I've been going through the book "The Definitive Guide to Grails" (Rocher/Brown) and in Chapter 04, this mysterious thing called a "layout" just appeared with no explanation. (And there's no "layout" in the index. As far as I know, it's never explained.)

How does the system know to "inherit" the pages from layout/main.gsp? There's nothing about "layouts" in the index, and it seems to have just appeared.

On their sample app, a simple store site, the URL mappings for the / homepage say

  "/"(controller:"store")

and store controller's empty "index" closure

package com.g2one.gtunes

class StoreController {

    def index = {
    }
}

simply tells it to render store/index.gsp

The store/index.gsp just has a few lines of HTML; no layout gets included with any directive

<html>
    <head>
        <meta http-equiv="Content-type" content="text/html; charset=utf-8">
        <meta name="layout" content="main">
        <title>gTunes Store</title>
        <g:javascript library="prototype"></g:javascript>
    </head>
    <body id="body">
        <h1>Your online music store and storage service!</h1>
        <p>Manage your own library, browse music and purchase new tracks as they become available</p>
    </body> 
</html>

When I run the sample the page shown for "/" isn't just this simple HTML, it's the contents of "layouts/main.gsp" with this information magically inserted inside it.

I don't see how the information in layout/main.gsp gets applied to the page, how the elements get mixed together. I've been following through the book page by page and this functionality just "appeared" with no explanation.

grails layout
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