How to capture mouse wheel in custom widget?

[bear24rw]

Version/Branch of Dear ImGui:

Version: v1.72
Branch: master

My Issue/Question:

I am trying to implement a custom widget which captures the mouse scroll wheel when hovered. When hovered the window should not scroll (similar to when hovered over a listbox or textarea widget). Is this easily achievable?

Screenshots/Video
Video showing undesired behavior of window scrolling while mouse wheeling over the custom widget:

Standalone, minimal, complete and verifiable example: (see #2261)

struct MyWidget {
    float wheel_counter = 0.0f;

    void Draw(const char *label, ImVec2 frame_size = ImVec2(0,0)) {
        ImGuiWindow* window = ImGui::GetCurrentWindow();
        if (window->SkipItems)
            return;

        ImGuiContext& g = *GImGui;
        const ImGuiStyle& style = g.Style;
        const ImGuiID id = window->GetID(label);

        const ImVec2 label_size = ImGui::CalcTextSize(label, NULL, true);
        if (frame_size.x == 0.0f) frame_size.x = ImGui::CalcItemWidth();
        if (frame_size.y == 0.0f) frame_size.y = frame_size.x;

        const ImRect frame_bb(window->DC.CursorPos, window->DC.CursorPos + frame_size);
        const ImRect inner_bb(frame_bb.Min + style.FramePadding, frame_bb.Max - style.FramePadding);
        const ImRect total_bb(frame_bb.Min, frame_bb.Max + ImVec2(label_size.x > 0.0f ? style.ItemInnerSpacing.x + label_size.x : 0.0f, 0));

        ImGui::RenderFrame(frame_bb.Min, frame_bb.Max, ImGui::GetColorU32(ImGuiCol_FrameBg), true, style.FrameRounding);

        ImGui::InvisibleButton(label, frame_size);

        char buffer[256];
        snprintf(buffer, sizeof(buffer), "Counter: %.3f", wheel_counter);
        window->DrawList->PushClipRect(inner_bb.Min, inner_bb.Max, true);
        window->DrawList->AddText(inner_bb.GetCenter(), ImGui::GetColorU32(ImGuiCol_Text), buffer);
        window->DrawList->PopClipRect();

        if (ImGui::IsItemHovered()) {
            wheel_counter += ImGui::GetIO().MouseWheel;
        }
    }
};

        ImGui::Begin("Issue");
        static MyWidget w;
        w.Draw("FooBar");
        ImGui::End();

[ocornut]

It's not easily achievable at the moment, other than using the window flags to make the parent window itself not react to mouse wheel.

[bear24rw]

Okay, thanks for the quick response. I came up with two workarounds:

1. Require the shift key to be held (arguably better anyway since it wont interrupt you scrolling down a window)
2. Use a child region and set its scrollmax to non-zero:

struct MyWidget {
    float wheel_counter = 0.0f;

    void Draw(const char *label, ImVec2 frame_size = ImVec2(0,0)) {
        ImGuiWindow* window = ImGui::GetCurrentWindow();
        if (window->SkipItems)
            return;

        const ImVec2 label_size = ImGui::CalcTextSize(label, NULL, true);
        if (frame_size.x == 0.0f)
            frame_size.x = ImGui::CalcItemWidth();
        if (frame_size.y == 0.0f)
            frame_size.y = frame_size.x;

        ImGui::BeginChild(label, frame_size);
        window = ImGui::GetCurrentWindow();
        window->ScrollMax.y = 1.0f;

        ImGuiContext& g = *GImGui;
        const ImGuiStyle& style = g.Style;
        const ImGuiID id = window->GetID(label);

        const ImRect frame_bb(window->DC.CursorPos, window->DC.CursorPos + frame_size);
        const ImRect inner_bb(frame_bb.Min + style.FramePadding, frame_bb.Max - style.FramePadding);
        const ImRect total_bb(frame_bb.Min, frame_bb.Max + ImVec2(label_size.x > 0.0f ? style.ItemInnerSpacing.x + label_size.x : 0.0f, 0));

        ImGui::RenderFrame(frame_bb.Min, frame_bb.Max, ImGui::GetColorU32(ImGuiCol_FrameBg), true, style.FrameRounding);

        ImGui::InvisibleButton(label, frame_size);

        char buffer[256];
        snprintf(buffer, sizeof(buffer), "Counter: %.3f", wheel_counter);
        window->DrawList->PushClipRect(inner_bb.Min, inner_bb.Max, true);
        window->DrawList->AddText(inner_bb.GetCenter(), ImGui::GetColorU32(ImGuiCol_Text), buffer);
        window->DrawList->PopClipRect();

        if (ImGui::IsItemHovered()) {
            wheel_counter += ImGui::GetIO().MouseWheel;
        }

        ImGui::EndChild();
    }
};

[ocornut]

Amend to this thread: in Dec 2020 we added ImGui::SetItemUsingMouseWheel() to claim the mouse wheel.

See #2891 for old discussion, and #4207 #3281 for some uses.

[ocornut]

Update: I have now pushed a more generic system for input ownership, and as I result obsoleted SetItemUsingMouseWheel(). There is still a redirection function but I expect to delete it sooner than the typical 2-3 years obsolescence schedule.

You can now use:

SetItemKeyOwner(ImGuiKey_MouseWheelY);

Note that SetItemUsingMouseWheel() used to lock both wheel axises.
If you need both you can call the function with ImGuiKey_MouseWheelX and ImGuiKey_MouseWheelY.

Also note that using SetKeyOwner() and SetItemKeyOwner() flags allow for variety of other behaviors outside the scope of this thread.

(cross posted to #2704, #2891, #5108)