Body content

We're used to passing body content to HTML tags. When you do this, the tag has control over where and when this content is rendered. A good example of this is the HTML <details> element:

<details>
  <summary>Hello <strong>World</strong></summary>
  This is some <em>content</em> that can be toggled.
</details>

This is what it renders (try clicking it):

Hello World

This is some content that can be toggled.

Custom tags can also receive content in the same way. This allows a component to give its user full control over how some section of the content is rendered, but control where, when, and with what data it is rendered. This feature is necessary to build composable components like overlays, layouts, dropdowns, etc. Imagine a <table> that didn't give you control over how its cells were rendered. That would be pretty limited!

Rendering body content

When a custom tag is passed body content, it is received as a special renderBody property on the component's input. You can include this content anywhere in your component by using the <${dynamic}> syntax.

components/fancy-container.marko

<div class="container fancy">
    <${input.renderBody}/>
</div>

div.container.fancy
  ${input.renderBody}

If we were to use this tag like this:

Marko Source

<fancy-container>
    <p>Content goes here...</p>
</fancy-container>

fancy-container
  p -- Content goes here...

The rendered output would be:

HTML Output

<div class="container fancy"><p>Content goes here...</p></div>

This is a pretty basic example, but you can imagine how this could be incorporated into a more advanced component to render passed content where/when needed.

ProTip: Body content can be rendered multiple times. Or not at all.

Passing attributes to body content

When rendering body content with <${dynamic}>, attributes may also be passed:

components/random-value.marko

<!-- heh, it's not actually random -->
<${input.renderBody} number=1337 />

<!-- heh, it's not actually random -->
${input.renderBody} number=1337

These attribute values can be received as a tag parameter:

<random-value|{ number }|>
    The number is ${number}
</random-value>

random-value|{ number }| -- The number is ${number}

ProTip: Some tags (like the above tag) may not render anything except their body content with some data. This can be quite useful, just look at the <for> and <await> tags!

Named body content

You can also pass named content sections to a tag using attribute tags which are denoted by the @ prefix.

<layout>
    <@heading>
        <h1>Hello Marko</h1>
    </@heading>
    <@content>
        <p>...</p>
    </@content>
</layout>

layout
  @heading
    h1 -- Hello Marko
  @content
    p -- ...

Like attributes, these attribute tags are received as input.heading and input.content, but they each have a renderBody property which we can now use:

components/layout.marko

<!doctype html>
<html>
    <body>
        <${input.heading.renderBody}/>
        <hr/>
        <${input.content.renderBody}/>
    </body>
</html>

<!doctype html>
html
  body
    ${input.heading.renderBody}
    hr
    ${input.content.renderBody}

ProTip: The renderBody property can be omitted. You could use <${input.heading}/>, for example.

Repeated body content

When an attribute tag is repeated, the child component can consume all instances using the iterable protocol.

This allows us to, for example, build a custom table component which allows its user to specify any number of columns, while still giving the user control over how each column is rendered.

Marko Source

<fancy-table data=people>
    <@column|person|>
        Name: ${person.name}
    </@column>
    <@column|person|>
        Age: ${person.age}
    </@column>
</fancy-table>

fancy-table data=people
  @column|person| -- Name: ${person.name}
  @column|person| -- Age: ${person.age}

Note For TypeScript the Marko.AttrTag helper should be used here.

Protip Since attribute tags are iterable you could pass input.column to a for of loop, or [...spread] it into an array. To .map, .filter or otherwise work with attribute tags as an array you can use the following pattern:
$ const columns = [...input.column || []];
$ const columns = [...(input.column || [])];

We can then use the <for> tag to render the body content into table, passing the row data to each column's body.

components/fancy-table/index.marko

<table class="fancy">
    <for|row| of=input.data>
        <tr>
            <for|column| of=input.column>
                <td>
                    <${column.renderBody} ...row/>
                </td>
            </for>        </tr>
    </for>
</table>

We now have a working <fancy-table>. Let's see what it renders:

Example Data

[
  {
    name: "Patrick",
    age: 63,
  },
  {
    name: "Austin",
    age: 12,
  },
];

HTML Output

<table class="fancy">
  <tr>
    <td>Name: Patrick</td>
    <td>Age: 63</td>
  </tr>
  <tr>
    <td>Name: Austin</td>
    <td>Age: 12</td>
  </tr>
</table>

Attributes on attribute tags

If you look at our previous example, we had to prefix each cell with the column label. It would be better if we could give a name to each column instead and only render that once.

Marko Source

<fancy-table>
    <@column|person| heading="Name">
        ${person.name}
    </@column>
    <@column|person| heading="Age">
        ${person.age}
    </@column>
</fancy-table>

fancy-table
  @column|person| heading="Name" -- ${person.name}
  @column|person| heading="Age" -- ${person.age}

Now, each attribute tag in input.column will contain a heading property in addition to its renderBody. We can use another <for> and render the headings in <th> tags:

components/fancy-table/index.marko

<table class="fancy">
    <tr>
        <for|column| of=input.column>
            <th>${column.heading}</th>
        </for>    </tr>
    <for|row| of=input.data>
        <tr>
            <for|column| of=input.column>
                <td>
                    <${column.renderBody} ...row/>
                </td>
            </for>
        </tr>
    </for>
</table>

We'll now get a row of headings when we render our <fancy-table>

HTML Output

<table class="fancy">
  <tr>
    <th>Name</th>
    <th>Age</th>
  </tr>
  <tr>
    <td>Patrick</td>
    <td>63</td>
  </tr>
  <tr>
    <td>Austin</td>
    <td>12</td>
  </tr>
</table>

Nested attribute tags

Continuing to build on our example, what if we want to add some custom content or even components into the column headings? In this case, we can extend our <fancy-table> to use nested attribute tags. We'll now have <@heading> and <@cell> tags nested under <@column>. This gives users of our tag full control over how to render both column headings and the cells within the column!

Marko Source

<fancy-table>
    <@column>
        <@heading>
            <app-icon type="profile"/> Name
        </@heading>
        <@cell|person|>
            ${person.name}
        </@cell>    </@column>
    <@column>
        <@heading>
            <app-icon type="calendar"/> Age
        </@heading>
        <@cell|person|>
            ${person.age}
        </@cell>
    </@column>
</fancy-table>

Now instead of rendering the heading as text, we'll render the heading's body content.

components/fancy-table/index.marko

<table class="fancy">
    <tr>
        <for|column| of=input.column>
            <th>
                <${column.heading.renderBody}/>            </th>
        </for>
    </tr>
    <for|row| of=input.data>
        <tr>
            <for|column| of=input.column>
                <td>
                    <${column.cell.renderBody} ...row/>
                </td>
            </for>
        </tr>
    </for>
</table>

Our headings can now include icons (and anything else)!

HTML Output

<table class="fancy">
  <tr>
    <th><img class="icon" src="profile.svg" /> Name</th>
    <th><img class="icon" src="calendar.svg" /> Age</th>
  </tr>
  <tr>
    <td>Patrick</td>
    <td>63</td>
  </tr>
  <tr>
    <td>Austin</td>
    <td>12</td>
  </tr>
</table>

Dynamic attribute tags

The flexibility of the <fancy-table> is great if you want to render columns differently or have columns that display the data in a special way (such as displaying an age derived from a date of birth). However, if all columns are basically the same, the user might feel they're repeating themselves. As you might expect, you can use <for> (and <if>) to dynamically render attribute tags.

$ const columns = [{
    property: "name",
    title: "Name",
    icon: "profile"
}, {
    property: "age",
    title: "Age",
    icon: "calendar"
}]

<fancy-table>
    <for|{ property, title, icon }|>
        <@column>
            <@heading>
                <app-icon type=icon/> ${title}
            </@heading>
            <@cell|person|>
                ${person[property]}
            </@cell>
        </@column>
    </for>
</fancy-table>

$ const columns = [
  {
    property: "name",
    title: "Name",
    icon: "profile",
  },
  {
    property: "age",
    title: "Age",
    icon: "calendar",
  },
];

fancy-table
  for|{ property, title, icon }|
    @column
      @heading
        app-icon type=icon
        -- ${" "}${title}
      @cell|person| -- ${person[property]}