I found a problem while using callback function,
My character has a collision callback function to detect is the fixture collided a valid platform to stand,
and this function will not be called while I generate a new fixture on my character.
While I generate a new fixture at the character's position,
it just make a collision so my character will be pushed out of the new fixture.
Is there any way to solve this, make callback works at the frame that creating a new fixture?