Dependency Management – Reducing Dependency Cycles and Coupling

So, what am I missing?

Dependency Injection decreases coupling between a class and its dependency. But it increases the coupling between a class and its consumer (since the consumer needs more info to create it) and the dependency and its consumer (since the consumer needs to know the dependency to use).

Very often, this is a good trade off. The class shouldn't know the details about its dependencies beyond an interface, and it should be the responsibility of the application to tie specific bits of code together.

If you have too many dependencies being passed around, the general technique is to eliminate those dependencies higher up in the call stack, by changing the order decisions are made. This is easiest to explain with an example:

getPath(config, user) {
  if (config.isB2B())
    return b2bpath(config, user);
  else
    return b2cpath(config, user);
}

b2bpath(config, user) {
  if (!config.allowedToAccessPath(user))
    return accessDeniedPage();
  else
    return "My fancy b2b page";
}

b2cpath(config, user) {
  if (!config.allowedToAccessPath(user))
    return accessDeniedPage();
  else
    return "My fancy b2c page";
}

You are repeating the authorization check down at the lowest levels of the call stack, so move it up:

getPath(config, user) {
  if (!config.allowedToAccessPath(user))
    return accessDeniedPage();

  if (config.isB2B())
    return b2bpath();
  else
    return b2cpath();
}

b2bpath() {
    return "My fancy b2b page";
}

b2cpath() {
    return "My fancy b2c page";
}

Then repeat to see if you can move some of the decisions into the code that calls getPath. This is a simple example, but I see the former kind of code all over the place with more layers. Start with decisions at your lowest layers, and try to figure out ways to move them up. Sometimes this requires judicious use of inheritance, like:

getPath(config, user) {
  module = config.isB2B() ? B2BModule() : B2CModule()
  if (!config.allowedToAccessPath(user))
    return module.accessDeniedPage();

  return module.getPath();
}

It's very rare not to be able to simplify dependencies this way. It's a matter of trying different arrangements until you find one that works.

If you have workflow-type dependencies, not just data dependencies, as in your first example, you can separate them out using something like this:

step1 = new ValidateHeaderId(inputHeaderId);
step2 = new FindShipment(retShipmentRepository);
step3 = new ValidateUserPermission(user, inputPostCode, inputEmail);
step4 = new SaveReturnOrder(inputLines, inputReference);
step5 = new CheckSaveStatus();
notSet = new NotSetPage(templateEngine, searchPage);
notPermitted = new NotPermittedPage(templateEngine, searchPage);
saveErrors = new SaveErrorsPage();
success = new SuccessPage();
requestTokenConfirmation = new RequestTokenConfirmation();

steps = [step1, [notSet, step2], [notSet, step3], [notPermitted, step4],
         [saveErrors, step5], [success, requestTokenConfirmation]];
executeSteps(steps);

This recognizes you have a series of steps which each produce some sort of result and choose the next step. executeSteps abstracts away the repetition of calling run() on each step, and passing the output from the previous step into the next step. This allows the steps to be stored in a data structure instead of a function, which can then be built up in several different ways, including by some sort of registration process or config file. Once each step object has been created, its dependencies no longer need to be tracked outside it. I believe the rules engines from BobDalgleish's answer are basically pre-existing libraries to help you do this more easily.