How can I create a custom module to display a calculated value on an already existing page/view? - drupal-8

Question

Trying get started with custom module development on Drupal 8. Don't have previous Drupal dev background. Been reading and watching some materials in the past 2 or so weeks.
I decided to dare doing a little more "complex" other than "Hello World", but I got stuck on how would I actually go about this.
This is what I have so far:
I created a content type for "people". It's very simple, all it has is the person's name and birthdate (date only), so you can use the form to input a few people.
I created a view that displays the list of people, their names and their birthdate. Of course everything so far is using the core admin thing.
Now, I wanted to create a custom module that calculates the person's age (based on the date stored in the DB). It takes today's date, subtracts the person's birthdate, and calculates their age.
Then, with some magic, it returns the age to the render/page output and now this calculated value (the age) will be displayed on the view as well along with the person's name and birthdate.
I assume I would need to "hook into" a place somewhere, where the list of people is returned from the database. Then I loop through the data, get the birthdate, calculate the age, stick the result (age) back into the data, and then the page/view will display this - somehow.
Of course I am already stuck on how would I go about doing this? Where would I need to hook into? With what API? And then of course, I created a view already, but the view doesn't have the "age" field - since that is calculated on the fly. So where and how would I display it?
So many questions...
If anyone would know some tutorial that is similar to this, I'd appreciate it. It's kinda tough getting started with the "custom" side of Drupal.
Thanks for some tips!

Answer 1

without custom programming
You can actually do this without custom coding. For this you can I would use either of the following modules field_token_value or computed_field.
They both create a field without giving the user a textbox to input any value.
The value of such fields are calculated from predefined rules/custom coding/tokens defined per field.
from your custom theme
The easiest way is to do this on the theming layer and not store this value in the database. You have to use hook_preprocess_node in your THEMENAME.theme
function THEMENAME_preprocess_node(&$variables){
//install debug and kint to be able to use
//kint($variables);
$node = $variables["node"];
if($node->getType() == "CONTENTTYPE" && $node->hasField('field_date')){
//get date value something like "2018-09-07T21:35:30"
$date = $node->field_date->value;
//your logic and age calculations
// ...
$age = 35;
//set variable to use in node.html.twig based templates
$variables["person_age"] = $age;
}
}
In order to get more information on what is available install debug module and enable kint (which is part of debug) to be able to see variables using kint function
Than copy node.html.twig from your custom theme (or parent theme or classy core theme) to your THEMENAME/templates folder and rename it node--CONTENTTYPE.html.twig.
In there you can include the variable you just created in proprocess.
person age: {{ person_age }}
Make sure you clear the cache for all those changes to be seen by Drupal.
FOR MORE INFO
To find out what twig templates to override see here
More on twig and getting available variables
Getting more info on which twig template is used can also be obtained by enabling debug mode on previous link.
I would suggest you pick up a book on drupal development to understand all those concepts in a more concrete way.