isSensor *after* b2CreateXYZShape?

[Daigoro96]

Is there a way to set the sensor after constructing the shape or is it only possible to create a sensor by providing ShapeDefinition.isSensor = true?

[HughPH]

There's no way to set the sensor flag after constructing the shape. It would be tricky to implement because the shape keeps a "sensorIndex" which points to an element in an array of sensors. The way removing from an array works is that the removed index is replaced with the last index, and the size is reduced by 1. So if you set isSensor = false then you wouldn't be able to remove it from the array without messing up the sensorIndex of other sensors.

[erincatto]

Events get weird if something stops being a sensor. Consider the end touch event:

struct b2SensorEndTouchEvent
{
  b2ShapeId sensorShapeId;
  b2ShapeId visitorShapeId;
};

I have two choices that both seem potentially buggy. Either I drop the event, which may confuse some users or I return the event but now the sensorShapeId is not a sensor.

I think I can manage the data structure issues, the main problem is the non-intuitive results.

[Daigoro96]

Thank you for the reply.
I only needed to enable the sensor (so the sensor would never be disabled afterwards) and will never do so in response to an event, so i decided to just expand it a bit.
However i will think about alternatives today and most likely find a better solution that modifying box2d slightly.