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.
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 order object, 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! By 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:
<cfcomponent extends="Model"> <cffunction name="init"> <cfset beforeValidationOnCreate("fixCreditCard")> <cfset afterValidation("calculateShippingCost")> </cffunction> <cffunction name="fixCreditCard"> Code for stripping out dashes in credit card numbers goes here... </cffunction> <cffunction name="calculateShippingCost"> Code for calculating shipping cost goes here... </cffunction> </cfcomponent>
The above code registers 2 methods to be run at specific points in the life cycle of all objects in your application.
When naming your callbacks you might be tempted to try and keep things (too) simple by doing something like
Do not do this.
If you do, CFWheels will fail silently and you might be left wondering why nothing is happening. (What is happening is that you, if there is a corresponding method named
afterValidation, unintentionally overrode an internal CFWheels method.)
It's best to name the methods so they describe what task they actually perform, such as
fixCreditCard as shown in the example above.
The following 16 functions can be used to register callbacks.
- afterNew() or afterFind()
- beforeValidationOnCreate() or beforeValidationOnUpdate()
- afterValidationOnCreate() or afterValidationOnUpdate()
- beforeCreate() or beforeUpdate()
- afterCreate() or afterUpdate()
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() or afterFind. The afterNew() callback methods are triggered when you create the object yourself for the very first time, for example, when using the new() method. afterFind is triggered when the object is created as a result of fetching a record from the database, for example, when 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.
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 callback 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.
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
checkSomethingElse method will be executed directly afterward.
When you read about the afterFind() callback above, you may have thought that it must surely only work for findOne()/ 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
NOTE: Because all afterFind() callbacks run when fetching records from the database, it's a good idea to check to make sure that the columns used in the method's logic exist before performing any operations. You mostly encounter this issue when using the
select argument on a finder to limit the number of columns returned. But no worries! You can use
StructKeyExists() and perform a simple check to make sure that the columns exists in the
<cfcomponent extends="Model" output="false"> <cffunction name="init"> <cfset afterFind("setFullName")> </cffunction> <cffunction name="setFullName"> <cfset arguments.fullName = ""> <cfif StructKeyExists(arguments, "firstName") and StructKeyExists(arguments, "lastName") > <cfset arguments.fullName = Trim( arguments.firstName & " " & arguments.lastName )> </cfif> <cfreturn arguments> </cffunction> </cfcomponent>
In our example model, an artist's name can consist of both a first name and a last name ("John Mayer") or just the band / last name ("Abba."). The
setFullName() method handles the logic to concatenate the names.
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.
Note that callbacks set on included models are not executed. Look at this example:
fooBars = model("foo").findAll(include="bars");
That will cause callback to be executed on the
Foo model but not 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.