<code>alpha</code> value for the dimming view used behind the presented
view controller. The color is black. Use <code>nil</code> to avoid adding a dimming
view behind the presented view controller.</p>

                      </div>
                      <div class="declaration">
                        <h4>Declaration</h4>
                        <div class="language">
                          <p class="aside-title">Swift</p>
                          <pre class="highlight swift"><code><span class="kd">pu