Twig Template Inheritance


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What about adding a layout to our page - like a header and a footer? Take a peek at the HTML for the page: it's just the HTML from the template. There's nothing special in Twig where a base layout with a header and a footer is automatically wrapped around our content. Whatever you have in your template is what you get on the page.

However, the Twig recipe did add a base layout file called base.html.twig. It's really simple now, but this is where we'll add our top nav, footer and any other things that should live on every page. The question is: how can we make our template use this?

Extending the Base Layout

With a cool feature called template inheritance. In homepage.html.twig, at the top, type {% extends then the name of the base template: base.html.twig. And notice: this is the do something tag. We're not printing this template, we're telling Twig that we want to extend it.

If we do nothing else and refresh, we get an error:

a template that extends another one cannot include content outside Twig blocks.

Hmm. When you extend a template, it tells Twig that you want to render your template inside that base layout. But... Twig has no idea where our content should go. Should it take our homepage HTML and put it down here? Or up here? Or right there? It doesn't know! So it throws that error.

The way we tell it is via these blocks. Blocks are holes into which a child template can put content. And you may have noticed one block called body... which is exactly where we want our content to go. To put it there, surround all the content with a {% block body %}... and at the bottom, {% endblock %}.

And now... it's alive! It doesn't look much different, but we are inside the base layout.

This is called template inheritance because it works exactly like PHP class inheritance. Imagine you have a Homepage class that extends a Base class. That Base class has a body() method, and we override that body() method in the Homepage class. It's the same concept in Twig.

Overriding the Page Title

And these block names - like javascripts, stylesheets and body - aren't special names... and they're not registered anywhere. Feel free to create new blocks however and whenever you want. For example, suppose we want to change the title of the page from a child template. In this case, the recipe already gave us a block called title to do that. And this block has default content... which is why we already see Welcome on the browser tab. Let's override this in our template.

Anywhere outside the body block, say {% block title %}, type something, then {% endblock %}.

Replacing vs Appending the Parent Block

And now, got it! New title! And notice that when we override a block, we override it completely. We don't see the word Welcome anymore. Occasionally, you may want to add to the parent block instead of replacing it. You can do that by saying {{ parent() }}.

This is really neat! The parent() function grabs the content from the title block of the parent template. Then we use {{ to print it. This time we see welcome and then our title.

But since we don't really want that, I'll remove it.

Status check: we're returning HTML and we have a base layout. Yeah, our site is still horribly ugly, but we'll fix that in a bit.

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