Origin of this ticket:

Currently, metadata caching fails if a constraint is not serializable. The only core constraint that is susceptible to this problem is the Callback constraint when used with an object method or closure:

// OK
new Callback(array('SomeClass', 'validateMe'));
new Callback('validateMeFunction');

// Not OK
new Callback(array($this, 'validateMe'));
new Callback(function ($value, $context) { ... });

When trying to serialize the last two constraints, an error is raised (#10083) that is a little hard to debug.

Two solutions come to mind:

• Disallow closures/objects methods in Callback and require all constraints to always be serializable. I don't quite like this, since I find passing closures to Callback very useful (e.g. in the "constraints" option of the Form component). After all, caching is not always necessary.
• Add a method isSerializable() to Constraint. This would return true by default and false in edge cases. If caching is turned on, GenericMetadata::addConstraint() checks isSerializable() and fails if an unserializable constraint is added.

The second solution adds minor overhead (the additional function call in addConstraint()), but only if metadata is not cached. I prefer that solution since it maintains the current flexibility, but returns a useful error message when a constraint is not serializable.

What do you think?