A callback with Custom Components?

whitphx · October 21, 2021, 4:22pm

Are there any way to implement the on_change callback with Custom Components?

whitphx · November 16, 2021, 11:37am

I understood it is currently impossible then I created a feature request:

github.com/streamlit/streamlit

A callback on a custom component

opened 01:21PM - 27 Oct 21 UTC

whitphx

enhancement

### Problem I am a developer of a custom component, [streamlit-webrtc](https:…//github.com/whitphx/streamlit-webrtc), which manages its state in Python code, and there is [a request](https://github.com/whitphx/streamlit-webrtc/issues/470) to make it possible to detect state changes through the `on_change` callback. However, currently a callback on a custom component is not supported. ### Solution * To introduce a mechanism to allow component developers to register and trigger a callback from a custom component. * It must not be an "automatic" trigger reacting to the changes of the frontend component state or session_state, such as a callback automatically invoked when the return value of `_component_func` or `session_state[key]` changes. * As stated above, my component manages the state on Python side and the component value differs from the returned value from the frontend component. Additionally, a value stored in the session state of its key is a mutable object so that it cannot be used to detect changes. So, I want a way to manually invoke the callback. * Ideally, it might be the best if we have a way to set arbitrary values to `WidgetMetadata.callback`, `WidgetMetadata.callback_args`, `WidgetMetadata.callback_kwargs`, and to forcefully call `call_callback(wid)` for a specific component. ### Additional context * Related feature request to my component: https://github.com/whitphx/streamlit-webrtc/issues/470

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A callback with Custom Components?

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