Flux

Flux automates integration of Fluid with TYPO3 and makes Fluid the entry point for developers. The name "Flux" has multiple meanings but in this context, it mainly refers to the gel-like fluid used when soldering components together in electronic hardware, making it easier to create a bond between components and improves the durability of the bond.

Flux has two main purposes:

Allow developers to configure TYPO3's page templates and add custom content types using Fluid templates, without the need for detailed knowledge about how TYPO3 usually requires such setup to be done.
Allow embedding of metadata such as which fields to show when editing a content element, descriptions of the content or page template, and more.

Bonus feature: nested content areas to create content grids.

Documentation

How it works

Flux has two main modes of operation - either you allow Flux to automatically create a site-wide "design" folder to contain your template files, which provides a 100% out of the box experience. Or you prefer to have a more advanced and controlled integration, in which case you disable the automatic creation of site-wide template folders and provide your own (through an extension).

The automatic mode of operation is the default. This means Flux is an ideal starting point if you have zero knowledge about how to set up TYPO3. You can start with the automation and as you learn more about TYPO3, you can refine your integration and continuously improve how it works. Or keep using the automation in case it fits all your needs.

To get started you only need to know how to install a TYPO3 extension either with Composer or for non-Composer sites, using the Extension Manager.

Types of integration

Flux has multiple ways to store your page- and content templates (which are also type definitions):

Fully automated through the site-wide "design" folder in the public root. This integration is easiest to use but forces your templates to render in Flux's context, and does not facilitate complex setups such as extension specific asset storage, and is not as portable as other methods - on the other hand it works immediately out of the box. Ideal for simple single-domain sites and gets you started in just a few minutes.
Extension-specific, meaning you use a TYPO3 extension (written by yourself) to store all page- and content templates and additional resources such as translations, icons, and so on. This method is the most portable, meaning you can package your entire site templating in an extension that you can reuse, and is the only method which allows you to also ship PHP classes (ViewHelpers, for example) along with your site templates.
For content types specifically, you can define new types using root-level database records (one per type). This method is possible to use completely without touching the file system, but has a lesser degree of portability. You can use this method to quickly create prototypes of content types using a more visual approach (through TYPO3 forms). This method also has an exporting feature which allows you to generate the necessary Fluid template that can then be used with either of the other methods.

Methods 1 and 3 are intended to get you started as quickly as possible. Method 2 is intended to serve more custom setups which ship more than just site templates, through use of a custom TYPO3 extension.

All three methods can be combined or used individually.

Composer Install

Recommended!

composer req fluidtypo3/flux
./vendor/bin/typo3 extension:install flux
# alternatively, instead of extension:install, activate in Extension Manager backend module

Non-Composer Install

NOT recommended!

In the Extension Manager backend module, search for flux
Choose to install the result with the extension key flux

Setup

There is no required setup to use Flux content types (but you almost certainly need to install fluid_styled_content to be able to render any content at all).
To use page templates without a content grid (which assumes you defined a grid with pageTSconfig or other) you only need to select the template to use in Page Layout when editing a page (start with the top page).
If your page template additionally contains a grid, you must also select Columns from selected "Page Layout" as the value of the two Backend Layout fields in the Appearance tab.

The remaining setup of labels, form fields, grid composition etc. can all be done from within your Fluid templates.

How does it work?

When Flux is installed and enabled in extension manager, and if automatic creation of site-wide Flux templates is enabled (which it is by default), the following happens automatically:

A folder named design is created in the public directory (this directory may differ between TYPO3 versions and can be changed with configuration, but in most recent TYPO3 versions it is public in the project public folder).
This folder is filled with a set of skeleton templates containing very basic embedded Flux metadata.
The file created in design/Templates/Page can be selected as page template (Flux adds a Page Layout tab to pages' editable properties).
The file created in design/Templates/Content becomes a custom content type which can be inserted just like the standard TYPO3 content types that create text, image, etc.

Renaming, removing or adding files in these folders automatically registers the file as either page template or content type, depending on location. Be careful when renaming or removing files: if the page- or content template is already in use this may break your system until you choose another page template and disable/delete removed or renamed content types. There is no warning given about types that are in use!

From that point on, you can create a completely styled site with custom content types to make sliders etc. using your favorite frontend framework (or none) - and you only need to know very basic Fluid (an XML based markup engine which comes with automatically rendered documentation for every tag you can use).

What does it NOT do?

Flux does not remove the need to learn "the TYPO3 way" of doing things - you should still aim to learn more about how TYPO3 works. Flux only makes it quicker to get started and provides a reasonable level of automation; complex sites will almost surely still require you to learn a bit about TYPO3 (such as, how to modify the <meta> section and how to use third party plugins for news etc.)

Flux is also not a replacement for things like fluid_styled_content (although it can work without it) - Flux creates custom content types, it does not replace TYPO3's native content types (although you can hide those and use only your custom types).

Lastly, Flux only has limited abstraction over how you define form fields. To know all the specific details of what each type of field does, you still need to know TYPO3's "TCA" (which is thoroughly documented). Flux tries as far as possible to use the same names of form field attributes as TCA. If you don't understand an attribute or aren't sure which field type to use, always consult the TCA documentation (keeping in mind not all field types will work: Flux fields are based on FlexForm fields. When FlexForm does not support a field type it is noted so in the TCA documentation).

Recommendation of VHS

VHS is another extension in the FluidTYPO3 family, which is highly recommended to use along with Flux. The reason VHS is mentioned here, is that it provides alternatives to TypoScript-based content- and menu-rendering instructions, allowing you to instead use Fluid.

Given that in particular menu rendering setup in TypoScript is notoriously difficult (due to a very old structure which has basically never changed), beginners may prefer to use a special XHTML tag and either a few CSS class properties, or a custom loop to output menu items and their links.

Flux form API

Flux lets you build and modify forms in Fluid, which become form fields in the form that edits content/page properties through the TYPO3 backend:

<flux:form id="myform">
  <flux:field.input name="myField" label="My special field" />
</flux:form>

Flux also lets you build a grid for content elements (nested content areas):

<flux:grid>
  <flux:grid.row>
    <flux:grid.column colPos="0" name="main" label="Main content area" />
  </flux:grid.row>
</flux:form>

Flux is then capable of extracting these embedded structures to read form fields, labels, content grids, backend preview output, and more - in short, your template files embed the instructions on how to both integrate and render templates.

Alternative APIs

As you create more complex projects they usually have more complex requirements - which may still benefit from Flux features such as a way to create Flux forms for custom plugins. Since Flux works by increasingly abstracting the API of TYPO3 core features (with the Fluid "flavor" as the most condensed and abstracted) Flux also declares these increasingly flexible layers of abstraction as public API.

This means Flux also has a good old PHP way to declare forms and so on:

$form = \FluidTYPO3\Flux\Form::create();
$form->setName('myform');
$form->createField('Input', 'myField', 'My special field');

And supports plain arrays (to allow sources like JSON):

$json = '{name: "myform", fields: [{"name": "myField", "type": "Input"}]}';
$asArray = json_decode($json, JSON_OBJECT_AS_ARRAY);
$form = \FluidTYPO3\Flux\Form::create($asArray);

And can use TypoScript:

plugin.tx_flux.providers {
  myextension_myplugin {
    tableName = tt_content
    fieldName = pi_flexform
    listType = myextension_myplugin
    extensionKey = Vendor.MyPlugin
    form {
      name = myform
      fields {
        myField {
          type = Input
          label = My special field
        }
      }
    }
  }
}

All of which create the same form with a single input field called myField with a label value of My special field. The last example shows the form structure nested in a Provider (another Flux concept) which connects the pi_flexform field of the related tt_content plugin record type to the form.

Flux feature highlights

Added features for content elements - add content grids (following the backend_layout approach) to any content/plugin.
Multiple APIs to access the same features from many different contexts using the same naming and nesting style.
Multiple levels of API abstraction - when you need more control, lower API abstraction levels can be used in your code.
Flexible ways to replace individual parts: templates, controller actions, etc.
Manipulation of properties of existing forms - change field labels, default values, add fields, sheets, etc.
Data type transformations - define the desired target type and let the TypeConverters of Extbase handle conversion.
Possibility for custom components of your own - with the same API support any other Flux component has.
Several Utility-type classes for advanced integrations with Fluid in particular.

Caveats

Keep In mind to have your PHP/HTTP configured correctly to accept a fairly large number of input fields. When nesting sections / objects the number of fields submitted, rises drastically. The php.ini configuration setting to think about is max_input_vars. If this number is too small then the TYPO3 Backend (being PHP) will decline the submission of the backend editing form and will exit with an "Invalid CSRF Token" message because of incomplete (truncated) POST data.
When working with custom Provider classes: 1) make sure you declare each Provider class as public: true in Service config of your extension. And 2) if the Provider needs to be used for more than one specific page/content/record type, make sure you also declare the Provider as shared: false. You can find an example of such configuration in the Configuration/Services.yaml file in Flux.
When working with custom Controller classes, make sure you declare each Controller class as public: true in Service config of your extension.
Note that both Providers and Controllers use constructor injection of dependencies. If you use custom Controllers and/or Providers and these override the __construct method, make sure that they take the same arguments as the parent class and that you call parent::__construct() with those arguments from your overridden __construct method.