RENDERER.md

Dash Renderer

The dash-renderer is the TypeScript/React frontend that powers Dash applications. Located in dash/dash-renderer/src/.

Initialization Flow

1. DashRenderer constructor
   └─ ReactDOM.createRoot('#react-entry-point')
      └─ <AppProvider />

2. AppProvider creates Redux store
   └─ Registers observers for callback processing

3. APIController fetches from server
   ├─ GET /_dash-layout → component tree
   ├─ GET /_dash-dependencies → callback definitions
   └─ Dispatches setLayout, setGraphs, setPaths

4. Config includes children_props from ComponentRegistry
   └─ Stored in window.__dashprivate_childrenProps

5. hydrateInitialOutputs()
   ├─ Validates callbacks against layout
   ├─ Triggers initial callbacks
   └─ Sets appLifecycle: 'HYDRATED'

6. DashWrapper renders component tree

Layout Traversal (crawlLayout)

The layout is traversed using crawlLayout (actions/utils.js). This algorithm:

1. For arrays: Iterates each child, following extra paths for nested components
2. For objects (components):
   • Applies the visitor function to the component
   • Follows props.children if present
   • Follows additional paths from children_props config

children_props

Each component class defines _children_props listing props that contain nested components. This is:

1. Generated from React component PropTypes/TypeScript during component generation
2. Stored on the Python component class as _children_props
3. Collected into ComponentRegistry.children_props when components are imported
4. Sent to frontend via config (dash.py:933)
5. Stored in window.__dashprivate_childrenProps on the frontend

# Example: Python component class
class Dropdown(Component):
    _children_props = ['options.[].label', 'options.[].title']
    # ...

Pattern Syntax

Pattern	Meaning	Example
children	Direct children prop	Standard
prop.[]	Array items are components	options.[]
prop.[].sub	sub prop of array items	options.[].label
prop.{}	Object values are components	Dynamic keys
prop.{}.sub	sub prop of object values	Nested dynamic

How crawlLayout Works

crawlLayout(object, func, currentPath, extraPath)

// For each component:
// 1. Call func(object, currentPath)
// 2. If props.children exists, crawl it
// 3. For each path in children_props[namespace][type]:
//    - Parse the pattern (handle [], {})
//    - Crawl that path to find nested components

Component Resolution

Components are resolved from window[namespace][type]:

// Component packages register on window
window.dash_html_components = { Div, Span, H1, ... };
window.dash_core_components = { Dropdown, Graph, Input, ... };

// Registry.js resolves {type, namespace} → React component
Registry.resolve({type: 'Div', namespace: 'dash_html_components'})
// → window.dash_html_components.Div

Callback Triggering

Callbacks are triggered by two sources:

1. Component setProps

When a component calls setProps, it triggers callbacks watching those props:

// Component calls setProps
this.props.setProps({ value: newValue });

// DashWrapper.tsx handles this:
// 1. dispatch(updateProps(...))     → Updates layout in Redux
// 2. dispatch(notifyObservers(...)) → Finds and queues callbacks

notifyObservers calls includeObservers to find callbacks with matching inputs:

// actions/index.js
export function notifyObservers({id, props}) {
  return async function (dispatch, getState) {
    const {graphs, paths} = getState();
    dispatch(
      addRequestedCallbacks(includeObservers(id, props, graphs, paths))
    );
  };
}

2. Callback Results

When a callback completes and updates component props, it also triggers dependent callbacks via includeObservers in the executedCallbacks observer.

Callback Processing

Once callbacks are added to the queue, observers process them through states:

REQUESTED → PRIORITIZED → EXECUTING → EXECUTED → STORED
                              ↓
                          WATCHED (promises)
                              ↓
                          BLOCKED (waiting on deps)

Observer Chain

1. requestedCallbacks: Deduplicates, checks dependencies, moves ready → prioritized
2. prioritizedCallbacks: Sorts by priority, executes (max 12 concurrent)
3. executingCallbacks: Tracks running callbacks, handles promises
4. executedCallbacks: Applies results to layout, triggers dependent callbacks
5. isLoading: Tracks loading state for dcc.Loading

Redux Store

Key Slices

{
  layout: { ... },              // Component tree

  layoutHashes: {               // Change tracking for memoization
    "path": { hash, changedProps }
  },

  paths: {                      // ID → path mapping
    strs: { "my-id": [...path] },
    objs: { "type,index": [...] }  // Wildcards
  },

  callbacks: {                  // Pipeline states
    requested, prioritized, blocked,
    executing, watched, executed, stored
  },

  graphs: {                     // Dependency graph
    inputMap: { "id": { "prop": [callbacks] } }
  },

  config: {
    children_props: { ... },    // From ComponentRegistry
    // ...
  },

  isLoading: boolean
}

Paths System

Maps component IDs to their location in layout:

// String IDs
paths.strs["my-dropdown"] = ["layout", "props", "children", 2, "props"]

// Wildcard IDs (pattern-matching)
paths.objs["type,index"] = [
  { values: ["filter", 0], path: [...] },
  { values: ["filter", 1], path: [...] }
]

window.dash_clientside

The clientside callback API (utils/clientsideFunctions.ts):

window.dash_clientside = {
  no_update,        // Return to skip output
  PreventUpdate,    // Throw to cancel callback
  callback_context, // Current callback info
  set_props,        // Update props from clientside
  clean_url,        // URL sanitization
  Patch             // Partial prop updates
}

callback_context

Available during callback execution:

window.dash_clientside.callback_context = {
  triggered: [{ prop_id: "btn.n_clicks", value: 1 }],
  triggered_id: "btn",
  inputs: { "input.value": "hello" },
  states: { "store.data": {...} }
}

Registering Clientside Functions

// In assets/clientside.js
window.dash_clientside = window.dash_clientside || {};
window.dash_clientside.my_namespace = {
  my_function: function(input_value) {
    return input_value.toUpperCase();
  }
};

set_props

Update component props directly from clientside:

// By string ID
window.dash_clientside.set_props('my-component', { value: 'new' });

// By pattern-matching ID
window.dash_clientside.set_props({ type: 'input', index: 0 }, { value: 'new' });

window.dash_component_api

API for components to interact with Dash (dashApi.ts):

window.dash_component_api = {
  ExternalWrapper,     // Render outside main tree
  DashContext,         // React Context
  useDashContext,      // Hook for context
  getLayout,           // Get props by ID/path
  stringifyId          // Convert wildcard IDs
}

getLayout

Retrieve component props:

const props = window.dash_component_api.getLayout('my-dropdown');
// → { id: 'my-dropdown', options: [...], value: 'a' }

useDashContext

Hook for components:

const {
  componentPath,
  isLoading,
  useSelector,
  useDispatch
} = useDashContext();

Memoization

DashWrapper uses hash-based memoization. layoutHashes tracks which components changed:

layoutHashes["0,props,children"] = {
  hash: 42,                      // Increments on change
  changedProps: { value: true }
}

Components only re-render when their hash changes.

Key Files

File	Purpose
DashRenderer.js	Entry point
AppProvider.react.tsx	Redux store setup
APIController.react.js	Fetches layout, hydrates app
wrapper/DashWrapper.tsx	Component rendering, setProps
actions/utils.js	crawlLayout algorithm
registry.js	Component resolution
reducers/layout.js	Layout state
reducers/callbacks.ts	Callback pipeline
reducers/config.js	Stores children_props
actions/index.js	notifyObservers
actions/dependencies_ts.ts	includeObservers, callback matching
observers/*.ts	Callback processing
utils/clientsideFunctions.ts	Clientside API
dashApi.ts	Component API