Makepad 2.0 Splash

Makepad 2.0 Splash Scripting Language

Splash is Makepad 2.0's core runtime UI scripting language, released February 12, 2026. It replaces the old compile-time live_design! macro system with a runtime script_mod! macro that enables hot reload, streaming evaluation, and AI-first code generation.

Core Concepts

Script Structure

Every Splash script starts with a use import and is embedded in Rust via the script_mod!{} macro:

use makepad_widgets::*;

app_main!(App);

script_mod! {
    use mod.prelude.widgets.*

    // let bindings, functions, state, and UI definitions go here

    startup() do #(App::script_component(vm)){
        ui: Root{
            main_window := Window{
                window.inner_size: vec2(800, 600)
                body +: {
                    // UI content
                }
            }
        }
    }
}

Syntax Rules

• No commas between properties -- whitespace-delimited
• No semicolons -- cleaner syntax optimized for LLM generation
• Property assignment: key: value
• Dot-path shorthand: draw_bg.color: #f00 (equivalent to draw_bg +: { color: #f00 })
• Merge operator: key +: { ... } extends parent without replacing
• Named children: name := Widget{...} (addressable, overridable per-instance)
• Let bindings: let MyTemplate = Widget{...} (local scope, must be defined before use)
• Rust binding: #(Struct::register_widget(vm)) connects Splash to Rust structs
• Debug logging: ~expression logs value during evaluation

State Management

State is managed via the mod.state object and reactive on_render callbacks:

// Define state
let state = { counter: 0 }
mod.state = state

// Reactive rendering -- re-runs when .render() is called
main_view := View{
    on_render: ||{
        Label{ text: "Count: " + state.counter }
    }
}

Event Handling

Events are handled both inline in Splash and from Rust:

// Inline event handlers in Splash
add_button := Button{
    text: "Add"
    on_click: ||{
        add_todo(ui.todo_input.text(), "")
        ui.todo_input.set_text("")
    }
}

// TextInput return key
todo_input := TextInput{
    on_return: || ui.add_button.on_click()
}

// Startup event
on_startup: ||{
    ui.main_view.render()
}

From Rust, use script_eval! to execute Splash code:

impl MatchEvent for App {
    fn handle_actions(&mut self, cx: &mut Cx, actions: &Actions) {
        if self.ui.button(cx, ids!(increment_button)).clicked(actions) {
            script_eval!(cx, {
                mod.state.counter += 1
                ui.main_view.render()
            });
        }
    }
}

Functions

fn tag_color(tag) {
    if tag == "dev" theme.color_highlight
    else if tag == "design" theme.color_selection_focus
    else theme.color_highlight
}

fn add_todo(text, tag) {
    todos.push({text: text, tag: tag, done: false})
    ui.todo_list.render()
}

Control Flow

// If/else
if todos.len() == 0
    EmptyState{}
else for i, todo in todos {
    TodoItem{ label.text: todo.text }
}

// For loops
for i, item in array {
    Label{ text: item.name }
}

// While
while condition { ... }

HTTP Requests

let req = net.HttpRequest{
    url: "https://api.example.com/data"
    method: net.HttpMethod.GET
    headers: {"User-Agent": "MakepadApp/1.0"}
}
net.http_request(req) do net.HttpEvents{
    on_response: |res| {
        let text = res.body.to_string()
        let json = res.body.parse_json()
    }
    on_error: |e| { /* handle error */ }
}

Streaming responses use is_streaming: true with on_stream and on_complete callbacks.

HTML Parsing

let doc = html_string.parse_html()
doc.query("p")              // all <p> elements
doc.query("#main")           // by id
doc.query("p.bold")          // by class
doc.query("div > p")         // direct children
doc.query("p[0]").text       // text content
doc.query("a@href")          // attribute value

Streaming Evaluation

Splash's parser supports checkpoint-based incremental parsing, designed for AI/LLM streaming code generation:

// Rust API for streaming evaluation
vm.eval_with_append_source(script_mod, &code, NIL.into())

This enables real-time UI updates as code is generated token-by-token, without requiring a complete script before evaluation.

Hot Reload & Script Mod Tracking

Splash scripts support hot reload via the --hot flag. The VM tracks each script_mod! block with a unique ScriptModKey (file, line, column):

// Internal: ScriptModKey uniquely identifies a script_mod! block
ScriptModKey { file: "src/app.rs", line: 5, col: 1 }

// Runtime substitution via overrides
ScriptCode::script_mod_overrides  // HashMap of ScriptModKey -> updated source

How hot reload works:

1. File watcher (makepad_live_reload_core) detects source file changes
2. script_mod! blocks are extracted from Rust source (handles raw strings, comments, char literals)
3. Rust placeholder counts (#(...)) are tracked -- adding/removing placeholders requires full rebuild
4. Validated script mods are applied via script_mod_overrides
5. IP-to-location mapping provides source maps for error reporting (fallback to nearest token for synthetic opcodes)

ScriptSource variants:

• ScriptSource::Mod -- Standard module evaluation (startup)
• ScriptSource::Streaming -- Incremental streaming evaluation (AI/LLM)

Critical Layout Rules

1. Always set height: Fit on containers -- default height: Fill causes invisible UI (0px height)
2. Use width: Fill on the root container -- never fixed pixel width at the top level
3. Set new_batch: true on any View with show_bg: true that contains text children
4. Use := for named children in templates -- without it, text overrides fail silently
5. draw_bg.border_radius takes a float, not an Inset -- draw_bg.border_radius: 16.0
6. Use styled Views (RoundedView, SolidView) instead of raw View{show_bg: true}

Widget Reference

Core containers: View, SolidView, RoundedView, RectView, RoundedShadowView, CircleView, GradientXView, GradientYView, ScrollXYView, ScrollXView, ScrollYView

Text: Label, H1-H4, P, TextBox, TextInput, LinkLabel, Markdown, Html

Controls: Button, ButtonFlat, ButtonFlatter, CheckBox, Toggle, RadioButton, Slider, DropDown

Layout: Splitter, FoldHeader, Hr, Vr, Filler

Lists: PortalList, FlatList

Navigation: Modal, Tooltip, PopupNotification, SlidePanel, ExpandablePanel, PageFlip, StackNavigation

Dock: Dock, DockSplitter, DockTabs, DockTab

Media: Image, Icon, LoadingSpinner, Vector, MathView, MapView

Canvas: Rendering Splash from Claude Code

Makepad Canvas (tools/canvas/) is a standalone app that renders Splash code received via HTTP/WS. Used by Claude Code for visual output.

HTTP API (recommended for sending Splash)

PORT=$(cat /tmp/makepad-canvas.port)

# Full render
curl -s -X POST "http://127.0.0.1:$PORT/splash" -d 'View{width:Fill height:Fit Label{text:"Hello"}}'

# Streaming render
curl -s -X POST "http://127.0.0.1:$PORT/splash/stream"                    # begin
curl -s -X POST "http://127.0.0.1:$PORT/splash/stream" -d 'View{...'      # append
curl -s -X POST "http://127.0.0.1:$PORT/splash/end"                       # end

# Clear
curl -s -X POST "http://127.0.0.1:$PORT/clear"

WS Event Listening (for receiving click events)

# Long-lived WS connection receives button click events as JSON
mkfifo /tmp/ws_fifo; (sleep 99999 > /tmp/ws_fifo &)
websocat ws://127.0.0.1:$PORT < /tmp/ws_fifo > /tmp/canvas_events &

# Events arrive as: {"event":"click","widget":"btn_name"}

Interactive Buttons

Use name := Button{...} to create clickable buttons. The name is sent in click events:

View{width:Fill height:Fit flow:Right spacing:12
  btn_save := Button{text:"Save"}
  btn_cancel := Button{text:"Cancel"}
}

When clicked: {"event":"click","widget":"btn_save"}

Vector Animations in Splash

// Pulsing dot (loop_:true = indefinite, NOT "indefinite"!)
Vector{width:16 height:16
  Circle{cx:8 cy:8 r:6 fill:#x44ddaa opacity:Tween{from:0.3 to:1.0 dur:1.5 loop_:true}}
}

// Moving dot with color change
Vector{width:Fill height:30
  Path{d:"M 20 15 L 400 15" stroke:#x222244 stroke_width:1.}
  Circle{cx:Tween{from:20 to:400 dur:3.0 loop_:true} cy:15 r:4 fill:Tween{from:#x44ddaa to:#xffaa44 dur:3.0 loop_:true}}
}

Canvas Splash Syntax (CRITICAL -- differs from script_mod!)

When generating Splash for Canvas HTTP rendering (POST /splash), use these EXACT patterns. Canvas Splash syntax differs from script_mod! macro context in several critical ways:

1. Properties use dot-path inline, NOT nested blocks:

// WRONG -- nested block syntax does not render backgrounds
RoundedView{height: Fit draw_bg: { color: #x1a1a2e border_radius: 8.0 } }

// CORRECT -- dot-path inline
RoundedView{width: Fill height: Fit draw_bg.color: #x1a1a2e draw_bg.radius: 8.}

2. Border radius is draw_bg.radius, NOT draw_bg.border_radius:

// WRONG
draw_bg.border_radius: 8.0

// CORRECT
draw_bg.radius: 8.

3. Padding uses explicit Inset{} type with trailing-dot floats:

// WRONG -- bare number or nested block
padding: 20
padding: { top: 20 bottom: 20 }

// CORRECT
padding: Inset{left: 20. right: 20. top: 16. bottom: 16.}

4. Align uses explicit Align{} type:

// WRONG
align: { y: 0.5 }

// CORRECT
align: Align{y: 0.5}
align: Center

5. Float values use trailing dot:

// WRONG       // CORRECT
8.0            8.
16.0           16.
0.5            0.5

6. SolidView and RoundedView do NOT need show_bg: true or new_batch: true -- they render backgrounds out of the box.

7. Use --data-binary for multi-line Splash via curl -- plain -d strips newlines.

Proven Canvas Dashboard Template

Source: tools/canvas/examples/token-dashboard.splash

SolidView{width: Fill height: Fit draw_bg.color: #x0c0c18 flow: Down padding: Inset{left: 32. right: 32. top: 24. bottom: 24.} spacing: 20

    // Title
    Label{text: "Dashboard Title" draw_text.color: #xeeeeff draw_text.text_style.font_size: 20}

    // Card row
    View{width: Fill height: Fit flow: Right spacing: 16
        RoundedView{width: Fill height: Fit draw_bg.color: #x161628 draw_bg.radius: 8. padding: Inset{left: 20. right: 20. top: 16. bottom: 16.} flow: Down spacing: 6
            Label{text: "Metric Name" draw_text.color: #x888899 draw_text.text_style.font_size: 10}
            Label{text: "Value" draw_text.color: #xcc66ff draw_text.text_style.font_size: 28}
        }
    }

    // Horizontal bar chart row
    View{width: Fill height: Fit flow: Right spacing: 8 align: Align{y: 0.5}
        Label{text: "Label" width: 100 draw_text.color: #xbbbbbb draw_text.text_style.font_size: 10}
        RoundedView{width: 200 height: 12 draw_bg.color: #xf44336 draw_bg.radius: 2.}
        Label{text: "200" draw_text.color: #x777777 draw_text.text_style.font_size: 10}
    }

    // Vertical bar chart (bars bottom-aligned)
    View{width: Fill height: 130 flow: Right spacing: 2 align: Align{y: 1.0}
        View{width: Fill height: Fit flow: Down align: Center spacing: 4
            RoundedView{width: 14 height: 80 draw_bg.color: #x7733cc draw_bg.radius: 2.}
            Label{text: "Mon" draw_text.color: #x444455 draw_text.text_style.font_size: 7}
        }
    }
}

Canvas Tips

• HTTP for splash, WS for events — most reliable pattern
• Vector shape properties: fill, stroke, stroke_width — NOT draw_bg.*
• CJK/Chinese text: Supported in both body text and code blocks. CodeEditor uses double-width columns for CJK characters (fixed 2026-03-23). Theme font_code includes LXGWWenKai as Chinese fallback.
• Large POST bodies: Canvas supports up to 512KB HTTP body
• Pub/sub broadcast: All connected WS clients receive click events
• Health check: Use GET /ping (NOT /health)
• Monitor toggle: /tmp/canvas-monitor-active flag file controls statusline auto-refresh. Canvas sidebar has a toggle button.

Compiled vs Eval: Two Fundamentally Different Widget Creation Paths

Understanding this distinction is CRITICAL for debugging any Splash eval issue:

Compiled path (script_mod!, examples, studio):
  parse → compile → execute → create_widget
    → script_apply(FULL type default object with all vec + map entries)
    → on_after_apply(value = full object) ✓  ← ScriptHook runs
    → Templates registered, #[live] fields populated from type default

Eval path (POST /splash, Splash.set_text):
  parse → eval → create_widget_from_prototype
    → script_apply(EVAL VALUE ONLY, e.g. {body: "..."})
    → on_after_apply: SKIPPED (is_eval() guard in TextFlow/Markdown)
    → Templates NOT registered, #[live] fields use defaults (not type default)

Fix pattern for eval-path issues: Implement lazy initialization in draw_walk that detects missing state and looks up the type default via cx.with_vm(|vm| vm.bx.heap.type_default_for_object(...)). This is how Markdown's code_block template inheritance was fixed.

Splash Eval Pitfalls (CRITICAL -- learned 2026-03-23)

These issues only affect widgets created via Splash runtime eval (POST /splash, Splash.set_text()), NOT compiled script_mod! widgets:

1. ScrollYView does NOT work in Splash eval -- renders blank

// WRONG -- Splash eval renders nothing
ScrollYView{ width: Fill height: Fill flow: Down
    Label{ text: "invisible" }
}

// CORRECT -- use View, Canvas wraps it in its own ScrollYView
View{ width: Fill height: Fit flow: Down
    Label{ text: "visible" }
}

2. on_after_apply / ScriptHook is NOT called for eval-created widgets

When Splash eval creates a widget (e.g. Markdown{body: "..."}), the ScriptHook::on_after_apply callback is never invoked. Any initialization that depends on on_after_apply must have a fallback path (e.g. lazy init in draw_walk).

3. Type default properties are NOT fully inherited in eval path

When set_type_default() overrides a widget type with extra properties (like use_code_block_widget: true) or named children (like code_block := View{...}), instances created via Splash eval may not inherit these. The #[live] fields only get values from the eval apply value (e.g. {body: "..."}), not the full type default.

Workaround: Check for missing state in draw_walk and look up the type default via vm.bx.heap.type_default_for_object().

4. Type default vec entries (named children) may not copy to instances

Even though copy_type_default_vec exists, the vec entries from set_type_default() may lose entries between registration and instance creation. The auto-proto vec copy in new_with_proto_impl copies from the direct proto, which may have fewer entries than the type_default.

5. Nested Markdown{} inside Splash works but needs type default templates

When Canvas overrides mod.widgets.Markdown with code_block := View{CodeView{...}}, and Splash eval creates Markdown{body: "..."}, the code_block template is NOT automatically available. The fix (in widgets/src/markdown.rs) lazily looks up the type default at draw time and registers missing templates.

Debugging Splash

Command-Line Flags

Print Debugging

std.println() / std.print() are the primary debugging tools. Output goes to both terminal and Studio's Log View:

std.println("debug: state.counter = " + state.counter)

The ~expression debug log syntax also prints values during evaluation:

~state.counter    // prints the value of state.counter during eval

Makepad Studio Integration

Studio can run Splash scripts directly from the Run List panel (looks for makepad.splash in project root). Script errors appear in the Log View with file path and error details.

When running under Studio, Splash scripts get a hub module:

Current Limitations

• No breakpoint debugging -- Splash VM does not support breakpoints or stepping
• No AST dump flag -- Inspecting parse results requires adding logs in Rust source (script/)
• Print-based debugging -- std.println() and ~expr are the primary debugging tools

File References

• Full language manual: splash.md (2559 lines)
• Migration guide: AGENTS.md (815 lines)
• Counter example: examples/counter/src/app.rs
• Todo example: examples/todo/src/app.rs
• Detailed reference: skills/makepad-2.0-splash/references/splash-language-reference.md
• Patterns guide: skills/makepad-2.0-splash/references/splash-scripting-patterns.md

Practical Splash Lessons (learned 2026-03-31, from building Vox voice input app)

Lesson 1: instance Fields Cannot Be Added in +: Blocks

The most critical Splash limitation. Adding instance my_var: 0.5 inside a draw_bg +: { } block causes a runtime error: "cannot push to frozen vec".

// WRONG — runtime crash
draw_bg +: {
    instance hover: 0.0          // CRASH: cannot push to frozen vec
    pixel: fn() { ... }
}

// CORRECT — override pixel function only, use built-in variables
draw_bg +: {
    pixel: fn() {
        let t = self.draw_pass.time    // built-in, always available
        return Pal.premul(vec4(t, 0.0, 0.0, 1.0))
    }
}

Workarounds:

• Use self.draw_pass.time for time-based animation
• Use self.pos, self.rect_size (always available)
• For custom instance variables, create a Rust DrawQuad subtype (see makepad-2.0-shaders skill)
• Use LoadingSpinner widget for simple animated indicators

Lesson 2: Transparent Floating Window Recipe

Creating a truly transparent overlay window requires three things:

my_window := Window{
    show_caption_bar: false
    window.transparent: true             // 1. Window-level transparency
    pass.clear_color: #x00000000         // 2. Render pass clear to fully transparent
    body +: {
        // 3. Do NOT set draw_bg on body — it overrides transparency
        View{
            width: Fill height: Fill
            // Only visible elements show; rest is see-through
        }
    }
}

Common mistakes:

• Setting only window.transparent: true — window stays opaque (gray background)
• Setting draw_bg.color: #x00000000 on body — does NOT make it transparent, just black
• Must also configure as floating panel from Rust: MacosWindowConfig::floating_panel() + Borderless

Reference implementation: Makepad tools/canvas/src/app.rs uses this exact pattern.

Lesson 3: Property Names That Don't Exist

Splash silently ignores unknown properties. These caused us real debugging time:

Lesson 4: Emoji Rendering

Makepad's text renderer supports some emoji but not all:

If an emoji shows as a box or garbage character, try a different one. Stick to basic emoji from the BMP (Basic Multilingual Plane).

Lesson 5: width: Fit + Large border_radius = Spiky Shape

RoundedView with border_radius: 28.0 on a height: 56 capsule produces diamond-shaped ends instead of half-circles. The underlying sdf.box() formula breaks when radius >= min(w,h)/2.

Fix: Use a custom SDF capsule shader instead of RoundedView:

View{
    show_bg: true
    draw_bg +: {
        pixel: fn() {
            let r = self.rect_size.y * 0.5
            let px = self.pos.x * self.rect_size.x
            let py = self.pos.y * self.rect_size.y
            let cx = clamp(px, r, max(r, self.rect_size.x - r))
            let d = length(vec2(px - cx, py - self.rect_size.y * 0.5)) - r
            let alpha = 1.0 - smoothstep(-1.0, 1.0, d)
            return Pal.premul(vec4(0.1, 0.1, 0.18, alpha * 0.82))
        }
    }
}

Lesson 6: Multi-Window App — Window Visibility Control

Windows declared in script_mod! auto-show on startup. Makepad WindowRef has no close()/open() method.

Working pattern: Declare windows at normal size. Use configure_window() to bring to front when needed.

// Show: configure_window triggers makeKeyAndOrderFront on macOS
let settings = self.ui.window(cx, ids!(settings_window));
settings.configure_window(cx, dvec2(480.0, 560.0), dvec2(500.0, 200.0), false, "Settings".into());

// Hide: resize to 1x1 (no close/minimize on WindowRef)
let capsule = self.ui.window(cx, ids!(capsule_window));
capsule.resize(cx, dvec2(1.0, 1.0));

Note: reposition(cx, dvec2(-9999, -9999)) does NOT reliably hide macOS floating panels.

Lesson 7: new_batch: true and Widget Z-Order

Adding a LoadingSpinner (or any child widget with its own draw shader) inside a custom-shader View can cause bleed-through at the edges — the child widget draws outside the parent's SDF mask.

Fix: Either:

1. Draw animation in the parent's own pixel shader (no child widget z-order issues)
2. Use clip_x: true + clip_y: true on the parent (may not fully solve it)

Lesson 8: Continuous Redraw for Time-Based Shader Animation

self.draw_pass.time in a shader only advances when the widget is redrawn. Without explicit redraw, time freezes.

fn handle_next_frame(&mut self, cx: &mut Cx, _e: &NextFrameEvent) {
    if self.inner.state != STATE_IDLE {
        // Redraw the WINDOW (not just the view) to update draw_pass.time
        self.ui.widget(cx, ids!(my_window)).redraw(cx);
        self.inner.next_frame = cx.new_next_frame();
    }
}

Key: Redraw the Window widget, not just the inner View. The draw pass time is per-window.

Lesson 9: #[rust] Fields with Complex Types in Script-Derived Structs

#[derive(Script, ScriptHook)] structs can only have #[rust] fields whose types implement Default. For complex non-Default types (channels, handles, etc.), wrap them in an Inner struct:

#[derive(Default)]
struct Inner {
    timer: Timer,
    rx: Option<crossbeam_channel::Receiver<u64>>,
    handle: Option<SomeNonDefaultHandle>,
}

#[derive(Script, ScriptHook)]
pub struct App {
    #[live] ui: WidgetRef,
    #[rust] inner: Inner,    // Single Default-able wrapper
}