
<p>The root type of our component is an <a href="qml-qtquick-item.html">Item</a> with the <code>id</code> <i>container</i>. An <a href="qml-qtquick-item.html">Item</a> is the most basic visual type in QML and is often used as a container for other types.</p>
<pre class="qml">

      property <span class="type">alias</span> <span class="name">cellColor</span>: <span class="name">rectangle</span>.<span class="name">color</span>

</pre>
<p>We declare a <code>cellColor</code> property. This property is accessible from <i>outside</i> our component, this allows us to instantiate the cells with different colors. This property is just an alias to an existing property - the color of the rectangle that compose the cell (see <a href="../qtqml/qtqml-syntax-propertybinding.html">Property Binding</a>).</p>
<pre class="qml">

      signal <span class="type">clicked</span>(color cellColor)

</pre>
<p>We want our component to also have a signal that we call <i>clicked</i> with a <i>cellColor</i> parameter of type <i>color</i>. We will use this signal to change the color of the text in the main QML file later.</p>
<pre class="qml">

      <span class="type"><a href="qml-qtquick-rectangle.html">Rectangle</a></span> {
          <span class="name">id</span>: <span class="name">rectangle</span>
          <span class="name">border</span>.color: <span class="string">&quot;white&quot;</span>
          <span class="name">anchors</span>.fill: <span class="name">parent</span>
      }

</pre>
<p>Our cell component is basically a colored rectangle with the <code>id</code> <i>rectangle</i>.</p>
<p>The <code>anchors.fill</code> property is a convenient way to set the size of a visual type. In this case the rectangle will have the same size as its parent (see <a href="qtquick-positioning-anchors.html">Anchor-Based Layout</a>).</p>
<pre class="qml">

      <span class="type"><a href="qml-qtquick-mousearea.html">MouseArea</a></span> {
          <span class="name">anchors</span>.fill: <span class="name">parent</span>
          <span class="name">onClicked</span>: <span class="name">container</span>.<span class="name">clicked</span>(<span class="name">container</span>.<span class="name">cellColor</span>)
      }

</pre>
<p>In order to change the color of the text when clicking on a cell, we create a <a href="qml-qtquick-mousearea.html">MouseArea</a> type with the same size as its parent.</p>
<p>A <a href="qml-qtquick-mousearea.html">MouseArea</a> defines a signal called <i>clicked</i>. When this signal is triggered we want to emit our own <i>clicked</i> signal with the color as parameter.</p>
<a name="the-main-qml-file"></a>
<h3 >The Main QML File</h3>
<p>In our main QML file, we use our <code>Cell</code> component to create the color picker:</p>
<pre class="qml">

  import QtQuick 2.0

  <span class="type"><a href="qml-qtquick-rectangle.html">Rectangle</a></span> {
      <span class="name">id</span>: <span class="name">page</span>
      <span class="name">width</span>: <span class="number">320</span>; <span class="name">height</span>: <span class="number">480</span>
      <span class="name">color</span>: <span class="string">&quot;lightgray&quot;</span>

      <span class="type"><a href="qml-qtquick-text.html">Text</a></span> {
          <span class="name">id</span>: <span class="name">helloText</span>
          <span class="name">text</span>: <span class="string">&quot;Hello world!&quot;</span>
          <span class="name">y</span>: <span class="number">30</span>
          <span class="name">anchors</span>.horizontalCenter: <span class="name">page</span>.<span class="name">horizontalCenter</span>
          <span class="name">font</span>.pointSize: <span class="number">24</span>; <span class="name">font</span>.bold: <span class="number">true</span>
      }

      <span class="type"><a href="qml-qtquick-grid.html">Grid</a></span> {
          <span class="name">id</span>: <span class="name">colorPicker</span>
          <span class="name">x</span>: <span class="number">4</span>; <span class="name">anchors</span>.bottom: <span class="name">page</span>.<span class="name">bottom</span>; <span class="name">anchors</span>.bottomMargin: <span class="number">4</span>
          <span class="name">rows</span>: <span class="number">2</span>; <span class="name">columns</span>: <span class="number">3</span>; <span class="name">spacing</span>: <span class="number">3</span>

          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;red&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;green&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;blue&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;yellow&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;steelblue&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;black&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }
      }
  }

</pre>
<p>We create the color picker by putting 6 cells with different colors in a grid.</p>
<pre class="qml">

          <span class="type">Cell</span> { <span class="name">cellColor</span>: <span class="string">&quot;red&quot;</span>; <span class="name">onClicked</span>: <span class="name">helloText</span>.<span class="name">color</span> <span class="operator">=</span> <span class="name">cellColor</span> }

</pre>
<p>When the <i>clicked</i> signal of our cell is triggered, we want to set the color of the text to the <i>cellColor</i> passed as a parameter. We can react to any signal of our component through a property of the name <i>'onSignalName'</i> (see <a href="../qtqml/qtqml-syntax-objectattributes.html#signal-attributes">Signal Attributes</a>).</p>
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