Nemo Plugin Patterns

Use this skill when creating, modifying, or debugging Nemo plugins (native or WASM).

Three-Tier Extension Model

XML (declarative) → Rhai scripts (lightweight logic) → Native/WASM plugins (full power)

PluginContext API

Available to all plugins at runtime:

trait PluginContext: Send + Sync {
    fn get_data(&self, path: &str) -> Option<PluginValue>;
    fn set_data(&self, path: &str, value: PluginValue) -> Result<(), PluginError>;
    fn emit_event(&self, event_type: &str, payload: PluginValue);
    fn get_config(&self, path: &str) -> Option<PluginValue>;
    fn log(&self, level: LogLevel, message: &str);
    fn get_component_property(&self, component_id: &str, property: &str) -> Option<PluginValue>;
    fn set_component_property(&self, component_id: &str, property: &str, value: PluginValue) -> Result<(), PluginError>;
}

PluginValue Types

enum PluginValue {
    Null, Bool(bool), Integer(i64), Float(f64),
    String(String), Array(Vec<PluginValue>),
    Object(HashMap<String, PluginValue>),
}

Native Plugin Pattern

Cargo.toml

[package]
name = "my-plugin"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]  # CRITICAL — must be cdylib

[dependencies]
nemo-plugin-api = { path = "../../crates/nemo-plugin-api" }
semver = { workspace = true }

src/lib.rs

use nemo_plugin_api::*;

fn init(registrar: &mut dyn PluginRegistrar) {
    let ctx = registrar.context_arc();
    ctx.log(LogLevel::Info, "Plugin initialized");

    // Register capabilities
    // registrar.register_data_source("name", schema);
    // registrar.register_transform("name", schema);
    // registrar.register_action("name", schema);

    // For background data production:
    std::thread::spawn(move || {
        loop {
            std::thread::sleep(std::time::Duration::from_secs(2));
            let _ = ctx.set_data("my.value", PluginValue::Float(42.0));
        }
    });
}

declare_plugin!(
    PluginManifest::new("my-plugin", "My Plugin", semver::Version::new(0, 1, 0))
        .with_description("What it does")
        .with_capability(Capability::DataSource("my".to_string())),
    init
);

Build

cargo build -p my-plugin --release
# Output: target/release/libmy_plugin.dylib (macOS) / .so (Linux) / .dll (Windows)

Lifecycle

1. ExtensionLoader scans plugin directories for .dylib/.so/.dll
2. PluginHost calls libloading::Library::new(path)
3. Host calls nemopluginmanifest() → gets manifest
4. Host calls nemopluginentry(registrar) → plugin registers + starts
5. Plugin uses PluginContext via registrar.context_arc() throughout lifetime

WASM Plugin Pattern

Cargo.toml

[package]
name = "my-wasm-plugin"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]  # CRITICAL — must be cdylib

[dependencies]
nemo-wasm-guest = { path = "../../crates/nemo-wasm-guest" }
wit-bindgen = { workspace = true }

src/lib.rs

wit_bindgen::generate!({
    path: "../../crates/nemo-wasm/wit",
    world: "nemo-plugin",
});

struct MyPlugin;

impl Guest for MyPlugin {
    fn get_manifest() -> PluginManifest {
        PluginManifest {
            id: "my-wasm-plugin".to_string(),
            name: "My WASM Plugin".to_string(),
            version: "0.1.0".to_string(),
            description: "What it does".to_string(),
        }
    }

    fn init() {
        host_api::log(LogLevel::Info, "WASM plugin initialized");
    }

    fn tick() -> u64 {
        // Do periodic work here
        let value = PluginValue::Float(42.0);
        let _ = host_api::set_data("my.value", value);

        2000  // Return ms until next tick. 0 = stop ticking.
    }
}

export_nemo_plugin!(MyPlugin);

Build

cargo component build -p my-wasm-plugin --release
# Output: target/wasm32-wasip1/release/my_wasm_plugin.wasm

Lifecycle

1. ExtensionLoader scans for .wasm files
2. WasmHost creates Component::from_binary() via wasmtime
3. Host calls get-manifest() export
4. Host calls init() export
5. Host calls tick() at plugin-specified intervals
6. Plugin returns ms until next tick (0 = stop)

Key Difference: Tick-Based vs Threaded

• WASM: No threads. Host drives execution via tick(). Return interval in ms.
• Native: Full thread access. Can std::thread::spawn for background work.

Rhai Script Pattern

Script File

// scripts/handlers.rhai

fn on_button_click(component_id, event_data) {
    log_info("Clicked: " + component_id);
    let value = get_data("some.path");
    set_component_text("label_id", "Updated: " + value);
}

fn on_input_change(component_id, event_data) {
    set_data("search.query", event_data);
}

Handler calling convention. Every XML-referenced handler (on-click,
on-change, on-load, …) is invoked as fn name(componentid, eventdata) —
two string arguments. Rhai resolves functions by name and arity, so a
mismatch (e.g. a zero-parameter handler) fails at runtime with
Function not found: <name>. Match the two-argument signature exactly. This
includes on-load, which is dispatched with component_id = "app" and
event_data = "load".

Available Rhai Functions

Data: getdata(path), setdata(path, value), getconfig(path) Components: getcomponentproperty(id, prop), setcomponentproperty(id, prop, value), getcomponentlabel(id), getcomponenttext(id), setcomponenttext(id, text), setcomponentlabel(id, label) HTTP: httpget(url), httppost(url, body), httpput(url, body), httpdelete(url) — synchronous, blocking; return {status, body, ok} on success or {error} on failure. Intended for UI event handlers (e.g. on-click), not async data-source contexts. JSON: jsonparse(str) → parsed value, jsonstringify(value) → string Math: abs, min, max, clamp, floor, ceil, round, sqrt, pow Strings: trim, toupper, tolower, startswith, endswith, contains, replace Type conversion: parsefloat, parseint, tostring, toint, tofloat Logging: logdebug, loginfo, logwarn, logerror, print

Sandboxing Limits

• Max operations: 100,000
• Max string size: 64KB
• Max array/map size: 10,000
• Max call stack depth: 64
• No eval, no file I/O. Network access IS available via the http_* functions above (file/system access remain gated off by default).

Plugin Capabilities

enum Capability {
    Component(String),     // Register a new component type
    DataSource(String),    // Register a data source
    Transform(String),     // Register a data transform
    Action(String),        // Register an action
    EventHandler(String),  // Register an event handler
}

Plugin Permissions

struct PluginPermissions {
    network: bool,          // Can make network requests
    filesystem: bool,       // Can access filesystem
    system: bool,           // Can access system resources
    data_read: bool,        // Can read from DataRepository
    data_write: bool,       // Can write to DataRepository
    component_read: bool,   // Can read component properties
    component_write: bool,  // Can modify component properties
    event_emit: bool,       // Can emit events
}

XML Plugin Configuration

<nemo>
  <plugin name="my-plugin" path="./plugins/my-plugin" />
  <!-- or for WASM: -->
  <plugin name="my-wasm-plugin" path="./plugins/my-wasm-plugin.wasm" />
</nemo>

Workspace Setup

Add to root Cargo.toml:

[workspace]
members = [
    # ...existing members...
    "plugins/my-plugin",
]

Debugging Plugins

1. Plugin not loading → Check crate-type = ["cdylib"], verify file extension matches platform
2. Manifest errors → Check declare_plugin! macro or WIT get-manifest return
3. Context API failures → Check that PluginContext methods are called correctly
4. WASM type issues → Complex types (arrays/objects) use JSON serialization in WIT bridge
5. Extension discovery → Check nemo-extension/src/loader.rs for path scanning logic