Include generic for world.createJoint

[alestiago]

Problem to solve

Avoid casting when doing something as:

PrismaticJoint prismaticJoint = world.createJoint(prismaticJointDef) as PrismaticJoint

Proposal

Use of generics to avoid casting:

PrismaticJoint prismaticJoint = world.createJoint<PrismaticJoint>(prismaticJointDef)

More information

Possible solutions

1. Updating function signature to T createJoint<T extends Joint>(JointDef def) and return to return joint as T; (Simplest Solution)
2. Refactoring joint.dart to perhaps benefit from OOP and simplify static method static Joint create(World world, JointDef def)
3. Open to further suggestions

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I'm interested in working on a PR for this.

[erickzanardo]

Sounds good to me! Feel free to put a PR for it 🚀

[spydon]

Sounds good to me too, there are a lot of these things that have been passed along through all the ports from C++. I assigned you to the issue.