Slint Development Skill

Use this skill when building, debugging, or reviewing applications that use Slint, a declarative GUI toolkit for native user interfaces across desktop, embedded, mobile, and web platforms.

When to Use This Skill

Use this skill when the task involves:

• Writing or debugging .slint files
• Integrating Slint with Rust, C++, JavaScript, or Python
• Investigating layout, binding, rendering, or event-handling issues
• Enabling the Slint MCP server for runtime inspection and UI debugging
• Explaining or reviewing Slint-specific code patterns

How to Help

When using this skill:

• Prefer idiomatic Slint patterns over manual UI workarounds
• Match guidance to the user's language binding and Slint version
• Watch for common pitfalls such as binding loops, missing layout constraints, and type mismatches
• Suggest the MCP server when runtime inspection or interaction would make debugging easier
• Prefer solutions that preserve Slint's declarative and reactive model

The .slint Language

Slint UIs are written in .slint markup files. The language is declarative and reactive.

Naming Conventions

Follow these project naming conventions when writing .slint and related Rust files:

1. Components and Structs: Use PascalCase for component names and struct types (e.g., QueuePage, SongModel, TextInputGroup, ModalSize).
2. Properties, Variables, Callbacks, and Functions: All Slint variables, properties, callbacks, and functions MUST be snake_case and NEVER kebab-case (e.g., in property <bool> is_active;, in-out property <string> new_playlist_name;, callback remove_clicked();, save_queue_as_playlist(string)).
3. Rust Function Names: Follow standard Rust snake_case, keep names simple and obvious, and avoid repeating struct/module context in function names (e.g., save_queue rather than save_queue_page_queue_songs).

Project Setup

Rust

# BUILD.bazel
load("@rules_rust//rust:defs.bzl", "rust_binary")
load("@rules_rust//cargo:cargo_build_script.bzl", "cargo_build_script")

cargo_build_script(
    name = "build_script",
    srcs = ["build.rs"],
    # Ensure the build script watches the .slint files for rebuilds
    data = glob(["ui/**/*.slint"]),
    deps = [
        "@crates//:slint-build",
    ],
)

rust_binary(
    name = "my_app",
    srcs = ["main.rs"],
    deps = [
        ":build_script",
        "@crates//:slint",
    ],
)

// build.rs
fn main() {
    slint_build::compile("ui/main.slint").unwrap();
}

// main.rs
slint::include_modules!();

fn main() -> Result<(), slint::PlatformError> {
    let app = MainWindow::new()?;
    // Set up callbacks, models, etc.
    app.run()
}

Debugging Slint Applications

Common Issues

1. Binding loops: A property depends on itself through a chain of bindings. The compiler warns about these. Break the cycle by introducing an intermediate property or restructuring.
2. Elements not visible: Check width, height (may be 0 if not in a layout), visible, opacity, and parent clipping.
3. Layout sizing: Elements outside layouts need explicit width/height. Inside layouts, they get sized automatically. Use preferred-width, min-width, max-width to constrain.
4. Type mismatches: length and int/float are different types. Use 1px * my_int to convert, or my_length / 1px to get a number.
5. Performance: Use ListView (not for in ScrollView) for long lists because it virtualizes. Use image-rendering: pixelated only when needed. Avoid deeply nested opacity or clip layers.

Debug Helpers

• debug("message", expression) prints to stderr at runtime
• SLINT_DEBUG_PERFORMANCE=refresh_lazy,console enables performance diagnostics
• Run with SLINT_BACKEND=winit-skia or other backend variants for testing

MCP Server for AI-Assisted Debugging

Slint includes an embedded MCP (Model Context Protocol) server that lets you inspect and interact with a running Slint application in real time. The server provides tools for exploring the UI tree, taking screenshots, clicking elements, dragging, typing, and more.

Once enabled, an AI coding assistant can connect to the MCP endpoint to inspect and interact with the running UI.

Enabling the MCP Server

Step 1: Build with SLINT_EMIT_DEBUG_INFO=1 so that element IDs and source locations are preserved in the compiled output. Without this, elements will lack the debug metadata needed for meaningful introspection. Set SLINT_MCP_PORT to an available port when running via Bazel:

SLINT_EMIT_DEBUG_INFO=1 SLINT_MCP_PORT=9315 bazel run //:my_app

In a Bazel setup, ensure the mcp feature is enabled for the slint crate within your workspace dependencies (e.g., via crate.annotation in bzlmod or annotations in crates_repository). Since Bazel does not have a direct equivalent to Cargo's ad-hoc --features command-line flag, you must configure it ahead of time in your build graph.

Step 2: Connect to the running application's MCP server at http://localhost:9315/mcp using Streamable HTTP transport and use the available tools to inspect and interact with the UI.

When scripting or verifying the server from the command line, use curl — it is the most reliable approach for raw JSON-RPC. Prefer curl over built-in HTTP fetch tools, which agents sometimes reach for but which are less predictable for this use case:

# Initialize (confirms the server is up and prints available tools)
curl -s -X POST http://127.0.0.1:9315/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'

# List windows
curl -s -X POST http://127.0.0.1:9315/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"list_windows","arguments":{}}}'

# Take a screenshot (response contains a base64-encoded PNG in the "data" field)
curl -s -X POST http://127.0.0.1:9315/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"take_screenshot","arguments":{"windowHandle":{"index":"1","generation":"1"}}}}'

Version Requirements

Slint Version	MCP Support
< 1.17.0	Not available
>= 1.17.0	Enable via the mcp crate feature in your Bazel Rust dependencies configuration

When to Suggest MCP

Suggest enabling the MCP server when the user is:

• Debugging layout or visual issues
• Trying to understand the runtime element hierarchy
• Testing interactions programmatically
• Verifying accessibility properties
• Diagnosing event handling problems

Translations & Internationalization (i18n)

Annotating Strings in .slint Files

Use @tr(...) to mark user-facing string literals for translation:

// Simple text
Text { text: @tr("Songs"); }

// String formatting
Text { text: @tr("Adjust the color for {}", item.name); }

// Plural forms
Text { text: @tr("I have {n} item" | "I have {n} items" % count); }

Extracting Translatable Strings

A Bazel rule is provided to extract all @tr(...) strings into ui/slint/locales/en_US/LC_MESSAGES/slint_app.po:

# Extract strings across all Slint files
bazel run //tools:extract_translations

# Or extract from specific Slint files
bazel run //tools:extract_translations -- ui/slint/src/app.slint

Note: This rule uses slint-tr-extractor (installable via cargo install slint-tr-extractor).

Bundled Translations Directory Structure

Translation catalogs follow the standard Gettext hierarchy under ui/slint/locales/:

ui/slint/locales/
├── en_US/LC_MESSAGES/slint_app.po
├── de_DE/LC_MESSAGES/slint_app.po
├── es_ES/LC_MESSAGES/slint_app.po
├── fr_FR/LC_MESSAGES/slint_app.po
└── ...

Build & Bundling Setup

1. build.rs:

   let config = slint_build::CompilerConfiguration::new()
       .with_bundled_translations("locales")
       .with_default_translation_context(slint_build::DefaultTranslationContext::None);
   slint_build::compile_with_config("src/app.slint", config).unwrap();
   

2. ui/slint/BUILD:

   • build_script_env must specify "CARGO_PKG_NAME": "slint_app".
   • data in cargo_build_script and compile_data in rust_library must include "locales/**/*.po".

Documentation Reference

Full documentation for the latest version is at https://slint.dev/docs. Key sections:

• Language guide: concepts, syntax, and coding patterns
• Reference: elements, properties, types, and standard widgets
• Translations: https://docs.slint.dev/latest/docs/slint/guide/development/translations/
• Language integrations: Rust, C++, Node.js, and Python API docs
• Tutorials: step-by-step guides for each language

The documentation can be found at https://snapshots.slint.dev/master/docs/slint/.