8.19.34. Window size constraints

ImGui’s SetNextWindowSizeConstraints clamps the next window between a min and max corner size. The 2-arg form is enough for a static box constraint; the 3-arg form takes an ImGuiSizeCallback — a C function pointer ImGui invokes on every resize so the callback can re-shape the requested size (aspect-ratio lock, fixed-step quantization, “always square”, etc.).

The boost module imgui/imgui_window_constraints_builtin ships a daslang wrapper around the 3-arg form: an ImGuiSizeConstraints struct that wraps a daslang lambda + the existing C++ trampoline (SetNextWindowSizeConstraintsCallback → das_invoke_lambda<void> in src/dasIMGUI.main.cpp). End-user code passes the wrapper:

// Module scope: ImGui keeps the pointer for the NEXT Begin(), so the
// wrapper must outlive the SetNextWindowSizeConstraints call.
let private ASPECT_RATIO = 16.0f / 9.0f

var private ASPECT_CN : ImGuiSizeConstraints <- ImGuiSizeConstraints(
    @ capture(= ASPECT_RATIO) (var data : ImGuiSizeCallbackData) : void {
        data.DesiredSize = float2(data.DesiredSize.x,
                                  data.DesiredSize.x / ASPECT_RATIO)
    })

SetNextWindowSizeConstraints(float2(0.0f, 0.0f), float2(FLT_MAX, FLT_MAX),
                             ASPECT_CN)

Seed the wrapper at its module-scope declaration and pass the same struct on every frame — the lambda carries its own capture state. A stack local will not do: auto-fit invokes the callback from inside Begin(), after the enclosing function has already returned.

The 2-arg SetNextWindowSizeConstraints(min, max) form stays on the boost-surface allow-list — only require this module when you need the callback form.

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

8.19.34.1. Walkthrough

The recording drags each window by its bottom-right corner - the real “drag border” gesture - to a shape that violates its constraint, and the callback snaps it back: the first window, dragged to a wide-short rectangle, springs to a 400x400 square; the second, dragged tall, collapses to 480x270 (16:9); the third, dragged off-grid, lands on 350x200 (nearest 50). Each snapped size is asserted - a drag that didn’t resize, or a callback that didn’t reshape the request, aborts the recording.

options gen2
options _comment_hygiene = 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_window_constraints_builtin
require imgui/imgui_visual_aids
require math

// =============================================================================
// TUTORIAL: window_size_constraints — lambda-callback overload for
//           ImGui::SetNextWindowSizeConstraints.
//
// The 2-arg form `SetNextWindowSizeConstraints(min, max)` is plain-allowed
// at user-call sites — clamp the window between two corner sizes. The 3-arg
// form takes a `ImGuiSizeCallback`, a C function pointer ImGui invokes on
// every resize so the callback can re-shape the requested size (aspect-ratio
// lock, fixed step quantization, "always square", ...).
//
// The boost module `imgui/imgui_window_constraints_builtin` provides a
// daslang-side `ImGuiSizeConstraints` struct + ctor that wraps a daslang
// lambda; the matching 3-arg overload threads the wrapper through the
// existing C++ trampoline at `src/dasIMGUI.main.cpp:361-384`. End-user code
// can then write:
//
//   let aspect = @ capture(= ratio) (var data : ImGuiSizeCallbackData) : void {
//       data.DesiredSize.y = data.DesiredSize.x / ratio
//   }
//   SetNextWindowSizeConstraints(float2(0,0), float2(FLT_MAX, FLT_MAX),
//                                ImGuiSizeConstraints(aspect))
//
// This tutorial mounts three resizable windows side-by-side — square,
// aspect-16:9, fixed-step-50 — each with a body line printing its current
// (width, height) so resizing visibly snaps to the constraint.
//
// STANDALONE: daslang.exe modules/dasImgui/examples/tutorial/window_size_constraints.das
// LIVE:       daslang-live modules/dasImgui/examples/tutorial/window_size_constraints.das
// =============================================================================

// Module-scope holders for the three ImGuiSizeConstraints. ImGui stores the
// callback pointer + user-data for use by the NEXT Begin(); the wrapper
// struct's address must outlive the SetNextWindowSizeConstraints call.
// Stack-locals don't survive long enough — auto-fit invokes the callback
// from inside Begin AFTER the wrapper function has returned. Module
// scope keeps the struct alive for the lifetime of the program.
// Each is seeded once, right here: daslang lambdas hold their own capture
// state, so a single per-shape struct serves every future Begin().
let private ASPECT_RATIO = 16.0f / 9.0f
let private FIXED_STEP = 50.0f

var private SQUARE_CN : ImGuiSizeConstraints <- ImGuiSizeConstraints(
    @(var data : ImGuiSizeCallbackData) : void {
        let s = max(data.DesiredSize.x, data.DesiredSize.y)
        data.DesiredSize = float2(s, s)
    })
var private ASPECT_CN : ImGuiSizeConstraints <- ImGuiSizeConstraints(
    @ capture(= ASPECT_RATIO) (var data : ImGuiSizeCallbackData) : void {
        data.DesiredSize = float2(data.DesiredSize.x,
                                  float(int(data.DesiredSize.x / ASPECT_RATIO)))
    })
var private STEP_CN : ImGuiSizeConstraints <- ImGuiSizeConstraints(
    @ capture(= FIXED_STEP) (var data : ImGuiSizeCallbackData) : void {
        let rx = float(int(data.DesiredSize.x / FIXED_STEP + 0.5f)) * FIXED_STEP
        let ry = float(int(data.DesiredSize.y / FIXED_STEP + 0.5f)) * FIXED_STEP
        data.DesiredSize = float2(rx, ry)
    })

[export]
def init() {
    live_create_window("dasImgui window_size_constraints tutorial", 980, 560)
    live_imgui_init(live_window)
    // Relies on FirstUseEver window sizes; disable imgui.ini so the documented
    // layout (and recording) is deterministic every run instead of restoring a
    // prior session. Tutorial-scoped on purpose — not a live_imgui_init change.
    DisableIniPersistence()
    let io & = unsafe(GetIO())
    GetStyle().FontScaleMain = 1.2
}

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

    ImGui_ImplGlfw_NewFrame()
    apply_synth_io_override()
    NewFrame()

    // -------- 1. Square — max(w, h) wins both dims. --------
    SetNextWindowPos(ImVec2(20.0f, 60.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSize(ImVec2(240.0f, 240.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSizeConstraints(float2(120.0f, 120.0f), float2(FLT_MAX, FLT_MAX),
                                 SQUARE_CN)
    window(SQUARE_WIN, (text = "Square", closable = false,
                        flags = ImGuiWindowFlags.None)) {
        let s = SQUARE_WIN.size
        text(SQUARE_TEXT, (text = "Drag border - always square.\n"
                                + "Size: {int(s.x)} x {int(s.y)}"))
    }

    // -------- 2. Aspect ratio 16:9 — height = width / aspect. --------
    SetNextWindowPos(ImVec2(290.0f, 60.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSize(ImVec2(320.0f, 180.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSizeConstraints(float2(160.0f, 0.0f), float2(FLT_MAX, FLT_MAX),
                                 ASPECT_CN)
    window(ASPECT_WIN, (text = "Aspect 16:9", closable = false,
                        flags = ImGuiWindowFlags.None)) {
        let s = ASPECT_WIN.size
        text(ASPECT_TEXT, (text = "Drag border - height tracks width / 16:9.\n"
                                + "Size: {int(s.x)} x {int(s.y)}"))
    }

    // -------- 3. Fixed step 50 — both dims snap to nearest 50 px. --------
    SetNextWindowPos(ImVec2(640.0f, 60.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSize(ImVec2(300.0f, 250.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSizeConstraints(float2(100.0f, 100.0f), float2(FLT_MAX, FLT_MAX),
                                 STEP_CN)
    window(STEP_WIN, (text = "Step 50", closable = false,
                      flags = ImGuiWindowFlags.None)) {
        let s = STEP_WIN.size
        text(STEP_TEXT, (text = "Drag border - both dims snap to 50 px.\n"
                              + "Size: {int(s.x)} x {int(s.y)}"))
    }

    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()
}

8.19.34.1.1. Requires

One extra module on top of the baseline boost layer:

imgui/imgui_window_constraints_builtin — the ImGuiSizeConstraints struct + ctor + 3-arg overload of SetNextWindowSizeConstraints.

8.19.34.1.2. The three callback patterns

The tutorial mounts three resizable windows demonstrating the canonical callback shapes:

Square — max(w, h) wins both dims:

let sq <- @ (var data : ImGuiSizeCallbackData) : void {
    let s = max(data.DesiredSize.x, data.DesiredSize.y)
    data.DesiredSize = float2(s, s)
}

Aspect 16:9 — height tracks width / aspect_ratio:

let ratio = 16.0f / 9.0f
let aspect <- @ capture(= ratio) (var data : ImGuiSizeCallbackData) : void {
    data.DesiredSize = float2(data.DesiredSize.x,
                               data.DesiredSize.x / ratio)
}

Fixed step — both dims snap to the nearest multiple:

let step = 50.0f
let snap <- @ capture(= step) (var data : ImGuiSizeCallbackData) : void {
    let rx = float(int(data.DesiredSize.x / step + 0.5f)) * step
    let ry = float(int(data.DesiredSize.y / step + 0.5f)) * step
    data.DesiredSize = float2(rx, ry)
}

Capture rules follow the standard daslang lambda surface — capture(= var) for by-value, capture(& var) for by-reference. A plain @(...) with no capture clause is enough for callbacks that read only data.

8.19.34.1.3. Struct layout

The ImGuiSizeConstraints daslang struct mirrors das::DasImGuiSizeConstraints in src/dasIMGUI.main.cpp field-by-field:

struct ImGuiSizeConstraints {
    context : Context?
    callback : lambda<(var data : ImGuiSizeCallbackData) : void>
    at : LineInfo?
}

The C++ trampoline reads field offsets directly to dispatch the lambda through das_invoke_lambda<void>::invoke<ImGuiSizeCallbackData*>. Don’t re-order the fields — the binding is not symbolic.

8.19.34.1.4. Standalone vs live

Same convention as previous tutorials. Resize any of the three windows by dragging a border or corner — the callback fires on every drag delta and quantizes the result.