Write code that runs every time a given object is created, updated, or deleted.
Callbacks in Wheels allow you to have code executed before and/or after certain operations on an object. This requires some further explanation, so let's go straight to an example of a real-world application: the e-commerce checkout.
A Real-World Example of Using Callbacks
Let's look at a possible scenario for what happens when a visitor to your imaginary e-commerce website submits their credit card details to finalize an order:
- You create a new
orderobject using the
new()method based on the incoming form parameters.
- You call the
save()method on the
orderobject, which will cause Wheels to first validate the object and then store it in the database if it passes validation.
- The next day, you call the
update()method on the object because the user decided to change the shipping method for the order.
- Another day passes, and you call the
delete()method on the object because the visitor called in to cancel the order.
Let's say you want to have the following things executed somewhere in the code:
- Stripping out dashes from the credit card number to make it as easy as possible for the user to make a purchase.
- Calculating shipping cost based on the country the package will be sent to.
It's tempting to put this code right in the controller, isn't it? But if you think ahead a little, you'll realize that you might build an administrative interface for orders and maybe an express checkout as well at some point in the future. You don't want to duplicate all your logic in all these places, do you?
Object callbacks to the rescue! y using object callbacks to implement this sort of logic in your model, you keep it out of your controllers and ensure your code stays DRY (Don't Repeat Yourself).
Part of the Order.cfc model file:
Code for stripping out dashes in credit card numbers goes here...
Code for calculating shipping cost goes here...
The above code registers 2 methods to be run at specific points in the life cycle of all objects in your application.
Registering and Controlling Callbacks
The following 16 functions can be used to register callbacks.
Callback Life Cycle
As you can see above, there are a few places (5, to be exact) where one callback or the other will be executed, but not both.
The very first possible callback that can take place in an object's life cycle is either
afterNew() callback methods are triggered when you create the object yourself for the very first time, for example, using the
afterFind() is triggered when the object is created as a result of fetching a record from the database, for example, using
findByKey(). (There is some special behavior for this callback type that we'll explain in detail later on in this chapter.)
The remaining callbacks get executed depending on whether or not we're running a "create," "update," or "delete" operation.
Breaking a Callback Chain
If you want to completely break the callback chain for an object, you can do so by returning
false from your callback method. (Otherwise, always return
true or nothing at all.) As an example of breaking the chain, let's say you have called the
save() method on a new object and the method you've registered with the
beforeCreate() callback returns
false. As a result, because the method you've registered with the
beforeCreate() callback will exit the callback chain early by returning
false, no record will be inserted in the database.
Order of Callbacks
Sometimes you need to run more than one method at a specific point in the object's life cycle. You can do this by passing in a list of method names like this:
When an object is saved in your application, these two callbacks will be executed in the order that you registered them. The `checkSomething` method will be executed first, and unless it returns `false`, the `checkSomethingElse` method will be executed directly afterward.
Special Case #1:
findAll() and the
When you read about the
afterFind() callback above, you may have thought that it must surely only work for
findByKey() calls but not for
findAll() because those calls return query result sets by default, not objects.
Believe it or not, but callbacks are even triggered on
findAll()! You do need to write your callback code differently though because there will be no
this scope in the query object. Instead of modifying properties in the
this scope like you normally would, the properties are passed to the callback method via the
Does that sound complicated? This example should clear it up a little. Let's show some code to display how you can handle setting a
fullName property on a hypothetical
<cfcomponent extends="Model" output="false">
<cfset this.fullName = setFullName(this.firstName, this.lastName)>
<cfset arguments.fullName = setFullName(arguments.firstName, arguments.lastName)>
<cfargument name="fn" type="string">
<cfargument name="ln" type="string">
<cfset var fullName = "">
<cfset fullName = arguments.ln>
<cfset fullName = arguments.fn & " " & fullName>
In our example model, an artist's name can consist of both a first name and a last name like in "John Mayer" or just the band or last name like in "Abba."
setFullName method handles the concatenation of the names but the interesting stuff is in the
addFullName method. This is where we check if the
arguments struct is empty. If it is, then we know we're dealing with an object and can proceed to work with the
this scope. Otherwise, we do the changes directly on the
arguments struct and return it back.
Always remember to return the
arguments struct, otherwise Wheels won't be able to tell that you actually wanted to make any changes to the query.)
Special Case # 2: Callbacks and the
Please note that if you use the
updateAll() or the
deleteAll() methods in Wheels, they will not instantiate objects by default, and thus any callbacks will be skipped. This is good for performance reasons because if you update 1,000 records at once, you probably don't want to run the callbacks on each object. Especially not if they involve database calls.