Index

What is Aspect Oriented Programming?
AOP examples
Xbase++ Aspect Oriented API
    XppAspectEntry class

What is Aspect Oriented Programming (AOP)?

In few words, AOP is new paradigm to extend functionality of application, by allowing to run some advice code around target code.

In our implementation, advice code can be codeblock, function or special object (based on XppAspectEntry class). Target (adviced) code can be any function (procedure) or any method of any class. Wrapping over protected or hidden methods also possible without any restrictions.

Advice code have full control over target code, it can change any parameter for target code and replace return value by own value. Advice code can be called before, after and around of adviced code.

For more information about AOP concepts, see wikipedia article here.

AOP examples

Most popular example of AOP is logging.

The following example show, how to log parameters and result of method checkPayment of class Payment:

//LOG PARAMETERS EXAMPLE USING ADVICE FUNCTION 

//make advice for single object
aspects():wrapMethod(oPayment,"checkPayment","logPayment")

...


Func logPayment(oAspect,lBefore)

  If lBefore
   ?"object=",oAspect:getObject()
   ?"  paymentPars=",oAspect:getParamA()
  Else
   ?"paymentReturn=",oAspect:getReturn()
  End

Return NIL

Another good example of using AOP techniques is processing and changing parameters of target code.

Code below set up advice for XbpStatic:setCaption method. The custom advice object MyCaptions change captions to uppercase on the fly.

//TRANSLATE EXAMPLE USING CUSTOM ADVICE CLASS 

//create custom advice object
oAdvice:=MyCaptions():new()

//to trap calls from all objects 
//delivered from XbpStatic we set lFilter to .F.
aspects():wrapMethod(XbpStatic(),"setCaption",oAdvice,.F.)

...


//custom class,
//only implementing beforeCall method
Class MyCaptions From XppAspectEntry
Exported:

  //Uppercase all captions
  Inline Method beforeCall()

    If ::pCount()>0
        //change caption to uppercase
        ::setParam(1,Upper(::getParam(1)))
    End

  Return NIL

EndClass

Xbase++ Aspect Oriented API

aspects():wrapMethod(object|objectsList, methodName, functionName|codeblock|oAdvice, lFilter) -> oRetAdvice

Weave advice code over an method.

Parameter	Description
1. object|objectsList	single object or array of objects that will be adviced. If third parameter is oAdvice, object can be NIL.
2. methodName	name of adviced method
3. functionName codeblock/codeblockA oAdvice	name of advice function. This function receive two parameters, oAdvice and lBefore. oAdvice is object of XppAspectEntry type and if function called before target method, lBefore is .T., otherwise is .F..       NOTE: Return of advice function must be NIL. advice block, called before adviced method or array of two blocks, first for before action (can be NIL) and second for after action (also can be NIL). This block(s) receive two parameters, oAdvice and lBefore.       NOTE: Return of advice block must be NIL. This object must be delivered from XppAspectEntry class. Method ::beforeCall() called before run of adviced method and ::afterCall() immediately after. For more info, see XppAspectEntry topic below.
4. lFilter	If you need to contoll all objects of the same classes as object|objectsList, pass .F.. In such cases, use oAdvice:getObject() call to determine actual adviced object. By default (.T.), advice code only called for objects specified by object|objectsList.

Return value	Description
oRetAdvice	XppAspectEntry object for managing advice

 

aspects():wrapFunction(targetFunction, functionName|codeblock|oAdvice, lFilter) -> oRetAdvice

Parameter	Description
1. targetFunction	name of adviced function.
other parameters the same as in wrapMethod

Return value	Description
oRetAdvice	XppAspectEntry object for managing advice

 

XppAspectEntry class

This class used by Aspect API for advicing method/function

Advice methods automaticaly called by Aspect API

::beforeCall()

This method called before adviced code

::afterCall()

This method called after adviced code

::checkValid(object)

This method called by Aspect API to determine is object valid for this XppAspectEntry set of adviced objects.
You can implement you own logic for filtering.

NOTE: if you set lFilter parameter for aspects():wrapMethod(..) to .F., objects not filtered and ::checkValid() method will be never called.

Methods for maganing list of adviced objects

::addObjects(object|objectsList)

Add object|objectsList to set of adviced objects by this XppAspectEntry object. Only this objects will be adviced, and ::before/afterCall() methods will be called.

::clearObjects()

Disable any filtering without stopping advicing. All objects of the same classes as specified in aspects():wrapMethod(..) will be adviced. You can use ::getObject() call to determine actual adviced object.

Parameters and return methods

::PCount() -> numParams

Returns number of parameters passed to adviced code.

::getParamA() -> aParams

Returns all parameters as array.

::getParam(nParam) -> paramValue

Returns specified parameter of adviced code.

::setParam(nParam,value)

Change parameter to new value.

::getReturn() -> returnValue

Returns return value of adviced code.

::setReturn(returnValue)

Change return value of adviced code.

::getObject()

Returns currently adviced object. Note: this call is not valid for XppAspectEntry objects returned by aspects():wrapFunction(..) call.

Methods related to adviced code

::disableBody()

This call disables call to adviced code. Disabling call to body, do not disable aspect itself, ::beforeCall() and ::afterCall() will be called anyway.

::enableBody()

Enable call to adviced code.

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