在3D空间中具有2D对象的Unity IDropHandler

Question

I have implemented a Unity component which implements IDragHandler, and another that implements IDropHandler. The IDragHandler component is placed on an object that I want to drag, but the IDropHandler component is attached to objects that I can drop objects on , not objects that I can drop .

I'm working in 3D with a perspective camera.

My code below works fine for dropping 3D objects on 3D objects, and 3D objects on 2D UI objects, but not for dropping 2D UI objects on 3D objects or 2D UI objects on 2D UI Objects.

The problem I've had is that the object I'm dragging around blocks the OnDrop and takes it for itself due to EventSystem's Raycasting (even if I don't implement IDropHandler on that object).

For 3D objects I temporarily disable the Collider while dragging in my IDragHandler component, to make sure that the Raycast which decides which object to give OnDrop passes by the dragged object and on to the object behind it. However, 2D UI objects don't work with colliders for dragging and dropping, so there's no collider available to disable. I don't know how to make sure that the EventSystem Raycast bypasses the dragged object to pass on OnDrop to the object behind it.

    public void OnBeginDrag(PointerEventData eventData)
    {
        ...

        if (UIObject)
        {
            // What to do here?
        }
        else
        {
            var collider = gameObject.GetComponent<Collider>();
            collider.enabled = false;
        }

    }

    public void OnDrag(PointerEventData eventData)
    {
        // Move object and stuff
        ...
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        if (UIObject)
        {
            // What to do here?
        }
        else
        {
            var collider = gameObject.GetComponent<Collider>();
            collider.enabled = true;
        }
    }

The things I have tried so far is to:

1) Temporarily place the gameObject in the IgnoreRaycast layer

2) Add a canvas group to the object and temporarily turn off "Block Raycast" on it.

Nothing I do in OnBeginDrag seems to have an effect until I have actually dropped the object for some reason. Try #2 mentioned above resulted in it not turning off "Block Raycast" until after the object was dropped, which caused the object to not even be draggable anymore.

Answer 1

I did a workaround where the dragged object in OnEndDrag raycasts and find if it was dropped on something, and if it was, calls OnDrop on that object. The disabling and enabling of colliders on the 3D object is obsolete now, so it was removed.

    public void OnBeginDrag(PointerEventData eventData)
    {
        ...
    }

    public void OnDrag(PointerEventData eventData)
    {
        // Move object and stuff
        ...
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        // Currently using transform.position as raycast origin, could easily be something else if desired.
        var rayCastOrigin = transform.position;

        // Save the current layer the dropped object is in,
        // and then temporarily place the object in the IgnoreRaycast layer to avoid hitting self with Raycast.
        int oldLayer = gameObject.layer;
        gameObject.layer = 2;

        var raycastOriginInScreenSpace = Camera.main.WorldToScreenPoint(rayCastOrigin);
        var screenRay = Camera.main.ScreenPointToRay(new Vector3(raycastOriginInScreenSpace.x, raycastOriginInScreenSpace.y, 0.0f));

        // Perform Physics.Raycast from transform and see if any 3D object was under transform.position on drop.
        RaycastHit hit3D;
        if (Physics.Raycast(screenRay, out hit3D))
        {
            var dropComponent = hit3D.transform.gameObject.GetComponent<IDropHandler>();
            if (dropComponent != null)
                dropComponent.OnDrop(eventData);
        }

        // Perform Physics2D.GetRayIntersection from transform and see if any 2D object was under transform.position on drop.
        RaycastHit2D hit2D = Physics2D.GetRayIntersection(screenRay);
        if (hit2D)
        {
            var dropComponent = hit2D.transform.gameObject.GetComponent<IDropHandler>();
            if (dropComponent != null)
                dropComponent.OnDrop(eventData);
        }

        // Reset the object's layer to the layer it was in before the drop.
        gameObject.layer = oldLayer;
    }

The behaviour I wanted is that when the transform.position of an object is dropped on something, it is counted as dropped. The default behaviour, that when the mouse pointer is inside dropped object it triggers onDrop, could easily be achieved by exchanging

var raycastOriginInScreenSpace = Camera.main.WorldToScreenPoint(rayCastOrigin);
var screenRay = Camera.main.ScreenPointToRay(new Vector3(raycastOriginInScreenSpace.x, raycastOriginInScreenSpace.y, 0.0f));

with

var screenRay = Camera.main.ScreenPointToRay(new Vector3(eventData.position.x, eventData.position.y, 0.0f));