module Athena::Framework::Controller::ValueResolvers::Interface

Value resolvers handle resolving the argument(s) to pass to a controller action based on values stored within the ATH::Request, or some other source.

Custom resolvers can be defined by creating a service that implements this interface, and is tagged with ATHR::Interface::TAG. The tag also accepts an optional priority field the determines the order in which the resolvers execute. The list of built in resolvers and their priorities can be found on the ATH::Controller::ValueResolvers module.

Warning

Resolvers that mutate a value already within the ATH::Request#attributes, such as one from a route or query parameter MUST have a priority >100 to ensure the custom logic is applied before the raw value is resolved via the ATHR::RequestAttribute resolver.

The first resolver to return a value wins and no other resolvers will be executed for that particular parameter. The resolver should return nil to denote no value could be resolved, such as if the parameter is of the wrong type, does not have a specific annotation applied, or anything else that can be deduced from either parameter. If no resolver is able to resole a value for a specific parameter, an error is thrown and processing of the request ceases.

For example:

@[ADI::Register(tags: [{name: ATHR::Interface::TAG, priority: 10}])]
struct CustomResolver
  include ATHR::Interface

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata) : MyCustomType?
    # Return early if a value is unresolvable from the current *request* and/or *parameter*.
    return if parameter.type != MyCustomType

    # Return the resolved value. It could either come from the request itself, an injected service, or hard coded.
    MyCustomType.new "foo"
  end
end

Now, given the following controller:

class ExampleController < ATH::Controller
  @[ARTA::Get("/")]
  def root(my_obj : MyCustomType) : String
    my_obj.name
  end
end

# GET / # => "foo"

Since none of the built-in resolvers are applicable for this parameter type, nor is there a my_obj value in ATH::Request#attributes, assuming no customer listeners manually add it, the CustomResolver would take over and provide the value for that parameter.

Configuration

In some cases, the request and parameter themselves may not be enough to know if a resolver should try to resolve a value or not. A naive example would be say you want to have a resolver that multiplies certain Int32 parameters by 10. It wouldn't be enough to just check if the parameter is an Int32 as that leaves too much room for unexpected contexts to be resolved unexpectedly. For such cases a .configuration annotation type may be defined to allow marking the specific parameters the related resolver should apply to.

For example:

# The priority _MUST_ be `>100` to ensure the value isnt preemptively resolved by the `ATHR::RequestAttribute` resolver.
@[ADI::Register(tags: [{name: ATHR::Interface::TAG, priority: 110}])]
struct Multiply
  include ATHR::Interface

  configuration This

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata) : Int32?
    # Return early if the controller action parameter doesn't have the annotation.
    return unless parameter.annotation_configurations.has? This

    # Return early if the parameter type is not `Int32`.
    return if parameter.type != Int32

    request.attributes.get(parameter.name, Int32) * 10
  end
end

class ExampleController < ATH::Controller
  @[ARTA::Get("/{num}")]
  def multiply(
    @[Multiply::This]
    num : Int32,
  ) : Int32
    num
  end
end

ATH.run

# GET /10 # => 100

While this example is quite naive, this pattern is used as part of the ATHR::RequestBody to know if an object should be deserialized from the request body, or is intended be supplied some other way.

Extra Data

Another use case for this pattern is providing extra data on a per parameter basis. For example, say we wanted to allow customizing the multiplier instead of having it hard coded to 10.

In order to do this we can pass properties to the .configuration macro to define what we want to be configurable via the annotation. Next we can then use this value in our resolver, and when applying to a specific parameter:

# The priority _MUST_ be `>100` to ensure the value isnt preemptively resolved by the `ATHR::RequestAttribute` resolver.
@[ADI::Register(tags: [{name: ATHR::Interface::TAG, priority: 110}])]
struct Multiply
  include ATHR::Interface

  configuration This, multiplier : Int32 = 10

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata) : Int32?
    # Return early if the controller action parameter doesn't have the annotation.
    return unless (config = parameter.annotation_configurations[This]?)

    # Return early if the parameter type is not `Int32`.
    return if parameter.type != Int32

    request.attributes.get(parameter.name, Int32) * config.multiplier
  end
end

class ExampleController < ATH::Controller
  @[ARTA::Get("/{num}")]
  def multiply(
    @[Multiply::This(multiplier: 50)]
    num : Int32,
  ) : Int32
    num
  end
end

ATH.run

# GET /10 # => 500

A more real-world example of this pattern is the ATHR::Time resolver which allows customizing the format and/or location that should be used to parse the datetime string.

Handling Multiple Types

When using an annotation to enable a particular resolver, it may be required to handle parameters of varying types. E.g. it should do one thing when enabled on an Int32 parameter, while a different thing when applied to a String parameter. But both things are related enough to not warrant dedicated resolvers. Because the type of the parameter is stored within a generic type, it can be used to overload the #resolve method based on its type For example:

# The priority _MUST_ be `>100` to ensure the value isnt preemptively resolved by the `ATHR::RequestAttribute` resolver.
@[ADI::Register(tags: [{name: ATHR::Interface::TAG, priority: 110}])]
struct MyResolver
  include ATHR::Interface

  configuration Enable

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata(Int32)) : Int32?
    return unless parameter.annotation_configurations.has? Enable

    request.attributes.get(parameter.name, Int32) * 10
  end

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata(String)) : String?
    return unless parameter.annotation_configurations.has? Enable

    request.attributes.get(parameter.name, String).upcase
  end

  # :inherit:
  #
  # Fallback overload for types other than `Int32` and `String.
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata) : Nil
  end
end

class ExampleController < ATH::Controller
  @[ARTA::Get("/integer/{value}")]
  def integer(
    @[MyResolver::Enable]
    value : Int32,
  ) : Int32
    value
  end

  @[ARTA::Get("/string/{value}")]
  def string(
    @[MyResolver::Enable]
    value : String,
  ) : String
    value
  end
end

ATH.run

# GET /integer/10  # => 100
# GET /string/foo # => "FOO"

Free Vars

If more precision is required, a free variable can be used to extract the type of the related parameter such that it can be used to generate the proper code.

An example of this is how ATHR::RequestBody handles both ASR::Serializable and JSON::Serializable types via:

{% begin %}
  {% if T.instance <= ASR::Serializable %}
    object = @serializer.deserialize T, body, :json
  {% elsif T.instance <= JSON::Serializable %}
    object = T.from_json body
  {% else %}
    return
  {% end %}
{% end %}

This works well to make the compiler happy when previous methods are not enough.

Strict Typing

In all of the examples so far, the resolvers could be applied to any parameter of any type and all of the logic to resolve a value would happen at runtime. In some cases a specific resolver may only support a single, or small subset of types. Such as how the ATHR::RequestBody resolver only allows ASR::Serializable or JSON::Serializable types. In this case, the ATHR::Interface::Typed module may be used to define the allowed parameter types.

Warning

Strict typing is ONLY supported when a configuration annotation is used to enable the resolver.

@[ADI::Register(tags: [{name: ATHR::Interface::TAG}])]
struct MyResolver
  # Multiple types may also be supplied by providing it a comma separated list.
  # If `nil` is a valid option, the `Nil` type should also be included.
  include ATHR::Interface::Typed(String)

  configuration Enable

  # :inherit:
  def resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata) : String?
    return unless parameter.annotation_configurations.has? Enable

    "foo"
  end
end

class ExampleController < ATH::Controller
  @[ARTA::Get("/integer")]
  def integer(
    @[MyResolver::Enable]
    value : Int32,
  ) : Int32
    value
  end

  @[ARTA::Get("/string")]
  def string(
    @[MyResolver::Enable]
    value : String,
  ) : String
    value
  end
end

ATH.run

# Error: The annotation '@[MyResolver::Enable]' cannot be applied to 'ExampleController#integer:value : Int32'
# since the 'MyResolver' resolver only supports parameters of type 'String'.

Since MyResolver was defined to only support String types, a compile time error is raised when its annotation is applied to a non String parameter. This feature pairs nicely with the free var section as it essentially allows scoping the possible types of T to the set of types defined as part of the module.

Direct including types

Athena::Framework::Controller::ValueResolvers::DefaultValue Athena::Framework::Controller::ValueResolvers::Enum Athena::Framework::Controller::ValueResolvers::Interface::Typed(*SupportedTypes) Athena::Framework::Controller::ValueResolvers::QueryParameter Athena::Framework::Controller::ValueResolvers::Request Athena::Framework::Controller::ValueResolvers::RequestAttribute Athena::Framework::Controller::ValueResolvers::Time Athena::Framework::Controller::ValueResolvers::UUID

Constants

TAG = "athena.controller.value_resolver"

The tag name for ATHR::Interface services.

Methods

abstract #resolve(request : ATH::Request, parameter : ATH::Controller::ParameterMetadata)

Returns a value resolved from the provided request and parameter if possible, otherwise returns nil if no parameter could be resolved.

View source

Macros

configuration(name, *args)

Helper macro around ADI.configuration_annotation that allows defining resolver specific annotations. See the underlying macro and the configuration section for more information.

View source