Complex Example

In this more complex example we will be wrapping reactflow a library for building node based applications like flow charts, diagrams, graphs, etc.

Lets start by importing the library reactflow. Lets make a seperate file called reactflow.py and add the following code:

from reflex.components.component import Component
from typing import Any, Dict, List, Union
from reflex.vars import Var


class ReactFlowLib(Component):
    """A component that wraps a react flow lib."""

    library = "reactflow"

    def _get_custom_code(self) -> str:
        return """import 'reactflow/dist/style.css';
        """

Notice we also use the _get_custom_code method to import the css file that is needed for the styling of the library.

For this tutorial we will wrap three components from Reactflow: ReactFlow, Background, and Controls. Lets start with the ReactFlow component.

Here we will define the tag and the vars that we will need to use the component.

For this tutorial we will define EventHandler props on_nodes_change and on_connect, but you can find all the events that the component triggers in the reactflow docs.

from reflex import Component, EventHandler, Var
from typing import Any, Dict, List, Union


class ReactFlowLib(Component):
    ...


class ReactFlow(ReactFlowLib):
    tag = "ReactFlow"

    nodes: Var[List[Dict[str, Any]]]

    edges: Var[List[Dict[str, Any]]]

    fit_view: Var[bool]

    nodes_draggable: Var[bool]

    nodes_connectable: Var[bool]

    nodes_focusable: Var[bool]

    on_nodes_change: EventHandler[lambda e0: [e0]]

    on_connect: EventHandler[lambda e0: [e0]]

Now lets add the Background and Controls components. We will also create the components using the create method so that we can use them in our app.

from reflex.components.component import Component
from typing import Any, Dict, List, Union
from reflex.vars import Var


class ReactFlowLib(Component):
    ...


class ReactFlow(ReactFlowLib):
    ...


class Background(ReactFlowLib):
    tag = "Background"

    color: Var[str]

    gap: Var[int]

    size: Var[int]

    variant: Var[str]


class Controls(ReactFlowLib):
    tag = "Controls"


react_flow = ReactFlow.create
background = Background.create
controls = Controls.create

Now that we have our components lets build the app.

Lets start by defining the initial nodes and edges that we will use in our app.

import reflex as rx
from .react_flow import react_flow, background, controls
import random
from collections import defaultdict
from typing import Any, Dict, List


initial_nodes = [
    {
        'id': '1',
        'type': 'input',
        'data': {'label': '150'},
        'position': {'x': 250, 'y': 25},
    },
    {
        'id': '2',
        'data': {'label': '25'},
        'position': {'x': 100, 'y': 125},
    },
    {
        'id': '3',
        'type': 'output',
        'data': {'label': '5'},
        'position': {'x': 250, 'y': 250},
    },
]

initial_edges = [
    {'id': 'e1-2', 'source': '1', 'target': '2', 'label': '*', 'animated': True},
    {'id': 'e2-3', 'source': '2', 'target': '3', 'label': '+', 'animated': True},
]

Next we will define the state of our app. We have four event handlers: add_random_node, clear_graph, on_connect and on_nodes_change.

The on_nodes_change event handler is triggered when a node is selected and dragged. This function is used to update the position of a node during dragging. It takes a single argument node_changes, which is a list of dictionaries containing various types of metadata. For updating positions, the function specifically processes changes of type position.

class State(rx.State):
    """The app state."""
    nodes: List[Dict[str, Any]] = initial_nodes
    edges: List[Dict[str, Any]] = initial_edges

    def add_random_node(self):
        new_node_id = f'{len(self.nodes) + 1}'
        node_type = random.choice(['default'])
        # Label is random number
        label = new_node_id
        x = random.randint(0, 500)
        y = random.randint(0, 500)

        new_node = {
            'id': new_node_id,
            'type': node_type,
            'data': {'label': label},
            'position': {'x': x, 'y': y},
            'draggable': True,
        }
        self.nodes.append(new_node)

    def clear_graph(self):
        self.nodes = []  # Clear the nodes list
        self.edges = []  # Clear the edges list

    def on_connect(self, new_edge):
        # Iterate over the existing edges
        for i, edge in enumerate(self.edges):
            # If we find an edge with the same ID as the new edge
            if edge["id"] == f"e{new_edge['source']}-{new_edge['target']}":
                # Delete the existing edge
                del self.edges[i]
                break

        # Add the new edge
        self.edges.append({
            "id": f"e{new_edge['source']}-{new_edge['target']}",
            "source": new_edge["source"],
            "target": new_edge["target"],
            "label": random.choice(["+", "-", "*", "/"]),
            "animated": True,
        })

    def on_nodes_change(self, node_changes: List[Dict[str, Any]]):
        # Receives a list of Nodes in case of events like dragging
        map_id_to_new_position = defaultdict(dict)

        # Loop over the changes and store the new position
        for change in node_changes:
            if change["type"] == "position" and change.get("dragging") == True:
                map_id_to_new_position[change["id"]] = change["position"]

        # Loop over the nodes and update the position
        for i, node in enumerate(self.nodes):
            if node["id"] in map_id_to_new_position:
                new_position = map_id_to_new_position[node["id"]]
                self.nodes[i]["position"] = new_position

Now lets define the UI of our app. We will use the react_flow component and pass in the nodes and edges from our state. We will also add the on_connect event handler to the react_flow component to handle when an edge is connected.

def index() -> rx.Component:
    return rx.vstack(
        react_flow(
            background(),
            controls(),
            nodes_draggable=True,
            nodes_connectable=True,
            on_connect=lambda e0: State.on_connect(e0),
            on_nodes_change=lambda e0: State.on_nodes_change(
                e0
            ),
            nodes=State.nodes,
            edges=State.edges,
            fit_view=True,
        ),
        rx.hstack(
            rx.button(
                "Clear graph",
                on_click=State.clear_graph,
                width="100%",
            ),
            rx.button(
                "Add node",
                on_click=State.add_random_node,
                width="100%",
            ),
            width="100%",
        ),
        height="30em",
        width="100%",
    )


# Add state and page to the app.
app = rx.App()
app.add_page(index)

Here is an example of the app running: