Exploring Mixins And ColdFusion Components

I am not sure what the exact, technical definition of a "Mixin" is; but, from what I understand, a "mixin" is generically meant to define a scenario in which an object is composed of functionality that was not originally defined within it. This can be done either at compile time through includes or at runtime through method injection or includes. Earlier today, someone asked me about some mixin behavior and I didn't have an existing blog post to point them to. As such, I figured I would take lunch to throw together a few examples.

To start off with, let's create two ColdFusion templates, each of which contains a user defined function. The first one, public_udf.cfm, will define a method with "public" access:

The second one, private_udf.cfm, will define a method with "private" access:

Now that we have these methods in place, let's use them in a standard page. We're going to include them as we would any standard user defined function (UDF) and then invoke them:

When we run this, we get the expected output:

I am a public function.
I am a private function.

I am demonstrating this more to show that the concept of Public / Private methods don't really have a meaning outside of components. Technically, I believe components and templates are executed in similar methods in that they are each a page with its own context, so technically, I guess you could think of a page's VARIABLES scope as being tantamount to the private scope of a component. But, as a standard page doesn't have a public scope, the demo is rather moot.

OK, let's get onto the more exciting stuff - mixing these UDFs into a component.

The first thing I want to try is mixing them in at compile time of the component. By that, I mean we are going to execute the CFInclude in the pseudo constructor of the CFC (the area within the CFComponent tags but outside the CFFunction tags):

You can see in this code (and subsequent demos) that the Mixin.cfc (our test component) has an Init() method and a GetVariables() method that can return the private scope for debugging. You can also see that I am including our two UDFs at compile time when the component is being instantiated.

Now, let's test this component to see how it behaves:

When we instantiate the Mixin.cfc component and output it's public and private scopes, we get the following CFDump output:

As you can see, when we include the UDF mixins at compile time, the public and private properties of the user define methods are carried through such that our public method is available in our public THIS scope and our private method is available in our private VARIABLES scope. You'll also notice that the public methods are available in the private scope as well.

Ok, now let's try to include the UDF mixins at runtime from within the component's Init() method:

As you can see, all we've done is moved the CFInclude tags from the pseudo constructor to the explicit constructor (Init()). Now, when we run the same test as above, we get the following CFDump output:

As you can see, when we include the UDF mixins at runtime from within a CFC method, the methods end up in the CFC's private scope regardless of what their compile time access properties are.

Now, if we want to use this methodology but we want proper scoping, we can manually copy the public method to the public scope. To do this, after the UDFs are included, we need to explicitly copy the public one from the private scope to the public scope:

Now, with our copy line in place:

... when we run the above test, we get the following CFDump output:

As you can see, both methods ended up in the private scope as before, but now, our PublicFunction() has been copied manually to the THIS scope and is publicly accessible.

After includes, there is really only one other kind of mixin that I can think of and that is to inject a method into a component at run time. In the following demo, the Mixin.cfc is not going to do any of the including; rather, the instantiating pages is going to manually copy our functions references into the existing CFC instance. Here is our updated CFC:

And, in this demo, we are going to copy the UDFs from the page scope to the public scope of the instantiated CFC:

With this technique, when we run the above code, we get the following CFDump output:

As you can see, this time, both our UDFs are available in the public scope; but, unlike the CFInclude-based mixins (or actual pre-defined CFC methods), the private scope has no reference to these methods at all.

You can also inject methods into the private scope using a method injection proxy. In that kind of scenario, you would pass the method reference to a public method that would turn around and inject the given method into the VARIABLES scope. It achieves the same thing, only with a different target scope. Heck, you can even inject the method proxy that takes care of the variables injection.

Once a method is mixed into a CFC, it has access to all the same methods and properties that any pre-defined method would have. As far as ColdFusion is concerned, a mixin is exactly the same as a standard component method. This brings us to one final topic of discussion and that is method binding. When a UDF is used outside of a component and then called, the method is bound to the context of the calling page. By that, I mean that if you define a UDF in template A and then invoke it in template B, the UDF executes as if it were defined in template B (its calling context).

Once you copy a method into a CFC, however, the rules change. A method reference copied to a CFC is bound to the that CFC. And, when that method is invoked on that component, it uses the CFC as the context rather than that of the calling page. Of course, this only works if you call the method ON the component. If you were to copy the method reference out of the CFC and back into the page, the method take on the pervious behavior (bound to calling page context).

So there you go. I hope this sheds some light on mixin behaviors for anyone who was unclear as to what they were or how they worked.

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