Categories in Objective-PHP
About Categories
Categories allow the programmer to add methods to classes at runtime. Once a method is added to a class via a Category it is available to all instances of objects of that class from then on. Another important factor of Categories is that they allow the programmer to add methods to classes which are defined in external Frameworks or libraries.
Both class and instance methods can be added to objects, and new instance variables cannot be defined. This is a restriction of Objective-C (as once a class has been compiled its ivars become a C struct of which one is created per class instance and hence adding new ivars at runtime would mean having to change the size of this struct, and this isn’t trivial. Since this restriction does not exist in PHP it would be possible to add ivars via Categories but this features is not currently planned).
Categories override methods with the same Selector that are already defined in a class.
In ObjPHP categories are implemented in a way that means as the parser comes across a definition of one, it creates the necessary runtime calls in the output source to thus add the new methods to the specified objects at runtime.
Implementation Detail
- An anonymous function is created with the method definition given.
The method handle is added to the Class object into the Dispatch Table using
addMethodWithMethodName()defined in_runtimeclass, see Runtime for more).Now when
objphp_msgSendis called to send a message (and asks the receiver if it has the method that implements the given message), the dispatch table is checked and the message is either called from the pointer there or from the actual class methods (the ones that were known at compile time). Since categories can only be added after the class has been defined categories implicity override methods defined in the class.
Source Code Links
- POINT TO SOURCE THAT IS RELAVENT
- POINT TO SOURCE THAT IS RELAVENT
More Information
- LINK TO OBJC doc
Document status: COMPLETE for current version.