Add a confirmation message to forms and provide Ajax out of the box

[qzminski]

I have added an option to define a confirmation message in the form settings. There is no longer a need to create a dedicated confirmation page – the confirmation will appear on the same page after a reload.

The redirect feature is still in place, however. Moreover, I have added the {{form_message}} insert tag, which can be used on the target page to display the confirmation message defined in the form settings.

Another significant change is that the form now works with ajax out of the box. This concept was taken from the well-known extension https://github.com/terminal42/contao-ajaxform

Problems to discuss:

1. The JavaScript file is included without any URL fingerprint, which can cause cache issues. Perhaps we should switch to Webpack Encore to generate the assets?
2. I am not entirely sure if the insert tags should be replaced using the replace() or replaceInline() method. @ausi can you shed some light on this?
3. Shall we use Twig templates here already?

I'm looking forward to your feedback!

[Toflar]

Concept by yours truly 😇 I think it would be a great addition to the core, thanks for the PR @qzminski!

[ausi]

I am not entirely sure if the insert tags should be replaced using the replace() or replaceInline() method. @ausi can you shed some light on this?

Using replace() could result in having <esi> tags in the output, replaceInline() ensures that everything is replaced completely without <esi> tags. So replaceInline() is correct here I think.

[qzminski]

@ausi I have added your suggestions in one commit: 82d802c

[ausi]

#5307 (comment) needs to be addressed, rest approved ✅

[leofeyer]

Now that #5406 has been merged, this PR needs to be adjusted accordingly, doesn‘t it? @aschempp /cc

[qzminski]

I have just pushed the commit that uses Webpack to generate the JS assets. Please review and confirm.

[leofeyer]

The changes look all good, but the problem remains that as soon as there is a confirmation message, the form is never rendered in the front end (see #5307 (review)).

[fritzmg]

I can confirm the issue. The problem is that the DCA field for the confirmation message and the template variable have the same name. The template will automatically be filled with all the tl_form data here:

contao/core-bundle/contao/classes/Hybrid.php

Line 228 in f219456

$this->Template->setData($this->arrData);

And thus

contao/core-bundle/contao/templates/forms/form_wrapper.html5

Line 9 in 324cab3

<?php if ($this->confirmation): ?>

will always be true once a confirmation message has been set.

[leofeyer]

I have fixed the issue in 176c393.

[leofeyer]

Thank you @qzminski.