8.19.25. Live reload

Every earlier tutorial’s source banner had two run modes:

daslang.exe <script> — standalone; runs main() which loops init / update / shutdown until exit_requested().
daslang-live <script> — same script, hosted inside a wrapper process that watches the file, reruns the typer on save, and swaps the new program in without restarting the GLFW window. ImGui context, registered widget state, dock layout, slider values — anything @live or restored via a hook — carry across the reload.

This tutorial walks through the seams the live-reload framework exposes:

live_create_window / live_imgui_init — idempotent so they no-op on reload (the preserved ImGui context is reused).
live_begin_frame / live_end_frame — per-frame gate / commit. live_begin_frame returns false while the host is paused or in the middle of swapping programs; skip the frame.
@live annotation — preserves a global (or struct field) across reload via auto-generated [before_reload] / [after_reload] serializers in the live/live_vars module.
[live_command] — registers an HTTP endpoint that the running process exposes; called from curl or any other client. The imgui_force_set / imgui_click / imgui_snapshot surface is built from [live_command] declarations.
[before_reload] / [after_reload] — manual save/restore hooks for state @live can’t track (raw pointers, GL textures, C-owned resources).

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

8.19.25.1. Walkthrough

The recording can’t trigger a reload, so it exercises the observable surface and asserts each piece: it force-sets the @live VOLUME slider and verifies the value (force_set_verified), clicks the @live PING_BTN twice and verifies click_count (hold_through_voice), then calls the user-defined bump_counter and reset_counter [live_command] endpoints from outside and verifies the counter climbs to 7 and back to 0 (record_check_value on the snapshot readout). A command that stopped reaching the running program would abort 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_visual_aids
require daslib/json public
require daslib/json_boost public

// =============================================================================
// TUTORIAL: live_reload — the daslang-live workflow that every earlier
// tutorial implicitly relied on.
//
// Every tutorial's STANDALONE / LIVE blocks pointed at the same shape:
//
//   STANDALONE: daslang.exe <script>
//   LIVE:       daslang-live <script>
//
// In the live mode, the script is hosted inside daslang-live: a wrapper
// process that runs your `init` / `update` / `shutdown` exports, watches
// the source file for edits, and exposes an HTTP server for `imgui_force_set` /
// `imgui_click` / `imgui_snapshot` and any user-defined `[live_command]`.
// Save the file and daslang-live re-runs the typer/codegen, calls
// `[before_reload]` hooks to stash anything that needs surviving, swaps in
// the new program, then calls `[after_reload]` to restore. The GLFW
// window stays open; the ImGui context, registered widget state, dock
// layout, slider values — anything @live or restored via a hook — all
// carry across the gap.
//
// This tutorial demonstrates each piece:
//
//   1. `live_create_window` / `live_imgui_init` — idempotent, reload-safe
//   2. `live_begin_frame()` — per-frame gate; returns false while paused
//                              or reloading. Skip the frame if it does.
//   3. `@live` on a state struct — value preserved across reload
//   4. `[live_command]` — custom HTTP endpoint that the running process
//                          exposes; driven from curl just like imgui_force_set
//   5. `[before_reload]` / `[after_reload]` — explicit save/restore hooks
//                                              for state the framework
//                                              doesn't track automatically
//
// STANDALONE: daslang.exe modules/dasImgui/examples/tutorial/live_reload.das
// LIVE:       daslang-live modules/dasImgui/examples/tutorial/live_reload.das
//
// DRIVE (when running live):
//   curl -X POST -d '{"name":"imgui_snapshot"}'                                                       localhost:9090/command
//   curl -X POST -d '{"name":"bump_counter"}'                                                          localhost:9090/command
//   curl -X POST -d '{"name":"reset_counter"}'                                                         localhost:9090/command
//   curl -X POST                                                                                       localhost:9090/reload
// =============================================================================

// ---- Custom counter that survives reload via @live ----
// Plain module-scope var, marked @live so daslang-live's serializer hauls
// it across the reload boundary. Non-`@live` globals reinitialise on
// reload — see the demo_string field below for that contrast.
var private @live g_custom_counter : int = 0
// NOT @live — gets reset on each reload (intentional for the demo).
var private g_session_string : string = "fresh session"

// ---- Custom live_command — externally callable HTTP endpoint ----
// imgui_force_set / imgui_click / imgui_snapshot are all `[live_command]`
// internally. Users can add their own:
struct BumpArgs {
    @optional by : int = 1
}

[live_command(description = "Bump the custom counter by N (default 1).")]
def bump_counter(input : JsonValue?) : JsonValue? {
    let args = from_JV(input, type<BumpArgs>)
    g_custom_counter += args.by
    return JV((ok = true, counter = g_custom_counter))
}

[live_command(description = "Reset the custom counter to 0.")]
def reset_counter(_input : JsonValue?) : JsonValue? {
    g_custom_counter = 0
    return JV((ok = true, counter = 0))
}

// ---- Reload hooks — fire on reload boundary ----

// [before_reload] runs in the OLD program before the new one loads — stash
// non-`@live` data for [after_reload] to restore. Usually marking a var `@live`
// suffices; hooks are the escape hatch for raw pointers, file handles, GL textures.
[before_reload]
def private on_before_reload() {
    print("[live_reload tutorial] before_reload fired. g_custom_counter = {g_custom_counter}\n")
}

[after_reload]
def private on_after_reload() {
    print("[live_reload tutorial] after_reload fired. g_custom_counter = {g_custom_counter} (preserved by @live)\n")
}

[export]
def init() {
    // Both calls are idempotent on reload — cold start creates the window + ImGui
    // context; reload reuses the preserved context and skips the duplicate
    // CreateContext (see live_imgui_init in imgui_live.das).
    live_create_window("dasImgui live_reload tutorial", 1040, 720)
    live_imgui_init(live_window)
    DisableIniPersistence()
    let io & = unsafe(GetIO())
    GetStyle().FontScaleMain = 1.5

    // Re-running init resets g_session_string each reload. (Initial-value
    // assignments at module scope run once at program load, but `init` is
    // called both on cold-start AND on reload.)
    g_session_string = "fresh session"
}

[export]
def update() {
    // The frame-gate. Returns false while daslang-live is paused, in the
    // middle of swapping programs, or in any other state where rendering
    // would crash or produce garbage. Always early-out on false.
    if (!live_begin_frame()) return

    begin_frame()

    ImGui_ImplGlfw_NewFrame()
    apply_synth_io_override()
    NewFrame()

    SetNextWindowPos(ImVec2(30.0f, 30.0f), ImGuiCond.FirstUseEver)
    SetNextWindowSize(ImVec2(640.0f, 460.0f), ImGuiCond.FirstUseEver)
    window(LIVE_WIN, (text = "live_reload", closable = false,
                      flags = ImGuiWindowFlags.None)) {

        // ---- A slider whose value survives reload (@live in SliderStateFloat) ----
        text("VOLUME.value is @live - survives reload.")
        slider_float(VOLUME, (text = "Volume"))

        separator(LR_SEP_1)

        // ---- The custom counter, driven from outside via [live_command] ----

        // Mirror the @live counter into text_show so the snapshot can address it;
        // g_custom_counter stays the plain @live var, this is just the readout.
        CTR_TEXT.value = "g_custom_counter = {g_custom_counter}"
        text_show(CTR_TEXT)
        text("  - @live, preserved across reload")
        text("  - mutated externally by `bump_counter` / `reset_counter`")

        separator(LR_SEP_2)

        // ---- Contrast: session string resets each reload ----
        text("g_session_string = \"{g_session_string}\"")
        text("  - NOT @live; init() rewrites it each reload")

        separator(LR_SEP_3)

        // ---- A button whose click count is preserved (ClickState is @live) ----
        if (button(PING_BTN, (text = "Ping (click_count survives reload)"))) {}
        text("PING_BTN.click_count = {PING_BTN.click_count}")
    }

    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()

    // Always match live_begin_frame with live_end_frame to keep the
    // frame pump alive on the daslang-live side.
    live_end_frame()
}

[export]
def shutdown() {
    // Idempotent on reload - live_imgui_shutdown skips during reload so
    // the ImGui context is preserved; only the cold process exit runs
    // the full teardown.
    live_imgui_shutdown()
    live_destroy_window()
}

[export]
def main() {
    // Standalone entrypoint. daslang-live drives init/update/shutdown
    // directly and ignores main(); plain `daslang.exe <file>` runs this
    // loop.
    init()
    while (!exit_requested()) {
        update()
        maybe_collect_gc()
    }
    shutdown()
}

8.19.25.1.1. The reload boundary

A daslang-live reload runs in this order:

File watcher notices a source-tree edit (or an HTTP POST /reload request arrives).
The HOST collects every [before_reload] function and runs them in the OLD program. The live/live_vars module auto-generates one of these per @live global; the user can register more.
Typer + codegen run against the new source. If they fail, the reload aborts and the old program keeps running — get_last_error() surfaces the diagnostic in daslang, and the last_error live command surfaces it over HTTP.
The new program is loaded. [after_reload] hooks run, restoring the saved state (@live first, then user hooks).
The next update() call sees live_begin_frame() == true and normal rendering resumes.

The GLFW window and the OS-level ImGui context survive the swap — only the daslang program is replaced.

8.19.25.1.2. `@live` preservation

The simplest way to keep a value across reload is the @live annotation on the global (or on individual struct fields). The live/live_vars module synthesizes the matching save/restore hooks at compile time:

var private @live g_custom_counter : int = 0

The serializer uses daslib/archive; primitives, arrays, tables, strings, and any struct whose fields are themselves @live-friendly all work out of the box. Each @live target gets its own storage key, and the saved data carries a hash of the initialization expression — change the initializer in source, and the stale value is discarded automatically. (No “I changed the default to 10 and now my old value of 0 is wrong” foot-gun.)

Boost widget state types (ClickState, SliderStateFloat, ToggleState, WindowState, …) are already structured this way — their value-carrying fields are @live, their pending-flags fields are not. That’s why a slider’s value survives reload but pending_value doesn’t.

8.19.25.1.3. The frame gate

live_begin_frame() is the only way the host signals “do not render this frame”:

def update() {
    if (!live_begin_frame()) return
    // ... NewFrame, your draw calls, Render
    live_end_frame()
}

States that return false:

The host is paused (POST /pause from daslang-live or mcp__daslang__live_pause).
A reload is in progress (between [before_reload] and [after_reload]).
The most recent typer pass failed and the program is “frozen” on the prior version — the next save that compiles will revive it.

Always early-out on false and always pair with live_end_frame() on the success branch.

8.19.25.1.4. `[live_command]` — user-defined HTTP endpoints

The same [live_command] annotation that registers imgui_force_set / imgui_click / imgui_snapshot works for user functions. The function takes a JsonValue? (the request body’s args field) and returns JsonValue? (echoed back to the caller):

struct BumpArgs {
    @optional by : int = 1
}

[live_command(description = "Bump the custom counter by N (default 1).")]
def bump_counter(input : JsonValue?) : JsonValue? {
    let args = from_JV(input, type<BumpArgs>)
    g_custom_counter += args.by
    return JV((ok = true, counter = g_custom_counter))
}

The endpoint name (bump_counter) is the function name; the HTTP surface routes POST /command requests with {"name":"bump_counter"} to this handler. The handler runs on the GLFW main thread between frames, so it can safely touch daslang globals and ImGui state without locks.

8.19.25.1.5. Manual reload hooks

When @live doesn’t fit — typically because the state is a pointer to a C-owned resource that the new program won’t recognize — declare a pair of hooks explicitly:

[before_reload]
def private on_before_reload() {
    // Stash whatever ``@live`` can't serialize.
    // live_store_bytes / live_store_string store under a string key.
}

[after_reload]
def private on_after_reload() {
    // Re-read the stash and rebuild the in-memory state.
    // live_load_bytes / live_load_string read by the same key.
}

imgui_live.das itself is the canonical example: it serializes the live_imgui_ctx pointer as a uint64 in [before_reload] and re-binds it with SetCurrentContext in [after_reload].

8.19.25.1.6. Init / shutdown idempotence

init runs on both cold-start AND reload. shutdown runs on reload AND process exit. The framework helpers (live_imgui_init / live_imgui_shutdown) are idempotent — they detect the reload case and no-op accordingly — so the script’s init / shutdown exports can be written once without distinguishing cold-start from reload.

User globals initialized at module scope (var x = 0) only run their initializer once at program load — reload starts a NEW program, so that initializer runs again. Use @live (or a [before_reload] hook) for anything you want to preserve. Anything reset INSIDE init rebuilds each reload — the demo’s g_session_string shows that pattern.

8.19.25.1.7. Standalone vs live

Run standalone with daslang.exe — every part of this tutorial still works EXCEPT the [live_command] HTTP endpoints, which need the daslang-live host. The reload hooks are silent in standalone mode (they only fire on the reload boundary, which never happens).

8.19.25.1.8. Driving from outside

Standard live-command shape; the user-defined endpoints sit next to the built-in ones:

# built-in: snapshot every registered widget
curl -X POST -d '{"name":"imgui_snapshot"}' localhost:9090/command

# user-defined: bump the custom counter by 3
curl -X POST -d '{"name":"bump_counter","args":{"by":3}}' localhost:9090/command

# user-defined: reset
curl -X POST -d '{"name":"reset_counter"}' localhost:9090/command

# framework: trigger a reload (file edit also triggers this)
curl -X POST localhost:9090/reload

The imgui_snapshot payload reflects whatever the most recent bump_counter did — daslang globals and live-command results share one in-memory model.

8.19.25.1.9. Next steps

Now that the live-command surface is explicit, next is the driving-from-outside view: the JSON command set the boost layer ships (imgui_force_set / imgui_click / imgui_open / …) treated as its own programming model — a UI that responds to scripted external events the same way it responds to mouse clicks.