Passing preprocess function variables to a template

I am trying to include another template file with pre-process's theme hook variables in a template file but it failed. I checked theme debug and it does not include example's pre-process theme hook. Can you give me guidance?

I added {% include 'example--p1.html.twig' %} in the page--front.html.twig file. The example--p1.html.twig file has one variable, {{ test }}. Also, I created a pre-process function that adds variables to a page template. The code is the following.

function mytheme_preprocess_example__p1(array &$variables, $hook) { 
  $variables['test'] = 'please help';
  $variables['test_var'] = 'this is my second variable';
}

It doesn't appear in the example--p1.html.twig template.

Someone suggested using the following code in the page--front.html.twig file.

{% include 'example--p1.html.twig' with {'test' : 'this is my message'} %} 

I then added the following code in the following files.

mytheme.theme

function mytheme_preprocess_example__p1(array &$variables, $hook) { 
  $variables['content']['text'] = 'second';
}

page--front.html.twig

{% include 'example--p1.html.twig' with {'test' : 'content.text'} %} 

It works as it shows the text message value in example--p1.html.twig and page--front.html.twig.

How can I pass pre-process function's text messages' variables to page example--p1.html.twig and then I can display all the variables if I include example--p1.html.twig to page--front.html.twig?

For your front page twig: page--front.html.twig -- you would want to add a variable in the THEMENAME_preprocess_page function.

Preprocess functions would essentially match what the twig is targeting -- page data. If your twig was for a node, then you'd want a preprocess_node.

function THEMENAME_preprocess_page(array &$variables, $hook) { 
  if (\Drupal::service('path.matcher')->isFrontPage()) {
    $variables['some_variable'] = 'some value';
  } else {
    $variables['some_variable'] = 'a different value';
  }
}

And then I think your include just needs to do this

{% include 'example--p1.html.twig' with {'test' : some_variable } %} 

Preprocess functions are part of a render setup that includes at least:

• hook_theme() to register the template to Drupal.
• hook_preprocess_HOOK to preprocess template variables before it's handed to Twig.
• The twig template to represent the HTML of the template.
• Something that "builds" the interface using the above setup.
  • This involves creating a render array with #type set to the registered template's name to render the data using the template. Examples of this are field formatters, controllers, blocks, etc.

The general flow of a render is build -> preprocess -> twig = HTML

Essentially a long-winded way of saying not all twig templates have preprocess functions. Twig templates that are just "included" are not part of this mechanism and won't have preprocess function.