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Code Block

public class Customer {                                                           // target

    @Value @Accessors(fluent = true)                   
    public class PlaceOrderParameters {                                           // to assist supporting methods 
        @Parameter() @MemberOrder(1)
        Product product;
        @Parameter() @MemberOrder(2)
        int quantity;
    }

    public static class PlacerOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer placeOrder(PlaceOrderParameters params) { ... }               // execution    

    public boolean hidePlaceOrder() { ... }                                       // supporting methods use PlaceOrderParameters
    public String disablePlaceOrder() { ... }
    public String disable1PlaceOrder(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0PlaceOrder() { ... }              
    public Product default0PlaceOrder() { ... }
    public int default1PlaceOrder() { ... }
    public String validate1PlaceOrder(PlaceOrderParameters params) { ... }
    public String validatePlaceOrder(PlaceOrderParameters params) { ... }
}

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this removes the duplication between the placeOrder(...) parameter list and the list of members in PlaceOrderParameters class.
the @Parameter and @MemberOrder syntax would be required by the framework to identify PlaceOrderParameters as a container of parameters (as opposed to a reference object or custom value type)

As a mixin, this becomes:

Code Block

@Action
public class Customer_placeOrder {

    private final Customer target;                                                // target
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    // to assist supporting methods
        Product product;@Parameter() @MemberOrder(1)
        Product product;
        @Parameter() @MemberOrder(2)
        int quantity;
    }

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer act(PlaceOrderParameters params) { ... }                             // execution

    public boolean hideAct() { ... }                                              // supporting methods
    public String disableAct() { ... }
    public String disable1Act(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0Act() { ... }              
    public Product default0Act() { ... }
    public int default1Act() { ... }
    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

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The previous syntax only passes in parameters to some of the supporting methods. For consistency, we could imagine it being passed in always:.With the parameters object passed in everywhere, I could see myself starting to move functionality onto that object. So as an idiom, we might see the following sort of code (in a mixin)

Just focusing on the mixin syntax, this would become:

Code Block

@Action
public class Customer_placeOrder {

    private final Customer target;                                                // target
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    // to assist supporting methods
        @Parameter() @MemberOrder(1)
        Product product;
        @Parameter() @MemberOrder(2)
        int quantity;
    }

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer act(PlaceOrderParameters params) { ... }                      // execution

    public boolean hideAct(PlaceOrderParameters params) { ... }                   // supporting methods
    public String disableAct(PlaceOrderParameters params) { ... }
    public String disable1Act(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0Act(PlaceOrderParameters params) { ... }              
    public Product default0Act(PlaceOrderParameters params) { ... }
    public int default1Act(PlaceOrderParameters params) { ... }
    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

Discussion

With the parameters object passed in everywhere, I could see myself starting to move functionality onto that object. So as an idiom, we might see the following sort of code (in a mixin):

Code Block

@Action
public class Customer_placeOrder {

    private final Customer target;                                                
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {          
    public static class PlaceOrderParameters {                  
        @Parameter() @MemberOrder(1)
        Product product;
        Product product;@Parameter() @MemberOrder(2)
        int quantity;
    }

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  
    public Customer act(PlaceOrderParameters params) { return params.act(this); } 

    public boolean hideAct(PlaceOrderParameters params) { return params.hide(this); }                                              
    public String disableAct(PlaceOrderParameters params) { return params.disable(this); }
    public String disable1Act(PlaceOrderParameters params) { return params.disable1(this); }
    public Collection<Product> choices0Act(PlaceOrderParameters params) { return params.choices0(this); }              
    public Product default0Act(PlaceOrderParameters params) { return params.default0(this); }
    public int default1Act(PlaceOrderParameters params) { return params.default1(this); }
    public String validate1Act(PlaceOrderParameters params) { ...return params.validate1(this); }
    public String validateAct(PlaceOrderParameters params) { ...params.validate(this); }
}

which would then beef up the parameters object:

Mixins and Parameters combined (proposed)

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Mixins and Parameters combined (proposed)