Learn how to implement the intercepting filters design pattern. Use in both aspect oriented and object oriented design.

1. Intercepting Filters Design
Pattern

2. Overview
Objective
Learn how to use the intercepting filter design pattern.
Requirements
Array
Extending an Object
Estimated Time
10 minutes
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3. Follow Along With Code
Example
1. Download a copy of the example code at
www.prodigyview.com/source.
2. Install the system in an environment you feel
comfortable testing in.
3. Proceed to examples/design/Filters.php

4. What are Filters?
Intercepting Filters is a design pattern that wraps execution
and intercepts the transmissions and/or request of
response. Intercepting filters can be stacked in a chain and
alter execution without changing the source code.
A simpler explanation would be modifying the data that is
passed into or returned from a method. This may also
possibly affect the execution of that method without directly
altering the source code.

5. Intercepting Filters Visual
MyObject::doSomething($mydata);
My Filter Object
Filter passes data to another object to filter
Filter is reached
Filter Data
Continue
Execution
Output

6. PVPatterns and
PVStaticPatterns
The classes that contain the functions for using filters is the
PVPatterns and PVStaticPatterns classes.
PVPatterns is for instances and PVStaticPatterns is for static
methods.
Both PVObject and PVStaticObject extend the pattern classes.

7. Vending Machine Example
We are going to start off with a vending machine example.
Our VendingMachine extends PVObject. PVObject
extends PVPatterns which has the filter methods we
need.
Extending PVObject
Compile the data to
filter
Make the method
filterable by adding
the filters

8. Adding A Filter
If you noticed in our filter class, we had an explicit section
where the filters are were defined.
Checks if there is a filter for this class and this method
Apply the filter for this class and this method Executes only if the event type has been set
Filters are suppose to be tied to a specific function in a specific class. We can simply
use get_class and __Function__ to get the current class and function. The method
_hasFitler first determines if they method has any filters. If true, lets execute them.
The function has two different filters because there are two different events being
performed, a check and a selection. The filter is being applied by passing in the class
name, the function name, the data to be filtered, and options. Keep in mind that the
filter can only filter one piece of data.

9. Item Check
Now for our vending we have items like beer which need to
be checked before buying. Lets add in a class that handles
that.
Options passed about the filter
Data passed from the filter
Return the data

10. Item Chooser
Soda sounds generic. What kind of soda? Next we are
going to add a class to handle what kind of item we are
picking.
Options passed
Data passed from about the filter
the filter
Return the data

11. Vend! Round 1
So we have our vending machine set up. Lets run the
function with some data and look at the results. Remember
the parameters was item, money and age.

12. Round 1 Results
Oh man, that’s not good. We are having minors getting
beer and anyone putting any price and getting what they
want. And all the items are generic! Let’s fix this.

13. Round 2- Add Some filters
So let’s add some filters in. The first two arguments is the
class and method to tie the filter too. The second two
arguments is the class and method that will do the filtering.
The last in the options in which you can tie the filter being
called to a particular event. Let’s give this a try.
Filter works only on
The class to attach the filter to The method to filter this event
The class that will filter the data The method in the class to filter the data

14. Round 2 Results
Ok a little better, at least we are getting some none generic
items back. But we still need to verify the age and amount
tendered.

15. Round 3 – Verification Filters
Now we are going to add the last set filters on, the same
way we added the previous filters.
Calls an instance

16. Round 3 Result
Finally, the results we need. So that is filters in a nutshell.
We modified the data in the method without ever modifying
the method.

17. Challenge!
Below is an optional challenge designed to help you gain a
better understanding of the design pattern.
1. Create 3 classes with 2 methods each. Make sure
each method accepts 2 parameters and has at least
one filter.
2. Attach a filter to each method that filters out by calling a
filter from another class.
3. Design the filters in a way THAT DOES NOT CAUSE
AN INFINITE LOOP.
4. Execute code.

18. Summary
1. Apply a filter using the _applyFilter method. The name
of the class and the name of the function calling the
filter should be used. The function only accepts 1
parameter for data to be filters.
2. Add a filter before execution using the _addFilter
method. Specify the class and method to be filter and
the class and method to run the filter through.

19. API Reference
For a better understanding of the Collections and the
Iterator, check out the api at the two links below.
PVStaticPatterns
PVPatterns
More Tutorials
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http://www.prodigyview.com/tutorials
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