Dynamic Apps with the Render Decorator

The components and event listeners you define in a Blocks so far have been fixed - once the demo was launched, new components and listeners could not be added, and existing one could not be removed.

The @gr.render decorator introduces the ability to dynamically change this. Let's take a look.

Dynamic Number of Components

In the example below, we will create a variable number of Textboxes. When the user edits the input Textbox, we create a Textbox for each letter in the input. Try it out below:

import gradio as gr

with gr.Blocks() as demo:
    input_text = gr.Textbox(label="input")

    @gr.render(inputs=input_text)
    def show_split(text):
        if len(text) == 0:
            gr.Markdown("## No Input Provided")
        else:
            for letter in text:
                gr.Textbox(letter)

demo.launch()

See how we can now create a variable number of Textboxes using our custom logic - in this case, a simple for loop. The @gr.render decorator enables this with the following steps:

Create a function and attach the @gr.render decorator to it.
Add the input components to the inputs= argument of @gr.render, and create a corresponding argument in your function for each component. This function will automatically re-run on any change to a component.
Add all components inside the function that you want to render based on the inputs.

Now whenever the inputs change, the function re-runs, and replaces the components created from the previous function run with the latest run. Pretty straightforward! Let's add a little more complexity to this app:

import gradio as gr

with gr.Blocks() as demo:
    input_text = gr.Textbox(label="input")
    mode = gr.Radio(["textbox", "button"], value="textbox")

    @gr.render(inputs=[input_text, mode], triggers=[input_text.submit])
    def show_split(text, mode):
        if len(text) == 0:
            gr.Markdown("## No Input Provided")
        else:
            for letter in text:
                if mode == "textbox":
                    gr.Textbox(letter)
                else:
                    gr.Button(letter)

demo.launch()

By default, @gr.render re-runs are triggered by the .load listener to the app and the .change listener to any input component provided. We can override this by explicitly setting the triggers in the decorator, as we have in this app to only trigger on input_text.submit instead. If you are setting custom triggers, and you also want an automatic render at the start of the app, make sure to add demo.load to your list of triggers.

Dynamic Event Listeners

If you're creating components, you probably want to attach event listeners to them as well. Let's take a look at an example that takes in a variable number of Textbox as input, and merges all the text into a single box.

import gradio as gr

with gr.Blocks() as demo:
    text_count = gr.State(1)
    add_btn = gr.Button("Add Box")
    add_btn.click(lambda x: x + 1, text_count, text_count)

    @gr.render(inputs=text_count)
    def render_count(count):
        boxes = []
        for i in range(count):
            box = gr.Textbox(key=i, label=f"Box {i}")
            boxes.append(box)

        def merge(*args):
            return " ".join(args)
        
        merge_btn.click(merge, boxes, output)


    merge_btn = gr.Button("Merge")
    output = gr.Textbox(label="Merged Output")

demo.launch()

Let's take a look at what's happening here:

The state variable text_count is keeping track of the number of Textboxes to create. By clicking on the Add button, we increase text_count which triggers the render decorator.
Note that in every single Textbox we create in the render function, we explicitly set a key= argument. This key allows us to preserve the value of this Component between re-renders. If you type in a value in a textbox, and then click the Add button, all the Textboxes re-render, but their values aren't cleared because the key= maintains the the value of a Component across a render.
We've stored the Textboxes created in a list, and provide this list as input to the merge button event listener. Note that all event listeners that use Components created inside a render function must also be defined inside that render function. The event listener can still reference Components outside the render function, as we do here by referencing merge_btn and output which are both defined outside the render function.

Just as with Components, whenever a function re-renders, the event listeners created from the previous render are cleared and the new event listeners from the latest run are attached.

This allows us to create highly customizable and complex interactions!

Putting it Together

Let's look at two examples that use all the features above. First, try out the to-do list app below:

import gradio as gr

with gr.Blocks() as demo:
    
    tasks = gr.State([])
    new_task = gr.Textbox(label="Task Name", autofocus=True)

    def add_task(tasks, new_task_name):
        return tasks + [{"name": new_task_name, "complete": False}], ""

    new_task.submit(add_task, [tasks, new_task], [tasks, new_task])

    @gr.render(inputs=tasks)
    def render_todos(task_list):
        complete = [task for task in task_list if task["complete"]]
        incomplete = [task for task in task_list if not task["complete"]]
        gr.Markdown(f"### Incomplete Tasks ({len(incomplete)})")
        for task in incomplete:
            with gr.Row():
                gr.Textbox(task['name'], show_label=False, container=False)
                done_btn = gr.Button("Done", scale=0)
                def mark_done(task=task):
                    _task_list = task_list[:]
                    _task_list[task_list.index(task)] = {"name": task["name"], "complete": True}
                    return _task_list
                done_btn.click(mark_done, None, [tasks])

                delete_btn = gr.Button("Delete", scale=0, variant="stop")
                def delete(task=task):
                    task_index = task_list.index(task)
                    return task_list[:task_index] + task_list[task_index+1:]
                delete_btn.click(delete, None, [tasks])

        gr.Markdown(f"### Complete Tasks ({len(complete)})")
        for task in complete:
            gr.Textbox(task['name'], show_label=False, container=False)




demo.launch()

Note that almost the entire app is inside a single gr.render that reacts to the tasks gr.State variable. This variable is a nested list, which presents some complexity. If you design a gr.render to react to a list or dict structure, ensure you do the following:

Any event listener that modifies the state variable in a manner that should trigger a re-render should return a new value, not simply modify the state variable directly. A gr.State change event listener will not trigger if the state is provided as input and then modified directly. A new variable must be returned that can be compared against. That's why in the mark_done method, we create a new list that we edit. Similarly, in delete, we create a new list instead of calling task_list.remove(task), and in add_task, we don't use task_list.append.
In a gr.render, if a variable in a loop is used inside an event listener function, that variable should be "frozen" via setting it to itself as a default argument in the function header. See how we have task=task in both mark_done and delete. This freezes the variable to its "loop-time" value.

Let's take a look at one last example that uses everything we learned. Below is an audio mixer. Provide multiple audio tracks and mix them together.

import gradio as gr
import numpy as np

with gr.Blocks() as demo:
    track_count = gr.State(1)
    add_track_btn = gr.Button("Add Track")

    add_track_btn.click(lambda count: count + 1, track_count, track_count)

    @gr.render(inputs=track_count)
    def render_tracks(count):
        audios = []
        volumes = []
        with gr.Row():
            for i in range(count):
                with gr.Column(variant="panel", min_width=200):
                    gr.Textbox(placeholder="Track Name", key=f"name-{i}", show_label=False)
                    track_audio = gr.Audio(label=f"Track {i}", key=f"track-{i}")
                    track_volume = gr.Slider(0, 100, value=100, label="Volume", key=f"volume-{i}")
                    audios.append(track_audio)
                    volumes.append(track_volume)

            def merge(data):
                sr, output = None, None
                for audio, volume in zip(audios, volumes):
                    sr, audio_val = data[audio]
                    volume_val = data[volume]
                    final_track = audio_val * (volume_val / 100)
                    if output is None:
                        output = final_track
                    else:
                        min_shape = tuple(min(s1, s2) for s1, s2 in zip(output.shape, final_track.shape))
                        trimmed_output = output[:min_shape[0], ...][:, :min_shape[1], ...] if output.ndim > 1 else output[:min_shape[0]]
                        trimmed_final = final_track[:min_shape[0], ...][:, :final_track[1], ...] if final_track.ndim > 1 else final_track[:min_shape[0]]
                        output += trimmed_output + trimmed_final
                return (sr, output)
            
            merge_btn.click(merge, set(audios + volumes), output_audio)

    merge_btn = gr.Button("Merge Tracks")
    output_audio = gr.Audio(label="Output")
                    
demo.launch()

Two things to not in this app:

Here we provide key= to all the components! We need to do this so that if we add another track after setting the values for an existing track, our input values to the existing track do not get reset on re-render.
When there are lots of components of different types and arbitrary counts passed to an event listener, it is easier to use the set and dictionary notation for inputs rather than list notation. Above, we make one large set of all the input gr.Audio and gr.Slider components when we pass the inputs to the merge function. In the function body we query the component values as a dict.

The gr.render expands gradio capabilities extensively - see what you can make out of it!