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Actions have four parts to them (responsibliities, I suppose):

  • execution
  • supporting methods for perform the validation etc
  • target object that they act upon
  • event publishing

Our programming model could provide several syntaxes to put these responsibilities in different places.  It already supports two (standard actions, and mixins).

To compare these syntaxes, we'll use a concrete example:

public class Customer {                                                                                            // target
    ...
    public static class PlaceOderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                                                   // event publishing 
    public Customer placeOrder(Product p, int quantity) { ... }                                                    // execution

    public boolean hidePlaceOrder() { return this.isBlackListed(); }                                               // supporting methods
    public String disablePlaceOrder() { return clockService.outsideShoppingHours(); }
    public String disable1PlaceOrder(Product p) { return p.isOutOfStock() ? "Out of stock": null; }                
    public Collection<Product> choices0PlaceOrder() { ... }                                                        
    public Product default0PlaceOrder() { return orderService.findLastProductPurchasedBy(this); }
    public int default1PlaceOrder() { return 1; }
    public String validate1PlaceOrder(int quantity) { return quantity <= 0 ? "Can only order +ve amounts": null; }
    public String validatePlaceOrder(Product p, int quantity) { return catalogService.runningLowOn(p) && quantity > 4 ? "We're running low on that product, no more than 4" : null; }

    ...
    @Inject CatalogService catalogService;
    @Inject ClockService clockService;
    @Inject OrderService orderService;
}

Notes:

  • For disable1PlaceOrder(), we could also hide individual parameters similarly
  • Instead of choices0PlaceOrder(), we could have used autoComplete0PlaceOrder(String)

Standard syntax

The standard syntax uses regular methods on the target object.  Naming conventions are used to associate the action with supporting methods (default, choices, hide, disable and validate).

Here's the same example as before, but with implementation stripped out so that it is easier to compare with the alternate syntaxes that follow:

public class Customer {                                                              // target
    
    public static class PlaceOderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                     // event publishing
    public Customer placeOrder(Product p, int quantity) { ... }                      // action execution

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


Mixins syntax

Mixins change the target, by allowing this set of methods to be moved to a different object.  In other words, the target responsibility changes.

So the real target is simply:

public class Customer {}


and the action itself moves onto the mixin:

@Action                                                        
public class Customer_placeOrder() {                           

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

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer act(Product p, int quantity) { ... }                          // action execution

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

Notes:

  • Instead of @Action, the @Mixin(method="act") could also be used, with additional annotations on the act(...) method.  I've chosen the version with the least boilerplate here.
  • Mixins are also used for derived properties or collections (ie query-only actions with no side-effects).  These are specified using @Property or @Collection 
  • We now have two classes here: the mixin, and the domain event within.

Parameters syntax (proposed)

Per this thread on slack, we could introduce a Parameters object (in Java 14+, this might be a record) to bring together all of the parameters into a single object.  This would make it easier to avoid issues with numbering etc.

This syntax changes the way in which supporting methods are associated back to the main execution method.

public class Customer {                                                           // target

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

    public static class PlacerOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer placeOrder(Product p, int quantity) { ... }                   // 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) { ... }
}

Notes:

  • The @Value @Accessors(fluent=true) allows us to use a syntax that is very similar to Java 14 records.
  • There is some duplication here: the list of the parameter types appears both in the placeOrder(...) method, as well as in the PlacerOrdersParameters class.

The above would also be supported with mixins:

@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;
        int quantity;
    }

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action(domainEvent = PlaceOrderEvent.class)                                  // event publishing
    public Customer act(Product p, int quantity) { ... }                          // 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) { ... }
}

Notes:

  • we now have three classes here: the mixin, the domain event, and the parameters object.

Parameters on Act syntax (proposed)

This is a variant of the previous, but uses the parameters class in the action as well:

public class Customer {                                                           // target

    @Value @Accessors(fluent = true)                   
    public class PlaceOrderParameters {                                           // to assist supporting methods 
        Product product;
        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) { ... }
}

Notes:

  • this removes the duplication between the placeOrder(...) parameter list and the list of members in PlaceOrderParameters class.

As a mixin, this becomes:

@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;
        int quantity;
    }

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

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

Notes:

  • we still have three classes here (mixin, parameters and domain event), but we have removed the duplication between the act(...) parameter list and the list of members of PlaceOrderParameters class

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):



Mixins and Parameters combined (proposed)

yada yada


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