Enhance yii\behaviors\AttributeBehavior

[Kolyunya]

Q	A
Is bugfix?	no
New feature?	yes
Breaks BC?	no
Tests pass?	yes
Fixed issues	#11923

AttributeBehavior can preserve current attribute values now.
Basic tests coverage is included.

[samdark]

Hmm... this makes https://github.com/yiisoft/yii2/blob/master/framework/behaviors/SluggableBehavior.php#L88 pointless so either immutable should be moved into base AttributeBehavior or immutable should be deprecated.

@Kolyunya what do you think would be the best?

[Kolyunya]

@samdark I'm not sure but I think it is important to maintain backward compatibility. If it is, we should choose the first option. Flag names (skipUpdateOnClean and immutable) won't be consistent though.

[samdark]

Both my suggestions are backwards compatible, aren't they?

[Kolyunya]

@samdark yes, they are. I just thought that deprecated properties and methods are subject for removal.

[samdark]

In 2.1 — yes.

[samdark]

Thinking about it more, condition for not touching a field could be anything: only certain existing value, value of another field etc.

[samdark]

Since closures are supported for values and we have dirty attributes support, we can achieve it using existing code:

'value' => function($event) {
    if (empty($this->create_time)) {
        return time();
    }

    return $this->create_time;
}

[samdark]

So I'd decline this pull request and document ↑

[Kolyunya]

@samdark I'm sorry but we can't achieve this using existing code in a general scenario when we are populating multiple attributes via single behavior.

Use-case: Page entity has three attributes: meta_title, meta_description, meta_keywords. We need to copy title to each of the attributes only if they are empty. This will allow users to set some of them manually and to let the behavior fill the others.

The only workaround is to add three behaviors for each attribute with custom value which will check if the corresponding attribute is empty or not which is quite inelegant.

[samdark]

$skipUpdateOnFilled is per-behavior switch. Code above is the same.

[samdark]

Right, it's not convenient, at least...

[DrDeath72]

just want to write about it, i want use BlameableBehavior with beforeValidate without rewrite filled user_id

[Faryshta]

how about a $when callable that can return false if you don't want the attribute to update?

[Kolyunya]

@samdark @cebe what is the status of this PR? Is it under discussion or does it need some modifications?

[samdark]

Changelog line needed.

[samdark]

2.0.12

[Kolyunya]

@samdark I wasn't able to push to this PR (since I already deleted that branch and repo) and I made a new one.