ActionParameters

Our programming model currently provides two different syntaxes: standard, and mixins. This page recaps on those options and suggests several others. To compare the 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

    @Action
    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 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.

Analysis

Actions have three parts (or responsibilities) to them:

execution
the target object that they act upon
the set of parameters/arguments that are passed to the execution and to the supporting methods that perform validation etc.

(Actually, there's also event publishing, and an earlier version of this page also discussed that ... but we didn't see any point in changing how that worked).

The standard model and the mixin model have a quite different "feel" to them, though they only subtly change where these responsibilities reside: for the standard model, the target object is implicit (ie "this") whereas with mixins the target object is explicit (passed into the constructor). In other respects the programming models are the same.

Playing around with where these responsibilities live allow us to create a number of other programming models. The table below summarises and names these options::

	target	behaviour	parameter values	Notes
standard	implicit	Y		The target is implicit ("this"), and the set of parameter values (arguments) are only implicit in the signatures of the execute action and the supporting methods
mixins	Y	Y		The target is explicit, being the constructor of the mixin.
Parameters model			Y	Separate class that captures the set of parameters that are passed to the supporting methods
Parameters on Act			Y	Minor variation
Parameters Everywhere			Y	Another variation
Mixins + Parameters	Y	Y	Y	Combines the concepts of a mixin along with a parameters model
Targetless Mixins + Targeted Parameters	Y	Y	Y	Splits out state and behaviour
Command handlers + Commands	Y	Y	Y	Variation that splits behaviour into separate interfaces

The rest of the page describes these options in more detail.

Parameters model 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;
    }

    @Action
    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 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 
        @Parameter() @MemberOrder(1)
        Product product;
        @Parameter() @MemberOrder(2)
        int quantity;
    }

    @Action
    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.
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:

@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()
        Product product;
        @Parameter()
        int quantity;
    }

    @Action
    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) { ... }
}

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

Parameters everywhere syntax (proposed)

The previous syntax only passes in parameters to some of the supporting methods. For consistency, we could imagine it being passed in always.

Just focusing on the mixin syntax, this would become:

@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()
        Product product;
        @Parameter()
        int quantity;
    }

    @Action
    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):

@Action
public class Customer_placeOrder {

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

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters { ... }                                       // see below.                                   

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action
    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:

@Action
public class Customer_placeOrder {

    private final Customer target;                                         // target
    ...

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    

        @Parameter()
        Product product;
        @Parameter()
        int quantity;

	    public Customer act(Customer customer) { ... }                     // execution

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

    ...

    @Action
    public Customer act(PlaceOrderParameters params) { return params.act(this); }                         // remainder is just boilerplate

    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); }
}

Notes:

the target is still outside of the parameters object
Event publishing also outside
Everything else has moved inside the parameters object
This implies that we would need dependency injection for the parameters object
The rest of the code in the mixin is just boilerplate. It's possible that the Lombok @Delegate annotation might be used to remove some of this boilerplate, didn't investigate further.

Mixins and Parameters combined (proposed)

The previous section describes an idiom to work within the new Parameter object programming model. But the next step along the journey would be to formally recognise this pattern. This would amount to collapsing the mixin concept and the parameters concept into the same thing. Said another way, mixins start to become stateful, keeping track of the parameter argument values as well as the target object:

@Action
public class Customer_placeOrder {

    private final Customer target;                                              // target
    ...

    @Parameter() @MemberOrder(1)                                                // supporting methods support
    Product product;
    @Parameter() @MemberOrder(2)
    int quantity;

    @Action
    public Customer act() { ... }                                               // execution 

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

Notes:

here the supporting methods would simply read from the fields of the mixin that represent the parameters of the mixin itself.
the domain event class is still separate
@MemberOrder is required because the JVM does not guarantee the order in the bytecode is the same as in the source file.

Target-less Mixins + Targeted Parameters

Traditionally mixins hold all of the behaviour and a little bit of the state - namely the target object. Meanwhile parameters hold the rest of the state, but without the target.

Another way to divide the responsibilities would be to move the target from the mixin, and add it into the parameters object. In other words, the former would just be the behaviour, the latter would be just the state.

Thus we have an extended parameters object, that also takes the target:

@Value @Accessors(fluent = true)             
public class PlaceOrderParameters {
    @Target                          // a new annotation
    Customer customer;

    @Parameter()
    Product product;
    @Parameter()
    int quantity;
}

Meanwhile the mixin provides just the behaviour, of both the action and also the various supporting methods. The supporting methods all need to take the PlaceOrderParameters, because it now contains the target, at least

@Action
public class Customer_placeOrder {

    @Action
    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) { ... }
}

Command Handlers

Building on the previous example, having split up the behaviour from the state completely, we realise that there's no need to keep all of the methods of the mixin together.

We could rename the "parameters object" as a command:

@Value @Accessors(fluent = true)             
public class PlaceOrderCommand {    
    @Target                        
    Customer customer;

    @Parameter()
    Product product;
    @Parameter()
    int quantity;
}

and then we could have a number of handlers, for example for the execution:

public class CustomerPlaceOrderHandler {

    @Action
    public Customer act(PlaceOrderCommand command) { ... }                             // execution - infer the name of the action from the type
}

and for the preconditions (no need for the "Act" suffix):

public class CustomerPlaceOrderValidationHandler {

    public boolean hide(PlaceOrderCommand command) { ... }                          // supporting methods
    public String disable(PlaceOrderCommand command) { ... }
    public String disable1(PlaceOrderCommand command) { ... }
    public String validate1(PlaceOrderCommand command) { ... }
    public String validate(PlaceOrderCommand command) { ... }
}

and for the UI hints:

public class CustomerPlaceOrderUiHintsHandler {

    public Collection<Product> choices0(PlaceOrderCommand command) { ... }              
    public Product default0(PlaceOrderCommand command) { ... }
    public int default1(PlaceOrderCommand command) { ... }
}

Command Handler Contracts

Command handlers in other frameworks often have a single method, called something like "apply" or "accept". We can't quite get there because we not only need to execute the action, but also do the validation and UI hint stuff.

We could though introduce some API to define this contract.

public class CustomerPlaceOrderHandler implements CommandActHandler<PlaceOrderCommand> {

    public Customer act(PlaceOrderCommand command) { ... }              
}

To hide entire action:

public class CustomerPlaceOrderHideActHandler implements CommandHideActHandler<PlaceOrderCommand> {
    public boolean hide(PlaceOrderCommand command) { ... }                          
}

To hide individual parameters:

public class CustomerPlaceOrderHideParamHandler implements CommandHideParamHandler<PlaceOrderCommand> {
    public String hide(PlaceOrderCommand command, int paramNum) { ... }
}

to disable entire action:

public class CustomerPlaceOrderDisableActHandler implements CommandDisableActHandler<PlaceOrderCommand> {
    public String disable(PlaceOrderCommand command) { ... }
}

To disable individual parameters:

public class CustomerPlaceOrderDisableHandler implements CommandDisableParamHandler<PlaceOrderCommand> {
    public String disable(PlaceOrderCommand command, int paramNum) { ... }
}

To validate entire parameter set:

public class CustomerPlaceOrderValidateActHandler implements CommandValidateActHandler<PlaceOrderCommand> {
    public String validate(PlaceOrderCommand command) { ... }
}

To validate individual parameters:

public class CustomerPlaceOrderValidateParamHandler implements CommandValidateParamHandler<PlaceOrderCommand> {
    public String validate(PlaceOrderCommand command, int paramNum) { ... }
}

And we keep going for the UI hints.

To return choices:

public class CustomerPlaceOrderChoicesParamHandler implements CommandChoicesParamHandler<PlaceOrderCommand> {
    public Collection<Object> choices(PlaceOrderCommand command, int paramNum) { ... }                              // bit ugly
}

To provide an autoComplete:

public class CustomerPlaceOrderAutoCompleteHandler implements CommandAutoCompleteParamHandler<PlaceOrderCommand> {
    public Collection<Object> autoComplete(PlaceOrderCommand command, int paramNum, String search) { ... }           // bit ugly
}

To return defaults:

public class CustomerPlaceOrderDefaultParamHandler implements CommandDefaultParamHandler<PlaceOrderCommand> {
    public Object defaultOf(PlaceOrderCommand command, int paramNum) { ... }                                         // 'default' is a reserved word
}

Of course, there's nothing to prevent a single class from implementing all of these interfaces:

@Action
public class CustomerPlaceOrderHandler 
       implements CommandActHandler<PlaceOrderCommand>,
                  CommandHideActHandler<PlaceOrderCommand>,
                  CommandHideParamHandler<PlaceOrderCommand>, 
                  CommandDisableActHandler<PlaceOrderCommand>, 
                  CommandDisableParamHandler<PlaceOrderCommand>,
                  CommandValidateActHandler<PlaceOrderCommand>,
                  CommandValidateParamHandler<PlaceOrderCommand>,
                  CommandChoicesParamHandler<PlaceOrderCommand>,
                  CommandAutoCompleteParamHandler<PlaceOrderCommand>,
                  CommandDefaultParamHandler<PlaceOrderCommand> {

    public Customer act(PlaceOrderCommand command) { ... }   
    public boolean hide(PlaceOrderCommand command) { ... }                         
    public String hide(PlaceOrderCommand command, int paramNum) { ... }
    public String disable(PlaceOrderCommand command) { ... } {
    public String disable(PlaceOrderCommand command, int paramNum) { ... }
    public String validate(PlaceOrderCommand command) { ... }
    public String validate(PlaceOrderCommand command, int paramNum) { ... }
    public Collection<Object> choices(PlaceOrderCommand command, int paramNum) { ... } 
    public Collection<Object> autoComplete(PlaceOrderCommand command, int paramNum, String search) { ... } 
    public Object defaultOf(PlaceOrderCommand command, int paramNum) { ... }
}

Space shortcuts

Child pages

Standard syntax

Mixins syntax

Analysis

Parameters model syntax (proposed)

Parameters on Act syntax (proposed)

Parameters everywhere syntax (proposed)

Discussion

Mixins and Parameters combined (proposed)

Target-less Mixins + Targeted Parameters

Command Handlers

Command Handler Contracts