Unmounted component setState fix

[alexeuler]

This code in TransitionMotion was a source of bug for us in testing.

    if (this.unmounting) {
        return;
    }
    // TODO: when config is {a: 10} and dest is {a: 10} do we raf once and
    // call cb? No, otherwise accidental parent rerender causes cb trigger
    this.animationID = defaultRaf((timestamp) => {
      const propStyles = this.props.styles;
      ...
      this.setState({
        currentStyles: newCurrentStyles,
        currentVelocities: newCurrentVelocities,
        lastIdealStyles: newLastIdealStyles,
        lastIdealVelocities: newLastIdealVelocities,
        mergedPropsStyles: newMergedPropsStyles,
      });
      ...
    }

The problem is that if execution passes the conditional if (this.unmounting), then executes async defaultRaf and after that component unmounts and after that the callback of defaultRaf is called, then setState will be called on unmounted component.

This fix solves the problem.

[chenglou]

Thanks. Wouldn't the right fix be having the check in both locations? This way we bail early instead of allocating another callback.

[alexeuler]

@chenglou Done!