Need callback for form submit data

[hayatoito]

Title: [Custom]: Need callback for form submit data (bugzilla: 24603)

Migrated from: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603

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comment: 0
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c0
Erik Arvidsson wrote on 2014-02-10 16:09:15 +0000.

Right now there is no way to have custom elements include data in form submissions. We should add another callback for this

I believe we need to add a callback that is called before the submit event.

Strawman:

document.registerElement('input', {
prototype: {
proto: HTMLElement.prototype,
beforeSubmitCallback: function() {
switch (this.type) {
case 'checkbox';
if (this.checked)
return this.value;
return undefined;
...
}
}
}
});

Basically, the contract is that the return value of the callback is used a the form value. If undefined is returned nothing is serialized.

This is of course a bit too simplistic but it might be enough to get started.

Things to keep in mind:

• Radio buttons need to check outside itself
• input[type=file]. Return Blob|File|data url?
• input[multiple]. Array of values?

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comment: 1
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c1
Boris Zbarsky wrote on 2014-02-10 16:20:44 +0000.

If is a subclass of , why can't it just set the .value to the thing it wants to submit?

If it's not a subclass of , you have other problems too, like form validation not knowing anything about it and so forth...

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comment: 2
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c2
Erik Arvidsson wrote on 2014-02-10 16:30:48 +0000.

I agree it might be simpler to just subclass input but at some point we should explain how input is implemented too. That also includes explaining form validation and requestAutoComplete and probably tons of other things.

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comment: 3
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c3
Boris Zbarsky wrote on 2014-02-10 16:37:53 +0000.

Sure. If we do that we need to make all the relevant algorithms robust to script randomly doing unexpected things. E.g. getting the value from an input can change the values of other inputs if it's done in arbitrary script....

The way input is implemented in practice, in ES terms, is that it has private slot (closure variable, weakmap entry, whatever) that stores a value string, and various form operations can retrieve that string without running any untrusted (from the point of view of the form) script in the process. Whatever setup we come up with for explaining input would be best if it preserved those invariants....

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comment: 4
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c4
Anne wrote on 2014-02-12 18:16:49 +0000.

What about form association and label association? It seems you need hooks for those too.

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comment: 5
comment_url: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24603#c5
Dimitri Glazkov wrote on 2014-05-08 20:51:20 +0000.

There's another proposal here: http://lists.w3.org/Archives/Public/public-webapps/2014JanMar/0448.html

[StokeMasterJack]

Here is my proposal for addressing this issue:

http://dave-ford.blogspot.com/2016/06/web-components-and-form-submit-fields.html

[domenic]

Custom elements F2F: we would like this! Previously, https://lists.w3.org/Archives/Public/public-webapps/2014JanMar/0448.html

Current thinking is:

• Probably this should be FormData based
• We should refactor the HTML spec to have "form data steps" per element to make the design a bit clearer (https://html.spec.whatwg.org/multipage/forms.html#constructing-form-data-set)
• Ideally we'd like to have a setValue/getValue pair so that this could also handle form restoration when doing back/forward/reopening the browser/etc.
• We're not sure how to best guide authors to having unique names for their fields. (E.g. using the name attribute or some derivative of it like type=image does with name + "x"/"y".)

[rniwa]

The last point is why we decided to use string & JSON for the value so that we can use name attribute of the element as done for any builtin element.

Using FormData is slightly nicer because it can contain multiple name-value pairs for when a compound element such as a credit-card component needs to submit multiple data fields. But letting users of a component specify the value might be more valuable than each value being submitted as a separate name-value pair. Also, user of such a component can always split the JSON into individual name-value pairs during submit event although having to parse the JSON there is kind of gross.

Yet anther option is to let users of component specify names for each value the component supplies but this is very different from the way builtin elements work so probably not a good idea.

[lukasoppermann]

@domenic if it is FormData based it would still submit normally when pressing the submit button, correct?

[domenic]

Yes

[lukasoppermann]

Awesome. But since you tagged it v2: If it does come into the specs it will probably be a year or more, correct? Since v1 is just releases and not even really implemented anywhere.

[domenic]

No, "v2" is a misnomer. The specs are living and see continuous changes, and implementations implement those changes as they can prioritize them, according to each implementation's individual release cycle. In the issue tracker, "v2" means something roughly like "not part of the initial set of features that we reached consensus on in the January 2016 face to face meeting".

[lukasoppermann]

Ahh, I see. Thanks for the clarification. Where can I best track the progress of this, to see if/when it will come?

[domenic]

Watching this issue is the best way to get updated on spec progress. After that, the procedure is specific to each browser; places like the various browser status trackers or bug trackers will be where implementation-specific discussion happens. That's kind of outside the scope of this repo though.

[PleasantD]

Has there been any progress on this?

We want to develop internal components that are project/framework agnostic. Ideally we want to be able to support Angular, React, and MVC patterns. The last one is the sticking point. Several of the components I want to build are basically form elements so I should be able to drop them in a form and submit with no additional code needed.

Is there a workaround to support submitting the component's values with the enclosing form on submit?
What about self-validating components, where the component can prevent form submission if it is in an invalid state?

[treshugart]

@PleasantD the only workarounds we've been able to find are to replicate the behaviour of form elements (which requires you to also to replicate <form />) or to not use shadow DOM at all.

Replicating form and input behaviour is robust, and allows you to keep shadow DOM encapsulation, but can work well when implemented correctly and edge-cases are tested. We've documented roughly how to do this here: https://skatejs.gitbooks.io/skatejs/content/docs/recipes/form-behaviour-and-the-shadow-dom.html. This can work with any web component library, not just Skate. The downfall here is that your custom <my-form /> will be tightly coupled to your <my-input /> elements. Your custom input elements won't be able to be dropped into a standard <form /> element, thus not reusable unless your consumers also use your custom form element.

Not using shadow DOM works well for leaf nodes. Luckily, most form elements are leaf nodes. However, it falls short when you want to use the custom element in a library that takes over control of childNodes, like React. I know libraries like Incremental DOM and Preact have since lightened up how they destroy nodes they don't know about, but I'd err on the side of replicating form behaviour until this gets implemented.

You could try and use the is attribute. This would in fact be the most elegant approach, but you'll never be able to remove the polyfill because, at least, Safari will never implement it.

[rniwa]

We've made a concrete proposal three years ago in https://lists.w3.org/Archives/Public/public-webapps/2014JanMar/0448.html

[tkent-google]

First let met say that all of these things seem like they can be done on top of the current FACE (form-associated custom elements) pull request. That's a good sign :).

I also think so. We don't need incompatible changes against whatwg/html#4383 in order to resolve known issues.

So, we, Chromium project, would like to start shipping process of both of event-based participation and form-associated custom element early next week. Does anyone still want to review these APIs?

[domenic]

Thanks everyone who accompanied us on this four-year journey!! On top of the feature that just got merged into the HTML spec, we have the following follow-up issues to make custom elements EVEN MORE powerful and on par with built-in elements:

• Move autofocus to HTMLElement: Move autofocus content attribute to HTMLElement whatwg/html#4563
• Being a submit button: Form-associated custom elements: being a submit button #814
• Implicit submission: Form associated custom elements: implicit submission #815

[tkent-google]

I added a section to the proposal document for a list of follow-up issues.
https://docs.google.com/document/d/1JO8puctCSpW-ZYGU8lF-h4FWRIDQNDVexzHoOQ2iQmY/edit?pli=1#heading=h.wz1kbybdmdqe

[j9t]

I added a section to the proposal document for a list of follow-up issues.
https://docs.google.com/document/d/1JO8puctCSpW-ZYGU8lF-h4FWRIDQNDVexzHoOQ2iQmY/edit?pli=1#heading=h.wz1kbybdmdqe

@tkent-google, are you still looking for feedback on the doc here? Just two notes:

1. Major: It seems Docs auto-invalidated many code samples (note, for example, the wrong quotes in const n = getAttribute(‘name’);).
2. Minor: Typo in sentence fragment “e.g. Create”.

[dflorey]

I'm trying to build a form-associated web component and I've been able to get the example up and running.
But I'm not sure how to implement custom validation:
Let's say want to build a material design textfield and want to make it a drop-in replacement for existing <input type="text"> tag.
When the user submits a form with my textfield in it I'd like to report the validation in a custom way (red text below the input) instead of using the default browser tooltip.
So I'd expect to be able to plug my custom reportValidation() method into the internals so that it will get called instead of the default impl.
Right now I just can set the validity on the internals which is the wrong way round.
The custom element knows how to report validity, especially when it comes to composed / compound fields.
So I'd expect something along these lines:

this.internals = this.attachInternals();
this.internals.reportValidity = myReportValidityFunction
this.internals.checkValidity = myCheckValidityFunction

or a number of callbacks like
formReportValidity()
or even better just
reportValidity()
etc.

It will be interesting to see how this will get implemented from the MWC team:
material-components/material-web#971

[tkent-google]

@dflorey ElementInternals is a helper for custom element authors, and browsers never call its methods internally.

Overriding reportValidity() is easy. We can just add our own reportValidity() to the custom element class. However, we have no ways to override interactive validation UI. It's an issue of not only form-associated custom elements, but also built-in form controls.
I recommend to file a new issue on https://github.com/whatwg/html/issues .

[dflorey]

Not sure if I understand correctly.
When calling reportValidity() on a native input field it will display the error message using the built-in UI (e.g. error popup on Chrome).
In my custom element I'd implement reportValidity() in a way that matches the design guidelines (e.g. material design).
I'd assume that when submitting a form on a form-associated custom element my reportValidity() would be called instead of the built-in UI, but unfortunately this seems not to be the case.

In the given sample in the doc...

https://docs.google.com/document/d/1JO8puctCSpW-ZYGU8lF-h4FWRIDQNDVexzHoOQ2iQmY/edit?pli=1#heading=h.2izmbauztyja

....and in all other examples that I've found it is also a bit weird that the validity will be set whenever the input changes as this is may be a very expensive operation.
Think of a signature input fields that does all kinds of image analysis to check if the control is valid. So the validation should only be checked when the browser calls my checkValidity() or reportValidity() methods instead on each change of the input.

I'd also expect to have a formDataCallback(FormData) or something along those lines to add the contents of the custom element to the form data to be submitted.
Right now the only way is to call this.internals_.setFormValue('') to set the value, but when should I call this method? Whenever the value changes? Again this may take a lot of time to calculate the value based on the input, so I'd want to call this method only whenever the user submits the form. How to do that?
I tried to use the event based form participation:

https://docs.google.com/document/d/1JO8puctCSpW-ZYGU8lF-h4FWRIDQNDVexzHoOQ2iQmY/edit?pli=1#heading=h.je8c7y5qpgki

but this seems not to work (quite yet)?

It would be so cool to be able to implement a custom element that can be used as a drop-in replacement for native input fields or to add new exciting input controls, but right now I don't see a way to do so due to the issues above.
Hopefully I just got it all wrong. I'd be super-happy to find a code example that shows a basic example how to

• add the value to the formdata on form submission
• performs the validation check when the form gets validated (=user clicks on submit)
• reports an error in a custom way and avoids the default browser UI