Custom constraints

[AubreyHewes]

Allows adding custom constraints via

$validator->addConstraint('name', \Callable|className|instance)

Background
I ran in to the problem of custom constraints and via pr #143 by @Maks3w (the factory) the possiblity was available to easily accomplish adding custom constraints.. (this is moved from a @Maks3w fork)

Implementation
This allows applying custom constraints, by name, as either a callable/className/instance.
I am not saying this is how it should be --- all input welcome.

i.e. schema

{ 'myCustomCheck': value }

Can be constrained via $validator->addConstraint('myCustomCheck', $constraint)

Add a callable constraint

$validator->addConstraint('test', \Callable);

• Inherits current/default ctr params (mode, uriRetriever, factory) - what the factory does internally
• The callable is the ConstraintInterface->check signature function ($value, $schema = null, $path = null, $i = null)

Add by Constraint class-name

$validator->addConstraint('test', 'FQCN');

• Inherits current/default ctr params (mode, uriRetriever, factory) - what the factory does internally
• FQCN must be of type JsonSchema\Constraints\ConstraintInterface

Add by Constraint instance

$validator->addConstraint('test', new MyCustomConstraint(...));

• Possibly requires adding the correct ctr params (mode, uriRetriever, factory et al) - what the factory does internally; If not the mode will be the default and the uriRetriever/factory will be another instance (if this is a problem remains to be seen)
• MyCustomConstraint must be of type JsonSchema\Constraints\ConstraintInterface

[bighappyface]

Please explain the need for custom constraints outside of the JSON Schema spec?

Wouldn't a custom constraint in a given schema only work for this validator and not others?

[AubreyHewes]

The official spec is too concrete for normal usage; the change is mostly based on (fe) https://github.com/acornejo/jjv#custom-checks

• support same schema in fe/be

Also slightly based on the "format" spec:

• Implementations MAY add custom format attributes. Save for agreement between parties, schema authors SHALL NOT expect a peer implementation to support this keyword and/or custom format attributes. (http://json-schema.org/latest/json-schema-validation.html#rfc.section.7.2)

For optimal schema usage you should be able to define own constraints, outside of the spec;

{ 'myCustomCheck': expectation }

And be able to test your constraint ... this allows this. Even if it is not spec.. it just opens up the possibilities.

Yes you need to create a validator for the required schema validation... This may be different than a previous global validator..(if this is what you meant)? So also enhances creating a new validator specific to the requirement.

[AubreyHewes]

The PR is slightly superfluous as you can add your own factory (with own custom constraints) per validator instance.

[bighappyface]

@AubreyHewes thanks for your patience and insight. Would you please rebase and squash this PR down to a single commit?

[AubreyHewes]

@bighappyface I am leaning towards this being superfluous. As via the factory you can do your own stuff anyway.. Also directly supporting custom attributes/stuff does not help the json spec.
Own business/domain spec should not be part of a spec validation. I.e. apply jsonschema and then apply own business/domain spec? Will close this.