Jochen,
> On 8. 1. 2024, at 16:10, Jochen Theodorou <[email protected]> wrote:
> I would like to know from users on this list mostly if they are using
> specific features of the meta class system and MOP, but especially what for.
>
> (1) categories
> ...
> use (CategoryClass) {
> // some code and callstack here influenced by CategoryClass
> }
> I am especially interested in knowing if you try to add a method to code that
> is not part if the use-block here.
All the time, mostly to extend library classes, occasionally to split my own
class implementation to more source files. I never ever use the use-block.
My aim here is to have something as close to the Objective-C categories as
possible. Not entirely successful (the worst problem is that the Groovy
category-based extensions do not work properly through introspection, whilst
methods added by an ObjC category are real first-class citizens, not
distinguishable from the “normal” ones), but I am reasonably close and the
support is immensely useable to me.
For convenience, I did my own Extension ASTT closely based on the Alex
Tkachman's Category, but improved a bit, so that
- inside of my @Extension(String) class MyStringExtensions { ... }, I can put
both instance and static extension methods, and for the latter, my ASTT
automatically creates an appropriate static extension class;
- the extension class list is collected compile-time and (eventually when my
build script creates the application) automatically added to the
org.codehaus.groovy.runtime.ExtensionModule manifest, so that they just-work
without any extra ado.
> (2) per instance meta classes
> https://groovy-lang.org/metaprogramming.html#_per_instance_metaclass
> shows examples here
> Anyone using that?
Can't recall I ever needed that.
> (3) Custom meta class
> Anyway trying to force the usage of something else then MetaClassImpl
> for the meta class?
Well sort of, though a pretty trivial one. My goal is to get rid of NPEs; in my
personal opinion that darned thing is a proper disaster and the right behaviour
is a completely consistent null-propagation (essentially what Groovy calls a
“safe dispatch”). Among many other things, what I do is
===
def mc=new OCSNMC(org.codehaus.groovy.runtime.NullObject)
mc.initialize()
org.codehaus.groovy.runtime.NullObject.metaClass=mc
... ...
}
class OCSNMC extends DelegatingMetaClass {
OCSNMC(Class clazz){
super(clazz)
}
Object invokeMethod(Object object, String methodName, Object[] arguments) {
if (arguments.size()==1 && methodName=='is') return arguments[0]==null
if (arguments.size()==0 && methodName=='iterator') return [].iterator()
if (arguments.size()==0 && methodName=='hasZeroValue') return YES
null
}
// alas, does not seem to work for getProperty; that one must be supported
by ASTTs
}
===
Aside that, not sure if interesting to you, but I install my own methods into
metaclasses all the time, like e.g.,
===
NSMutableDictionary.metaClass.putAt<<{ key,value ->
if (value==nil) delegate.removeObjectForKey(key)
else delegate.setObjectForKey(value,key)
}
...
NSKeyValueCoding.NullValue.class.metaClass.asBoolean={ -> false }
===
and I even (of course having done ExpandoMetaClass.enableGlobally()) install
global handlers
===
// for some triple-weird reason simply defining propertyMissing in an
extension does NOT work (never gets called)
Object.metaClass.propertyMissing={name->
DPA.propertyMissing(delegate,name)
}
Object.metaClass.static.propertyMissing={name->
DPA.staticPropertyMissing(delegate,name)
}
===
DPA is my own rather non-trivial class, whose code dynamically fixes properties
of library classes, i.e., allows me to write SomeLibraryClassOrInstance.foo for
classes which have either static or instance method foo() and if so happens,
getFoo() { foo() } (essentially) is installed automatically to the metaclass
(the rationale here is that the WebObjects standard, which long long predates
the unlucky Java one, has setters setFoo(foo) and getters foo(), not getFoo()).
Thanks and all the best,
OC
