How do I use THasAttributeFilter in Spring4d

Question

I would like to query some attributes (aka annotations) in a class I'm writing.

I can do it the complex way, but I'd like to use Spring4D instead.

It has a unit called Reflection that looks like it has what I need, but I don't know how to use it.

I've looked on the net, but there are no examples on how to use this unit.

I found:

THasAttributeFilter

Which looks like this:

  THasAttributeFilter<T: TRttiObject> = class(TMemberSpecificationBase<T>)
  private
    fAttributeClass: TAttributeClass;
  protected
    function Accept(const member: T): Boolean; override;
  public
    constructor Create(attributeClass: TAttributeClass);
  end;

Great, so I create a filter like so:

type
  Capability = (CanDoA, CanDoB);
  TCapabilities = set of Capability;

[Capabilities(CanDoA)]
TMyClass = class(TParent)
strict private
  class var fCapabilities: TCapabilities;
....
public
  property Capabilities: TCapabilities read GetCapabilities;
end;

I can create a HasAttributeFilter like so:

HasAttr:= THasAttributeFilter<TMyClass>.Create(CapabilitiesAttribute);
... ?? now what ??

How do I use Spring4D to test the attribute without having to write the zillions of lines that classical RTTI writing (as e.g. per Nick Hodges' book) requires?

You've got some typos in the property line that the editor won't let us change because they're apparently too trivial. You might want to edit them. — David Schwartz, Jul 16 '14 at 15:57
@DavidSchwartz Attributes can be fed with everything that can be evaluated at compile time, not just "Integers and numbers". Furthermore, Embarcadero themselves don't prefix attributes with `T`. Nobody says they "must" start with a `T` — Günther the Beautiful, Jul 16 '14 at 16:00
@DavidSchwartz, fixed the typo. You need to brush up on Attributes though, have a look at: http://www.malcolmgroves.com/blog/?p=476 — Johan, Jul 16 '14 at 16:11

score 3 · Accepted Answer · answered Jul 16 '14 at 19:11

These filters are implemented using the Specification Pattern. See also Spring.DesignPatterns.pas.

They are used when you need delegates like in the IEnumerable<T>.Where() method.

Here is a code snipped from Spring4D itself (Spring.Container.Builder):

var
  condition: TPredicate<TRttiMethod>;
  method: TRttiMethod;
  ...
begin
  condition := TMethodFilters.IsInstanceMethod
    and TMethodFilters.HasAttribute(InjectAttribute)
    and not TMethodFilters.HasParameterFlags([pfOut, pfVar])
    and not TMethodFilters.IsConstructor;
  for method in model.ComponentType.Methods.Where(condition) do
    ...

You can see that different specifications can be combined with boolean logic to form one expression: method has to be an instance method and has to have the InjectAttribute, must not have out or var paramaters and should not be a constructor.

This combined specification can then be assigned to a TPredicate<T> which can then be passed to the Where method.

The TSpecification<T> which is returned by each of the TMethodFilters is a record with operator overloading which makes the boolean logic and assigning it to a TPredicate<T> possible.

Using the specification pattern leads to very readable and composable code because you keep everything separated. The looping does not get cluttered with the internals how to determine if its a constructor or what kinds of parameters the method has because every specification is encapsulated into its own class.

P.S. Methods is a property from the TRttiTypeHelper from Spring.Helpers which returns IEnumerable<TRttiMethod>.

Thanks Stefan, The TPredicate usage does look really cool. I assume it works because of the parameter capturing that's happening in the anonymous methods? I.e. this will not work in D7, but will in XE. — Johan, Jul 16 '14 at 19:18
Yes, in all versions that are supported by Spring4D (2010 and up). — Stefan Glienke, Jul 16 '14 at 19:30

How do I use THasAttributeFilter in Spring4d

1 Answers1