[FEATURE] extend any class with `@Graft.*` API

Extend existing classes by grafting bytecode from Transplant classes. Transplants are plain java classes; annotations define interactions with the target class.

A custom classloader is responsible for applying the transformations at load-time. The classloader is setup either using a javaagent or prior to referencing any targeted classes.

Sprung from #570, refer to it for the discussion leading up to this.

Use-cases

• Functionality for editors: callbacks and tighter tooling integration
• Additional debugging capabilities: logging, record stack traces, additional callbacks
• Extending existing functionality for final or otherwise non-extendable classes
• The odd bug fix in imported dependencies

Example Transplant for ComponentMapper

@Graft.Target(ComponentMapper.class)
public class ComponentMapperTransplant<A> {

    @Graft.Mock // only needed for compilation
    private World world;

    @Graft.Fuse // inject into original method; 
    public A create(int entityId) {
        LifecyclePlugin.dispatcher.onComponentPreCreate(world, entityId);
        A result = create(entityId); // create(entity) refers to original method
        LifecyclePlugin.dispatcher.onComponentPostCreate(world, entityId);
        return result;
    }
}

API

• @Graft.Target specifies which class to transplant to.
• @Graft.Fuse transplants bytecode over to @Graft.Target, translating any references to ComponentMapperTransplant -> ComponentMapper.
• @Graft.Mock to keep the compiler happy when you need to reference fields or methods in the target class. Mocked references point to target class after transplant.
• Any non-annotated methods or fields are copied over as-is.

Nice to have:

• @Graft.Replace: Like @Graft.Fuse but removes the original method. Better to resolve it when applying @Graft.Fuse.
• @Graft.Remove: Remove field or method from target class.

Caveats

• You’re working against internal implementation; there are no semver guarantees
• No retrofitting of interfaces or parent type on target class
• Annotations aren’t fused (is there a use-case?)
• No @Graft.Fuse for constructors; nice to have, but not initially
• No GWT support
• No android support (possible but not planned)

[junkdog]

PSA: a lot of stuff at work and outside work for the remainder of the week; prob won’t have time to do any work on it before the weekend

[junkdog]

Closing as junkdog/graftt is its own project