# Your First Component

This page is intended for developers of Crystal Toolkit who want to add a new `MPComponent` to support interactive viewing of an `MSONable` object, such as crystal structures, phase diagrams, etc.

Important

Crystal Toolkit is built on top of Dash by Plotly. Dash is a framework to build rich, interactive web apps purely from Python with no JavaScript knowledge required. If you’re not familiar with the Dash framework, please read their documentation and try their example app first.

The basic steps are to:

1. Sub-class MPComponent. Recognize that the component comes with a `dcc.Store` which contains the `MSON` representation of whatever the component is intended to render.

2. Implement its master `layout()` method to return the “default” layout for your component. You may also add additional `..._layout()` methods for optional layouts; for example, the `StructureMoleculeComponent` has `legend_layout()` to return the legend for the structure or molecule being displayed.

   The `_sub_layouts` method, which returns a dictionary with values corresponding to different parts of your component, is available for book-keeping if your component is complex.

4. For interactivity, implement the `generate_callbacks(app, cache)` method. Inside this method you can define callbacks to be associated with the component. At minimum, make sure that if the component’s main `dcc.Store` is updated with a new object (for example, a new crystal structure is set for `StructureMoleculeComponent`) that the state of the component is also updated appropriately.

   Try to keep the total number of callbacks to a minimum, and the size of the data stores small wherever possible, to improve performance of your component. Using multiple-output callbacks, and judicious use of `raise PreventUpdate` and `return no_update` can help with this, especially if checking for the initial callback firing on page load.

   If you have a long-running callback, or one that may be long-running, make sure to wrap it in a `dcc.Loading` component to improve the user experience.

At this point, the component should be working. It needs a simple example app to be added to `crystal_toolkit.apps.examples` and a corresponding test that the example app runs, and finally a short documentation page in `docs_rst`. Please copy from the existing examples as necessary.