The Law of Demeter
Now that you understand responsibilities, dependencies and interfaces, you're equipped to explore the Law of Demeter.
The Law of Demeter (LoD) is a set of coding rules which result in loosely coupled objects. Loose coupling is nearly always a virtue but is just one component of design and must be balanced against competing needs. Some Demeter violations are harmless, others expose a failure to correctly identify and define public interfaces.
Demeter restricts the set of objects to which a method may send messages. For example, according to Demeter, method depart of class Trip may only send messages to:
- The instance of Trip itself
- A parameter that was passed into the depart method
- An object created within depart
- Other objects directly held by Trip
Demeter is often paraphrased as 'only talk to your immediate neighbors' or 'use only one dot'. Imagine that Trip's depart method contains each of the following lines of code:
customer.bicycle.wheel.tire customer.bicycle.wheel.rotate hash.keys.sort.join(', ')
Every one of these lines violates Demeter. Each line is a message chain containing a number of dots (periods). The chains are colloquially referred to as 'train wrecks'; each method name represents a train car, the dots are the connections between them. Thus, Demeter violations can be recognized because they look like trains.
Consequences of Violations
Demeter become a 'law' because a human being decided so; don't be fooled by its grandiose name. As a law it's more like 'floss your teeth every day' than like gravity. You might prefer not to confess to your dentist but occasional violations will not collapse the universe.
Chapter 2 stated that code should be Transparent, Reasonable, Usable and Exemplary. Some of the message chains above fail when judged against TRUE:
- If wheel changes tire or rotate, depart may have to change. Trip has nothing to do with wheel yet changes to wheel might force changes in Trip. This unnecessarily raises the cost of change; the code is not Reasonable.
- Changing tire or rotate may break something in depart. Since Trip is distant and apparently unrelated, the failure will be completely unexpected. This code is not Transparent.
- Trip cannot be reused unless it has access to a customer with a bicycle that has a wheel and a tire. It requires a lot of context and is not easily Usable.
- This pattern of messages will be replicated by others, producing more code with similar problems. This style of code, unfortunately, breeds itself. It is not Exemplary.
The first two message chains are nearly identical, differing only in that one retrieves a distant attribute (tire) and the other invokes distant behavior (rotate). Even experienced designers argue about how firmly Demeter applies to message chains that return attributes. It may be cheapest, in your specific case, to reach through intermediate objects to retrieve distant attributes. Balance the likelihood and cost of change against the cost of removing the violation. If, for example, you are printing a report of a set of related objects, the most rational strategy may be to explicitly specify the intermediate objects and to change the report if it becomes necessary. Since the risk incurred by Demeter violations is low for stable attributes, this may be the most cost efficient strategy.
This tradeoff is permitted as long as you are not changing the value of the attribute you retrieve. If depart sends customer.bicycle.wheel.tire with the intent of altering the result, it is not just retrieving an attribute, it is implementing behavior that belongs in Wheel. In this case customer.bicycle.wheel.tire becomes just like customer.bicycle.wheel.rotate; it's a chain that reaches across many objects to get to distant behavior. The inherent cost of this coding style is high; this violation should be removed.
The third message chain, hash.keys.sort.join is perfectly reasonable and may be the most parsimonious and maintainable expression of this idea. If you apply Demeter to this line of code, your costs may well go up instead of down.
As you can see, Demeter is more subtle than first appears. Its fixed rules are not an end in themselves; like every design principle, it exists in service of your overall goals. Certain 'violations' of Demeter reduce your application's flexibility and maintainability, others make perfect sense.
One common way to remove 'train wrecks' from code is to use delegation to avoid the 'dots'. In object oriented terms, to delegate a message is to pass it on to another object, often via a wrapper method. The wrapper method encapsulates, or hides, knowledge that would otherwise be embodied in the message chain.
There are a number of ways to accomplish delegation. Ruby contains delegate.rb and forwardable.rb and the Ruby on Rails framework includes the delegate method. Each of these exists to make it easy for an object to automatically intercept a message sent to self and to instead send it somewhere else.
Delegation is tempting as a solution to the Demeter problem because it removes the visible evidence of violations. This technique is sometimes useful, but beware, it can result in code that obeys the letter of the law while ignoring its spirit. Using delegation to hide tight coupling is not the same as de-coupling the code.
Listening to Demeter
Demeter is trying to tell you something and it isn't 'Use more delegation'.
Message chains like customer.bicycle.wheel.rotate occur when your design thoughts are unduly influenced by objects you already know. Your familiarity with the public interfaces of known objects may lead you to string together long message chains to get at distant behavior.
Reaching across objects to distant behavior is tantamount to saying, 'there's some behavior way over there that I need right here and I know how to go get it'. The code knows not only what it wants (to rotate) but how to reach that behavior. Just as Trip, earlier, knew how Mechanic should prepare a bike, here the depart method knows how to navigate through a series of objects to make a wheel rotate.
This how knowledge causes all kinds of problems. The most obvious is that it raises the risk that Trip will be forced to change because of an unrelated change somewhere in the message chain. However, there's another problem here that is even more serious.
When the depart method knows this chain of objects, it binds itself to a very specific implementation and it cannot be reused in any other context. Customers must always have Bicycles which in turn must have Wheels that rotate.
Consider what this message chain would look like if you had started out by deciding what depart wants from customer. From a message based point of view, the answer is obvious:
The ride method of customer hides implementation details from Trip and reduces both its context and its dependencies, significantly improving the design. When FastFeet changes and begins leading hiking trips it's much easier to generalize from customer.ride to customer.depart or customer.go than to disentangle the tentacles of this message chain from your application.
The train wrecks of Demeter violations are clues that there are objects whose public interfaces are lacking. Listening to Demeter means paying attention to your point of view. If you shift to a message based perspective, the messages you find will become public interfaces in the objects they lead you to discover. However if you are bound by the shackles of existing domain objects, you'll end up assembling their existing public interfaces into long message chains and thus will miss the opportunity to find and construct flexible public interfaces.