How to notify dragdrop source event once in imgui loop ?

[ChivenZhang]

Just a Question. I want to add dragdrop source event into my message system. But i can't do this, as imgui would loop many times.
The code as below. Can anyone help?

if (ImGui::BeginDragDropSource(ImGuiDragDropFlags_None))
{
	auto data = (u8)CastRef(this);
	if (ImGui::SetDragDropPayload("imgui", &data, sizeof(data), ImGuiCond_None))
	{
		// ！！！ Here notify lots of times during one dragdrop action. 
		// What I want is to notify only once. Can I make it?
		onDragDroped(Event{ EventType::GUI_DRAG_UP, (u8)CastRef(this) });
	}
	ImGui::Text(m_IconText.getValue().c_str());
	ImGui::EndDragDropSource();
}

[PathogenDavid]

It's not clear from your description what the issue is.

SetDragDropPayload returns true when your payload is accepted, after which BeginDragDropSource should also stop returning true as the user is no longer dragging the associated widget.

Please create a minimal repro that we can copy+paste into one of the example apps.

[ChivenZhang]

It's not clear from your description what the issue is.

SetDragDropPayload returns true when your payload is accepted, after which BeginDragDropSource should also stop returning true as the user is no longer dragging the associated widget.

Please create a minimal repro that we can copy+paste into one of the example apps.

SetDragDropPayload returns true whenever I drag up the source widget, So It will notify event many times, if I don't drop the source widget. I want to let it notify event once during the time I drag up the widget.

The image below shows what problem I met when using event system : drag up many times and drop once.

[PathogenDavid]

Oh, sorry. I misremembered some of the details of how SetDragDropPayload works. (I forgot it also returned true for "Could be accepted if user stopped dragging".)

I don't think there's a super clean way to do this without poking at internals, the easiest solution would be to keep track of some extra state to identify if the payload was accepted during the previous frame when BeginDragDropSource stops returning true. For example:

static bool lastFrameWasAccepted = false; // You probably don't want to use a static local in your actual app, this is just an example.)
if (ImGui::BeginDragDropSource(ImGuiDragDropFlags_None))
{
	auto data = (u8)CastRef(this);
	lastFrameWasAccepted = ImGui::SetDragDropPayload("imgui", &data, sizeof(data), ImGuiCond_None);
	ImGui::Text(m_IconText.getValue().c_str());
	ImGui::EndDragDropSource();
}
else if (lastFrameWasAccepted)
{
	lastFrameWasAccepted = false;
	onDragDroped(Event{ EventType::GUI_DRAG_UP, (u8)CastRef(this) });
}

[ChivenZhang]

I'll try it later, thanks.

[ocornut]

It is unclear what you are trying to achieve with your GUI_DRAG_UP event. When it is supposed to be triggered, what is the purpose of it? What do you want to happen if you drag a payload over one destination, then move over another?

Two notes:

• You can check if GetDragDropPayload() == NULL to know if one has already been submitted.
• You can call SetDragDropPayload() with ImGuiCond_Once which will have a pretty similar effect as the above.

So I suspect in your case this may be enough:

if (ImGui::GetDragDropPayload() == NULL)
{
    ImGui::SetDragDropPayload("imgui", &data, sizeof(data), ImGuiCond_Once);
    onDragDroped(Event{ EventType::GUI_DRAG_UP, (u8)CastRef(this) }); 
}

Note of this checks if the payload is being "accepted" but if you were to be interested in that information, your GUI_DRAG_UP/GUI_DRAG_DOWN protocol is not going to be enough.

[PathogenDavid]

It is unclear what you are trying to achieve with your GUI_DRAG_UP event. When it is supposed to be triggered, what is the purpose of it? What do you want to happen if you drag a payload over one destination, then move over another?

My interpretation is they want their event to fire once when the payload is dropped. IE: The payload was accepted, a call somewhere to AcceptDragDropPayload returned said payload, and the user is no longer dragging.

I do agree though, it's not totally clear.

• You can call SetDragDropPayload() with ImGuiCond_Once which will have a pretty similar effect as the above.

Maybe I'm missing something, but cond here has to do with how Dear ImGui handles copying the payload and doesn't affect what SetDragDropPayload returns.

if (ImGui::GetDragDropPayload() == NULL)

My understanding is this condition can never be true within a BeginDragDropSource block as when BeginDragDropSource always sets ImGuiContext::DragDropActive to true before returning true, which means GetDragDropPayload won't return null.

[ocornut]

but cond here has to do with how Dear ImGui handles copying the payload and doesn't affect what SetDragDropPayload returns.

Correct.

My understanding is this condition can never be true within a BeginDragDropSource block as when BeginDragDropSource always sets ImGuiContext::DragDropActive to true before returning true, which means GetDragDropPayload won't return null.

Also correct, but I think this constitute a bug of GetDragDropPayload() then, it should be NULL before the first payload has been submitting. I will look into fixing this. Thanks!

[ocornut]

Pushed fix for GetDragDropPayload() not returning NULL 856c631 + test ocornut/imgui_test_engine@0446c87fe

[ChivenZhang]

Pushed fix for GetDragDropPayload() not returning NULL 856c631 + test ocornut/imgui_test_engine@0446c87fe

I can't thank you too much, thanks every one. It runs well. (one drag up, one drop down)

20221124034606.mp4