dash-startup-loading-plugin

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An installable Dash Hooks plugin that replaces Dash's initial loading presentation with a configurable full-screen overlay.

The plugin injects its CSS and JavaScript into Dash's normal index document before React mounts. Applications do not need to copy assets or replace index_string, and Dash's built-in <div class="_dash-loading"> remains in the document.

Requirements

• Python 3.9 or later
• Dash 3.0.3 or later

Installation

pip install "dash-startup-loading-plugin>=1.0.2"

Dash discovers the plugin through its dash_hooks entry point. Installing the package enables the default loading overlay without an explicit import.

When Dash Ant Design (dash_antd_components) is installed, the plugin detects it automatically and applies matching light and dark loading backgrounds.

Quick start

The default configuration requires no plugin-specific code:

from dash import Dash, html

app = Dash(__name__)
app.layout = html.Main(
    [
        html.H1("My Dash app"),
        html.P("The overlay closes after this layout is ready."),
    ]
)

if __name__ == "__main__":
    app.run(debug=True)

Call configure() before creating Dash when custom behavior is needed:

from dash import Dash, html
from dash_startup_loading_plugin import configure

configure(
    required_selectors=["#header", "#sidebar-menu"],
    pending_selector="[data-async-placeholder]",
    timeout_ms=6000,
    minimum_display_ms=250,
    fade_duration_ms=180,
)

app = Dash(__name__)
app.layout = html.Main(
    [
        html.Header("Header", id="header"),
        html.Nav("Sidebar", id="sidebar-menu"),
    ]
)

Component-library integrations

Component libraries are not dependencies of this package. Install only the libraries used by the application.

Dash Ant Design

Dash Ant Design is detected automatically. configure_dac() is only required to override its defaults:

pip install dash-ant-design

The plugin does not impose a Dash Ant Design version constraint. Use the version compatible with the application's Python and Dash versions.

from dash_startup_loading_plugin import configure_dac

configure_dac(
    background="#f5f5f5",
    dark_background="#202020",
)

Dash Mantine Components

configure_dmc() uses Mantine's active default theme and registers its pre-render color-scheme hook:

pip install dash-mantine-components

from dash_startup_loading_plugin import configure_dmc

configure_dmc()

feffery-antd-components

Use configure_fac() to match the loading overlay to AntdConfigProvider:

pip install feffery-antd-components

The plugin does not pin a feffery-antd-components version. Compatibility is determined by the installed component library.

from dash_startup_loading_plugin import configure_fac

configure_fac(required_selectors=["#fac-app-ready"])

Installed examples

The package includes four runnable examples:

# Dash
dash-startup-loading-plugin examples.dash

# Dash Mantine Components
dash-startup-loading-plugin examples.dash-mantine-components

# Dash Ant Design
dash-startup-loading-plugin examples.dash-ant-design

# feffery-antd-components
dash-startup-loading-plugin examples.feffery-antd-components

Install the selected example's component library separately. If it cannot be imported, the command reports the failed module and the corresponding installation command.

Server options are available on every example:

dash-startup-loading-plugin examples.dash \
    --host 127.0.0.1 --port 8050 --debug

Readiness behavior

The overlay closes when:

1. root_selector exists and no longer contains ._dash-loading.
2. The root contains rendered content.
3. Every required_selectors entry exists.
4. No pending_selector node remains under the root.
5. The conditions remain true for two animation frames.

timeout_ms is a forced-dismiss fallback. minimum_display_ms applies to ready and manual dismissal, but does not delay a timeout.

pending_selector delays dismissal while any matching element remains inside root_selector. It is useful for lazy or asynchronous placeholders that are mounted before the real content. Set it to an application-specific CSS selector, or use None when no pending-node check is needed:

configure(pending_selector="[data-async-placeholder]")
configure(pending_selector=None)

Configuration

configure(**changes) updates the process-wide immutable StartupLoadingConfig.

Option	Default	Description
enabled	True	Enable index injection.
overlay_id	"dash-loading"	Injected overlay ID.
aria_label	"Loading"	Accessible status label.
root_selector	"#react-entry-point"	Root observed for rendered content.
required_selectors	("#react-entry-point",)	Selectors that must exist before dismissal.
pending_selector	"[data-dac-async-placeholder]"	Selector checked under root_selector; dismissal waits until all matches disappear. Use None to disable.
timeout_ms	6000	Forced-dismiss timeout; use None to disable.
minimum_display_ms	0	Minimum display time.
fade_duration_ms	160	Fade-out duration.
z_index	9999	Overlay stacking order.
background	"#ffffff"	Light background.
dark_background	"#0f0f0f"	Dark background.
color	"#1677ff"	Light spinner color.
dark_color	"#4096ff"	Dark spinner color.
theme_mode	"auto"	"auto", "light", or "dark".
dash_theme_component_id	None	Preferred persisted Dash theme component.
spinner_size_px	28	Spinner width and height.
spinner_stroke_px	3	Spinner stroke width.
hide_default_loading	True	Hide the visual ._dash-loading indicator while the overlay exists.
custom_loader_html	None	Trusted HTML replacing the default spinner.

custom_loader_html is inserted verbatim and must never contain untrusted user input.

Python API

from dash_startup_loading_plugin import (
    StartupLoadingConfig,
    configure,
    configure_dac,
    configure_fac,
    configure_dmc,
    get_config,
    reset_config,
)

Browser API

// Recheck readiness.
window.dashLoading.check();

// Dismiss the default or a custom overlay.
window.dashLoading.finish();
window.dashLoading.finish("my-loading-overlay");

Before fading out, the overlay emits a bubbling dash-loading:ready event. event.detail.reason is "ready", "timeout", or "manual".

document.addEventListener("dash-loading:ready", function (event) {
    console.log(event.detail.reason);
});

Notes

• Dash's hook and plugin configuration is process-wide. Use one configuration per process.
• Resources are inlined, so strict Content Security Policy deployments must allow the injected style and script.
• The overlay is only for initial application startup. Use dcc.Loading or another callback-specific pattern for later callback execution.

License

MIT