The Aspect4Delphi consists of an library that enables the use of the concept of aspect-oriented programming (AOP) in Delphi.
AOP is a programming paradigm that aims to increase modularity by allowing the separation of cross-cutting concerns. It does so by adding additional behavior to existing code without modification of the code itself. Instead, we can declare this new code and these new behaviors separately.
Aspect4Delphi helps us implement these cross-cutting concerns.
- Before advice: advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).
- After advice: advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.
- Exception advice: Advice to be executed if a method exits by throwing an exception.
- Delphi Community Edition - The IDE used
To get a local copy up and running follow these simple steps.
To use this library an updated version of Delphi IDE (XE or higher) is required.
Clone the repo
git clone https://github.com/ezequieljuliano/Aspect4Delphi.git
Add the "Search Path" of your IDE or your project the following directories:
Aspect4Delphi\src
To provide the aspect orientation paradigm in your project with Aspect4Delphi you need:
- Implement a pointcut attribute.
- Implement a class that represents your aspect.
- Register your pointcut attribute and its aspect class in context.
- Set your classes methods to virtual.
- Write down your methods with the respective aspect.
To illustrate usage let's look at a solution for managing logs of an application.
Implement a pointcut attribute (preferably inherits from AspectAttribute):
interface
uses
Aspect.Core;
type
LoggingAttribute = class(AspectAttribute)
private
{ private declarations }
protected
{ protected declarations }
public
{ public declarations }
end;
implementation
end.
Implement the class that represents your aspect and contains the advices methods (you must implement the IAspect interface):
interface
uses
System.SysUtils,
System.Rtti,
Aspect,
Aspect.Core,
Logging.Attribute,
App.Context;
type
ELoggingAspectException = class(Exception)
private
{ private declarations }
protected
{ protected declarations }
public
{ public declarations }
end;
TLoggingAspect = class(TAspectObject, IAspect)
private
{ private declarations }
protected
procedure OnBefore(
instance: TObject;
method: TRttiMethod;
const args: TArray<TValue>;
out invoke: Boolean;
out result: TValue
); override;
procedure OnAfter(
instance: TObject;
method: TRttiMethod;
const args: TArray<TValue>;
var result: TValue
); override;
procedure OnException(
instance: TObject;
method: TRttiMethod;
const args: TArray<TValue>;
out raiseException: Boolean;
theException: Exception;
out result: TValue
); override;
public
{ public declarations }
end;
implementation
{ TLoggingAspect }
procedure TLoggingAspect.OnAfter(instance: TObject; method: TRttiMethod;
const args: TArray<TValue>; var result: TValue);
var
attribute: TCustomAttribute;
begin
inherited;
for attribute in method.GetAttributes do
if attribute is LoggingAttribute then
begin
LoggingFile.Add('After the execution of ' +
instance.QualifiedClassName + ' - ' +
method.Name
);
Break;
end;
end;
procedure TLoggingAspect.OnBefore(instance: TObject; method: TRttiMethod;
const args: TArray<TValue>; out invoke: Boolean; out result: TValue);
var
attribute: TCustomAttribute;
begin
inherited;
for attribute in method.GetAttributes do
if attribute is LoggingAttribute then
begin
LoggingFile.Add('Before the execution of ' +
instance.QualifiedClassName + ' - ' +
method.Name
);
Break;
end;
end;
procedure TLoggingAspect.OnException(instance: TObject; method: TRttiMethod;
const args: TArray<TValue>; out raiseException: Boolean;
theException: Exception; out result: TValue);
var
attribute: TCustomAttribute;
begin
inherited;
for attribute in method.GetAttributes do
if attribute is LoggingAttribute then
begin
LoggingFile.Add('Exception in executing ' +
instance.QualifiedClassName + ' - ' +
method.Name + ' - ' +
theException.Message
);
Break;
end;
end;
end.
Register your pointcut attribute and its aspect class in context (preferably keep this context in singleton instance):
interface
uses
Logging.Aspect;
Aspect.Context;
function AspectContext: IAspectContext;
implementation
var
AspectContextInstance: IAspectContext = nil;
function AspectContext: IAspectContext;
begin
if (AspectContextInstance = nil) then
begin
AspectContextInstance := TAspectContext.Create;
AspectContextInstance.RegisterAspect(TLoggingAspect.Create);
end;
Result := AspectContextInstance;
end;
end.
In your business rule class use the attribute to define your joins points. You can use the constructor and destructor to add your class at the Weaver.
interface
uses
System.SysUtils,
Logging.Attribute,
App.Context;
type
EWhatsAppMessageException = class(Exception)
private
{ private declarations }
protected
{ protected declarations }
public
{ public declarations }
end;
TWhatsAppMessage = class
private
{ private declarations }
protected
{ protected declarations }
public
constructor Create;
destructor Destroy; override;
[Logging]
procedure Send; virtual;
end;
implementation
{ TWhatsAppMessage }
constructor TWhatsAppMessage.Create;
begin
inherited Create;
AspectContext.Weaver.Proxify(Self);
end;
destructor TWhatsAppMessage.Destroy;
begin
AspectContext.Weaver.Unproxify(Self);
inherited Destroy;
end;
procedure TWhatsAppMessage.Send;
begin
//Execution of send.
end;
See the open issues for a list of proposed features (and known issues).
Contributions are what make the open source community such an amazing place to be learn, inspire, and create. Any contributions you make are greatly appreciated.
- Fork the Project
- Create your Feature Branch (
git checkout -b feature/AmazingFeature) - Commit your Changes (
git commit -m 'Add some AmazingFeature') - Push to the Branch (
git push origin feature/AmazingFeature) - Open a Pull Request
Distributed under the APACHE LICENSE. See LICENSE for more information.
To contact us use the options:
- E-mail : ezequieljuliano@gmail.com
- Twitter : @ezequieljuliano
- Linkedin: ezequiel-juliano-müller