For the past few months, at the recommendation of Jonah, I've been slowly working my way through Streamlined Object Modeling: Patterns, Rules, and Implementation by Jill Nicola, Mark Mayfield, and Mike Abney. Object Oriented Programming (OOP) is a fairly new concept for me. Sure, I've been creating "components" for years but, not in an object-oriented sense; more like an encapsulated procedural sense. As such, evolving my brain to use "Object Think" is going to be a long, difficult journey. I've created this project to help me work through some of the principles that were covered in the book.
While there's not really much to see in the demos, as far as visuals are concerned, you can see the results in the browser's console. Since there are several types of objects and many types of collaborations, I don't think any one example or exploration is going to help. Each one of these will help me look at different types of object models:
And, please, any feedback or input about any one of these examples would be extremely welcome! This stuff is so new and so foreign to me that I need all the help I can get.
Blog Post: Streamlined Object Modeling, October 14, 2013.
In the following sections, I am attempting to outline important notes about Streamlined Object Modeling that I can refer to as I am building out my examples. The book has a ton of information in it; so, what I have below is just the tip of the iceberg. Furthermore, what I have below is my interpretation of the principles and suggestions in the book, so feel free to take them with a grain of salt.
When it comes to defining objects within your domain model, there are four main categories:
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People: People play a huge role in any piece of software; but, people generally perform actions within a given context as defined by a given role. Each context should be modeled with a more specific type of person. Non-human entities, such as Companies, can be personified and are treated like "People" actors.
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Places: Actions that take place in a location should be modeled with a Place.
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Things: Things are typically the target of an action. They are often represented by a general type definition and a more specific form of the Thing.
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Events: Events tie people, places, and things together in a moment of time (or range of time).
As each of these objects is assembled into a object modeling pattern, they are referred to as "Pattern Players". Here are the pattern players, categorized by type (Location 1100):
- Actor
- Role
- Place
- Outer Place
- Item
- Specific Item
- Assembly
- Part
- Container
- Content
- Group
- Member
- Transaction
- Composite Transaction
- Line Item
- Follow-up Transaction
Within your application, your objects will often work together in order to model and fulfill business requirements. These relationships - these collaborations - follow 12 general patterns. Within each pattern, each object is known as a "pattern player" and has specific responsibilities when it comes to defining business logic and enforcing business constraints.
A Role is the representation of an Actor within a given context. An Actor may know zero or more roles (though the set of Roles is typically unique). A Role may only know one Actor (and the Actor can never be changed).
A Place is a location where things happen. An OuterPlace may know one or more Places - it cannot be empty. A Place may know at most one OuterPlace. This relationship can be hierarchical.
A SpecificItem is a specific representation of a generic Item. An Item may know zero or more SpecificItems. A SpecificItem must know exactly one Item (and cannot exist without it).
An Assembly is an aggregation of Parts. An Assembly must know at least one Part - it cannot be empty. A Part may know only one Assembly, but it can exist outside of an Assembly.
A Container is an collection of Content. A Container may know zero or more Contents - it can be empty. A Content may know at most one Container, but it can exist outside the Container and it can be moved into another Container. This relationship can be hierarchical.
A Group is a collection of Members. A Group knows zero or more members - it can be empty. A Member knows zero or more Groups. This relationship can be hierarchical.
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I'm a fan of rules and "best practices;" and, Streamlined Object Modeling has some really interesting best practices around the way that business rule checking should be implemented within Objects. I'll try to recap what I've learned so far, partly for your [reader] benefit; but, mostly because it will help drive these concepts into my head as I prepare to write some code.
I have always thought about "setters" as performing a single action. But, as they point out in the, book, setting a property actually consists of three distinct actions, each of which can be factored out into a smaller, more cohesive methods:
Method cohesiveness essentially means that a method should do only one thing. Setting a property involves doing three things: (1) checking the local validity of the new value, (2) checking the business rule validity of the new value, and (3) assigning the new value. To maximize the method cohesiveness of the property set accessor, isolate the business rule check and the value assignment into separate methods. This method-cohesive implementation has distinct advantages. Isolating the business rule check allows subclasses to extend and override property rules by writing their own test methods. Isolating the value assignment allows bypassing the business rules when necessary... Organizing your code this way keeps you from having to refactor it later on. (Location 4724)
According to this passage (and subsequent explanations), this means that setting a property leads to three methods:
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setValue( aValue ): Sets the value of the property in the target object. This is the "public" method called to coordinate the setting event.
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testSetValue( aValue ): Tests the value against the object's property rules and raises an exception if the rule checking fails (or returns Void if all tests pass). This method is also public because it can help with validation later on.
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doSetValue( aValue ): Assigns the value to the object's property variable. This method circumvents any validation as it assumes the "test" methods were already called. This method can also be "public" in order to help coordinate "collaboration" rules as well as to help object creation from a persistent data store.
When you're dealing with a single property, the test() methods may be verbose; but, they are fairly straightforward. When you have to perform validation across multiple objects, however, it gets a bit more complicated. When testing for validation across multiple objects, the a test must be performed in the current object; then, in the collaborating object (which may veto the validation and, therefore, the action).
Whereas property rules determine whether a property can be set, collaboration rules are the business rules that determine whether a relationship between two objects can be created or dissolved. And, like cross-property validation, collaboration rules must be validated by all of the objects involved in the collaboration. This is true regardless of which object initiates the collaboration request.
Like property accessors, it is recommended that collaboration accessors be split up into three smaller cohesive methods that allow for pluggability and extension:
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add( aCollaborator ): Adds the collaborator to the current object. This is the "public" method coordinates the cross-object validation and application.
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testAdd( aCollaborator ): Tests collaborator against the objects state and collaboration rules and raises an exception if the rule checking fails (or returns Void if all tests pass). This method is also "public" as it will give us the ability to streamline the cross-object validation (see below).
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doAdd( aCollaborator ): Assigns or adds the collaborator to the object's properties. This method circumvents validation as it assumes the "test" methods were already called. This method can also be "public" in order to help object creation from a persistent store.
NOTE: The use of "add" does not imply that all collaborators are collections of objects. Setting a single collaborator is an "add" so as to differentiate from a property accessor.
Since collaborative rule checking requires that all [involved] objects perform rule checking, the process can be streamlined by electing one object as the "coordinator." The other objects can then defer to this object to execute the majority of the work-flow. This means that one object will be responsible for calling the "test" and "do" methods for both objects.
Notice, however, that while this approach cuts down on code duplication, the work-flow still defers to the appropriate objects for logic and execution. We are not creating God Objects:
All the collaboration rules cannot be consolidated in one object. While putting all rules in one place may make the implementation easier in the short-term, it destroys pluggability, extensibility, and scalability. (Location 4921)
When determining which object will play the role of coordinator, follow the principle of, "Most Specific Carries the Load":
- Generic delegates to Specific.
- Whole delegates to a Part.
- Specific delegates to a Transaction.
- Rolling-back a partially-valid collaboration.
- Conflict collaboration rules.
- Others...
I've decided to execute this exploration in a JavaScript context because it is a very general and well-known language. And frankly, I'm gonna need a lot of help on this journey. By using JavaScript, I'm hoping that people will actually be encouraged to give me constructive feedback.
I've decided to use RequireJS as my asynchronous model loader. This way, I don't have to explicitly load each script; but, more importantly, RequireJS will keep me more honest about defining each class as its own cohesive module.
Each exploration will have its own Domain Model and will have its logic executed in the main.js file (once all of the JavaScript modules have loaded).