8.19.6. Custom widgets

ImGui core ships no rotary control; community ports vendor their own. This tutorial adds a rotary volume knob as a user-defined [widget] kind — the same annotation every built-in (button, slider_float, checkbox, …) uses. The knob plugs into pending_value_finalize unchanged: matching the slider state-struct convention is the only contract.

Source: modules/dasImgui/examples/tutorial/custom_widgets.das.

8.19.6.1. Walkthrough

The recording drives the custom knob with real synthetic input and self-verifies. It drags the Master knob along its 270° arc (asserting value rose off 0), drags Treble up a little (same assertion), then drives Bass via imgui_force_set — the one place force_set is the subject, so it counts as the real input — and asserts the value took. Finally it clicks the ordinary Reset button and asserts Bass snapped from −0.5 back to its default 0, proving the live API and a plain built-in both operate on the custom widget unchanged. (The arc drag rides a radius-25 circle around the knob disc’s centre, which sits in the top third of the hitbox — bbox.x + 36, bbox.y + 36 — clear of the bottom dead zone.)

options gen2
options _comment_hygiene = true
// TODO: this tutorial uses pre-v2 raw imgui calls (Text/Spacing/Separator/
// SameLine etc.) and needs a follow-up rewrite to use the v2 wrappers
// (text/spacing/separator/same_line). Opt out of the default-on lint until
// that pass lands so the file stays compileable.
options _allow_imgui_legacy = true
options gc

require imgui
require imgui_app
require opengl/opengl_boost
require live/glfw_live
require live/live_api
require live/live_commands
require live/live_vars
require live_host
require imgui/imgui_live
require imgui/imgui_boost_runtime
require imgui/imgui_boost_v2
require imgui/imgui_widgets_builtin
require imgui/imgui_containers_builtin
require imgui/imgui_visual_aids
require imgui/imgui_colors

require math
require strings

// =============================================================================
// TUTORIAL: custom_widgets — write your own widget kind with [widget].
//
// ImGui core ships no rotary control; here we add one. The knob:
//   - is a single [widget] def at module scope, ~30 lines including drawlist
//   - plugs into pending_value_finalize + one [widget_dispatch] — same
//     state-struct convention as slider_float, so imgui_snapshot reflects it
//     and imgui_force_set drives it, exactly like a built-in
//   - uses the canonical InvisibleButton + DrawList pattern for the body
//
// Read alongside :ref:`tutorial_widgets_tour` for the built-in catalog.
//
// STANDALONE: daslang.exe modules/dasImgui/examples/tutorial/custom_widgets.das
// LIVE:       daslang-live modules/dasImgui/examples/tutorial/custom_widgets.das
//
// DRIVE (when running live):
//   curl -X POST -d '{"name":"imgui_force_set","args":{"target":"MIXER_WIN/MASTER","value":0.75}}'   localhost:9090/command
//   curl -X POST -d '{"name":"imgui_force_set","args":{"target":"MIXER_WIN/TREBLE","value":-0.3}}'   localhost:9090/command
//   curl -X POST -d '{"name":"imgui_click","args":{"target":"MIXER_WIN/RESET_BTN"}}'           localhost:9090/command
// =============================================================================

//! State struct for the knob — same five-field convention as ``SliderStateFloat``
//! so :ref:`pending_value_finalize <function-imgui_boost_runtime_pending_value_finalize>`
//! can drive it without modification.
struct VolumeKnobState {
    @live value : float                   //! current knob value, preserved across reload
    @live bounds : tuple<float; float>    //! (min, max) — set per-frame at the call site
    @optional has_pending : bool          //! imgui_force_set queued; consumed next frame
    @optional pending_value : float       //! next-frame value queued by imgui_force_set
    @optional changed : bool              //! true on the frame the widget fired
}

// imgui_force_set's "set" action queues pending_value (drained in knob() step 1) —
// the dispatch half of the custom-widget rail. payload is JSON (write_json), parsed
// with sscan_json_at like the built-in value dispatchers (imgui_widgets_builtin.das).
[widget_dispatch]
def knob_dispatch(var state : VolumeKnobState; action : string; payload : string) {
    if (action == "set") {
        unsafe {
            sscan_json_at(payload, addr(state.pending_value),
                          *reinterpret<TypeInfo const?>(typeinfo rtti_typeinfo(type<float>)))
        }
        state.has_pending = true
    }
}

//! Rotary knob — vertical-drag value control with a 270° indicator arc.
//! ``widget_ident`` is injected at position 1 by the ``[widget]`` annotation;
//! pass it to ``pending_value_finalize`` at the bottom.
[widget]
def knob(var state : VolumeKnobState; text : string) : bool {
    // 1. Drain imgui_force_set: any pending dispatcher-side update lands here.
    if (state.has_pending) {
        state.value = state.pending_value
        state.has_pending = false
    }
    let (mn, mx) = state.bounds

    // 2. Reserve a hitbox. InvisibleButton is the "registered" item [widget]'s
    //    postlude attaches bbox/hex_id/hover/active/focus to. Hitbox spans disc +
    //    label + value — one fixed-width cell, so adjacent knobs stay aligned.
    let p = GetCursorScreenPos()
    let sz = ImVec2(72.0f, 112.0f)             // 72×72 knob + 24 label + 16 value
    let radius = 30.0f
    let center = ImVec2(p.x + sz.x * 0.5f, p.y + 36.0f)
    InvisibleButton(text, sz)

    // 3. Drag handling — mouse-vs-center drives the angle. atan2 returns (-π, π];
    //    shift by -3π/4 and wrap into [0, 2π) so the active arc is [0, 3π/2) and
    //    the bottom-gap dead zone [3π/2, 2π); dead-zone clicks hold the old value.
    var changed = false
    if (IsItemActive()) {
        let mp = GetIO().MousePos
        var th = atan2(mp.y - center.y, mp.x - center.x) - 3.0f * PI / 4.0f
        if (th < 0.0f) {
            th += 2.0f * PI
        }
        if (th < 3.0f * PI / 2.0f) {
            let f = th / (3.0f * PI / 2.0f)
            let new_val = clamp(mn + f * (mx - mn), mn, mx)
            if (new_val != state.value) {
                state.value = new_val
                changed = true
            }
        }
    }
    state.changed = changed

    // 4. Draw via the window's drawlist. Indicator sweeps a 270° arc with a 90°
    //    bottom gap (DAW convention): frac 0→3π/4, 0.5→3π/2, 1→9π/4. ImGui's y
    //    points down, so the formula is monotonic in θ and inverts step 3's map.
    let frac = (state.value - mn) / (mx - mn)
    let theta = 3.0f * PI / 4.0f + frac * 3.0f * PI / 2.0f
    let tip = ImVec2(
        center.x + cos(theta) * radius * 0.82f,
        center.y + sin(theta) * radius * 0.82f
    )
    let hovered = IsItemHovered()
    let rim_col = hovered ? rgba(190u, 200u, 220u, 255u) : rgba(120u, 130u, 150u, 255u)
    *GetWindowDrawList() |> AddCircleFilled(center, radius, rgba(40u, 42u, 48u, 255u), 32)
    *GetWindowDrawList() |> AddCircle(center, radius, rim_col, 32, 2.0f)
    *GetWindowDrawList() |> AddLine(center, tip, rgba(220u, 200u, 60u, 255u), 3.0f)

    // 5. Label + value readout, both drawlist (no ImGui layout impact).
    let label_size = CalcTextSize(text, false, -1.0f)
    let label_pos = ImVec2(center.x - label_size.x * 0.5f, p.y + 76.0f)
    *GetWindowDrawList() |> AddText(label_pos, rgba(220u, 220u, 220u, 255u), text)
    let val_str = build_string() <| $(var w) {
        fmt(w, ":.2f", state.value)
    }
    let val_size = CalcTextSize(val_str, false, -1.0f)
    let val_pos = ImVec2(center.x - val_size.x * 0.5f, p.y + 94.0f)
    *GetWindowDrawList() |> AddText(val_pos, rgba(170u, 170u, 180u, 255u), val_str)

    // 6. Register state + serializer (snapshot, and force_set draining). Same call
    //    any slider makes; the [widget_dispatch] above routes the live "set" in.
    pending_value_finalize(widget_ident, "knob", state)
    return changed
}

[export]
def init() {
    live_create_window("dasImgui custom_widgets", 800, 520)
    live_imgui_init(live_window)
    let io & = unsafe(GetIO())
    GetStyle().FontScaleMain = 1.5
}

[export]
def update() {
    if (!live_begin_frame()) return
    begin_frame()

    ImGui_ImplGlfw_NewFrame()
    apply_synth_io_override()
    NewFrame()

    SetNextWindowPos(ImVec2(60.0f, 60.0f), ImGuiCond.Always)
    SetNextWindowSize(ImVec2(680.0f, 380.0f), ImGuiCond.Always)
    window(MIXER_WIN, (text = "Mastering", closable = false,
                       flags = ImGuiWindowFlags.None)) {
        Text("Three knobs from one [widget] def - different state globals.")
        Spacing()

        // Each knob is one ImGui-layout cell (fixed 72×112 hitbox), so
        // SameLine spacing is constant regardless of value-text width.
        MASTER.bounds = (0.0f, 1.0f)
        knob(MASTER, (text = "Master"))
        SameLine()
        TREBLE.bounds = (-1.0f, 1.0f)
        knob(TREBLE, (text = "Treble"))
        SameLine()
        BASS.bounds = (-1.0f, 1.0f)
        knob(BASS, (text = "Bass"))

        Spacing()
        Separator()
        Spacing()

        // Ordinary built-in widget on the same panel — imgui_click works on it.
        if (button(RESET_BTN, (text = "Reset"))) {
            MASTER.value = 0.5f
            TREBLE.value = 0.0f
            BASS.value = 0.0f
        }
    }

    end_of_frame()
    Render()
    var w, h : int
    live_get_framebuffer_size(w, h)
    glViewport(0, 0, w, h)
    glClearColor(0.10f, 0.10f, 0.12f, 1.0f)
    glClear(GL_COLOR_BUFFER_BIT)
    live_imgui_render()

    live_end_frame()
}

[export]
def shutdown() {
    live_imgui_shutdown()
    live_destroy_window()
}

[export]
def main() {
    init()
    while (!exit_requested()) {
        update()
        maybe_collect_gc()
    }
    shutdown()
}

8.19.6.1.1. Requires

Same boost stack as Widgets tour, with two additions: imgui/imgui_boost for IM_COL32 (the int/uint color helpers from the legacy boost layer), and math for cos / sin / PI used by the indicator angle.

8.19.6.1.2. The state struct

VolumeKnobState mirrors SliderStateFloat field-for-field:

struct VolumeKnobState {
    @live value : float
    @live bounds : tuple<float; float>
    @optional has_pending : bool
    @optional pending_value : float
    @optional changed : bool
}

This shape is the contract. pending_value_finalize is generic on the state type — it reads has_pending / pending_value to consume queued imgui_force_set deliveries, and serializes the whole struct (value, bounds, changed) verbatim into the snapshot. Any widget kind that matches these field names plugs straight into the rails. @live keeps value and bounds preserved across reloads; @optional lets the dispatcher-managed fields stay zero-defaulted in older saved states.

8.19.6.1.3. The `[widget]` annotation

The annotation does two things to the function it decorates (widgets/imgui_boost_v2.das:32):

Injects a ``widget_ident : string`` parameter at position 1, between state and the user-facing args. Inside the body, widget_ident is the bare identifier string ("MASTER" at the call site knob(MASTER, ...)) — pass it to pending_value_finalize so the finalizer can build the registry path.
Registers a per-kind ``WidgetCallMacro`` that intercepts knob(IDENT, ...) calls. The macro auto-emits the named global (MASTER) on first use, parses dotted-suffix flags (.PUBLIC / .NOTLIVE), and rewrites the call to thread widget_ident through.

The body also gets widget_prelude(widget_ident) injected at the top — that pushes the ImGui ID stack and applies any pending focus from imgui_focus. The user never calls it directly.

8.19.6.1.4. The drawlist pattern

Custom widgets follow the InvisibleButton + DrawList pattern from the boost design (API_REWORK.md §4.9). Sequence:

InvisibleButton(text, sz) reserves a hitbox of size sz. This is the registered item — widget_finalize reads bbox / hex_id / hover / active / focus from it, so it must be the last ImGui item before the finalizer call. Hitbox includes the label and value-readout area below the disc, so adjacent knobs stay aligned regardless of value-text width.
While IsItemActive() is true, the body reads GetIO().MousePos and computes atan2(my - cy, mx - cx). The angle is shifted by -3π/4 and wrapped into [0, 2π) so the active arc lands in [0, 3π/2) and the bottom-gap dead zone in [3π/2, 2π). Dead-zone reads are ignored — value holds at its previous reading. InvisibleButton handles press / drag / release; the widget never has to track its own pressed state.
GetWindowDrawList() returns the per-window draw list. Primitives — AddCircleFilled / AddCircle / AddLine / AddText — render inside the bbox but are pure painting; they don’t advance the ImGui cursor or participate in input.

Indicator angle is the inverse of the input mapping: θ = 3π/4 + frac · 3π/2 sweeps 270° with a 90° gap at the bottom (DAW convention). ImGui’s y-axis points down so positive sin θ is down on screen; the formula reads clockwise visually (frac=0 at 7 o’clock, frac=0.5 at 12, frac=1 at 5). Mouse position drives state.value which drives the indicator angle — no delta accumulation, no wraparound bookkeeping.

8.19.6.1.5. Reusing `pending_value_finalize`

The last line of the knob body — pending_value_finalize(widget_ident, "knob", state) — is the same line every value-typed built-in uses (slider_float, drag_float, input_float, color_edit3, combo). It builds the two finalize lambdas:

Serializer: closure over widget_ident, returns state_jv(path, type<VolumeKnobState>) — JSON-ifies the live state every time imgui_snapshot asks.
Dispatcher: closure that handles imgui_force_set with action "set" — writes state.pending_value and flips has_pending. Next frame the body drains it (step 1).

Then widget_finalize installs both lambdas keyed on the widget’s path, and register_focusable makes the widget reachable by imgui_focus.

For widgets that don’t fit this shape — pure-action buttons, multi-stage inputs, plots — the escape hatch is to write your own one-screen <kind>_finalize modeled on click_finalize, toggle_finalize, or plot_finalize in widgets/imgui_widgets_builtin.das. The shape is always the same: construct ser and disp lambdas via state_jv / with_state, then call widget_finalize.

8.19.6.1.6. Standalone vs live

main() runs the loop when invoked as daslang.exe custom_widgets.das. Under daslang-live the host calls init / update / shutdown directly; live-reloading the source preserves MASTER.value / TREBLE.value / BASS.value (via @live on VolumeKnobState).

8.19.6.1.7. Driving from outside

The custom knob takes the same live commands every slider does:

# snapshot — knobs appear under "kind":"knob" with bbox + hex_id + payload
curl -X POST -d '{"name":"imgui_snapshot"}' localhost:9090/command

# set a value programmatically — pending_value_finalize handles the dispatch
curl -X POST -d '{"name":"imgui_force_set","args":{"target":"MIXER_WIN/MASTER","value":0.75}}' \
     localhost:9090/command

# click the built-in reset button next to the knobs
curl -X POST -d '{"name":"imgui_click","args":{"target":"MIXER_WIN/RESET_BTN"}}' \
     localhost:9090/command

The snapshot payload carries value, bounds, changed — whatever fields the state struct declares. No per-kind glue: the state_jv helper introspects the struct at compile time and serializes every field.

8.19.6.1.8. Next steps

This is the same pattern every built-in widget uses. To wire a wholly new kind that needs its own dispatcher action (e.g. a 2-D pad with set_xy), copy a *_finalize helper from widgets/imgui_widgets_builtin.das and rename one action key.