Designing App UI
Design your app's UI by including markup in the .app resource.
Each part of your UI corresponds to a component, which can in turn contain nested
components. Compose components to create a sophisticated app.
An app’s markup starts with the <aura:application> tag.
Let's look at a sample.app file, which starts with the <aura:application> tag.
<aura:application extends="force:slds">
<lightning:layout>
<lightning:layoutItem padding="around-large">
<h1 class="slds-text-heading_large">Sample App</h1>
</lightning:layoutItem>
</lightning:layout>
<lightning:layout>
<lightning:layoutItem padding="around-small">
Sidebar
<!-- Other component markup here -->
</lightning:layoutItem>
<lightning:layoutItem padding="around-small">
Content
<!-- Other component markup here -->
</lightning:layoutItem>
</lightning:layout>
</aura:application>
The sample.app file contains HTML tags, such as <h1>, as well as components, such as <lightning:layout>. We won't go into the details for all the components here but note how simple the markup is. The <lightning:layoutItem> component can contain other components or HTML markup.