Creating threads to update visualization asynchronously (#2656)

This PR introduces a thread-based mechanism to handle model updates and visualization rendering concurrently. It ensures smooth execution of simulations by separating model step execution and visualization updates into independent threads, improving performance and responsiveness during simulations.
Fixes #2604 

### Motive
Previously, the visualization process could become a bottleneck during rapid simulations, as rendering was tightly coupled with model updates. This caused delays and UI responsiveness issues. By separating these operations into threads, the model execution is no longer hindered by the visualization process.

### Implementation

1. Introduced separate thread for visualisation:
Handles visualization updates triggered by threading.Event (visualization_pause_event) to synchronize rendering with model steps.
2. Thread Synchronization:
Implemented visualization_pause_event to signal the visualization thread after each model step is completed, ensuring rendering happens efficiently without blocking the simulation. Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

mesa/visualization/solara_viz.py

@@ -25,6 +25,8 @@
 
 import asyncio
 import inspect
+import threading
+import time
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Literal
 
@@ -57,6 +59,7 @@ def SolaraViz(
     simulator: Simulator | None = None,
     model_params=None,
     name: str | None = None,
+    use_threads: bool = False,
 ):
     """Solara visualization component.
 
@@ -76,6 +79,8 @@ def SolaraViz(
             This controls the speed of the model's automatic stepping. Defaults to 100 ms.
         render_interval (int, optional): Controls how often plots are updated during a simulation,
             allowing users to skip intermediate steps and update graphs less frequently.
+        use_threads: Flag for indicating whether to utilize multi-threading for model execution.
+            When checked, the model will utilize multiple threads,adjust based on system capabilities.
         simulator: A simulator that controls the model (optional)
         model_params (dict, optional): Parameters for (re-)instantiating a model.
             Can include user-adjustable parameters and fixed parameters. Defaults to None.
@@ -114,6 +119,7 @@ def SolaraViz(
     reactive_model_parameters = solara.use_reactive({})
     reactive_play_interval = solara.use_reactive(play_interval)
     reactive_render_interval = solara.use_reactive(render_interval)
+    reactive_use_threads = solara.use_reactive(use_threads)
     with solara.AppBar():
         solara.AppBarTitle(name if name else model.value.__class__.__name__)
         solara.lab.ThemeToggle()
@@ -136,12 +142,25 @@ def SolaraViz(
                 max=100,
                 step=2,
             )
+            if reactive_use_threads.value:
+                solara.Text("Increase play interval to avoid skipping plots")
+
+            def set_reactive_use_threads(value):
+                reactive_use_threads.set(value)
+
+            solara.Checkbox(
+                label="Use Threads",
+                value=reactive_use_threads,
+                on_value=set_reactive_use_threads,
+            )
+
             if not isinstance(simulator, Simulator):
                 ModelController(
                     model,
                     model_parameters=reactive_model_parameters,
                     play_interval=reactive_play_interval,
                     render_interval=reactive_render_interval,
+                    use_threads=reactive_use_threads,
                 )
             else:
                 SimulatorController(
@@ -150,6 +169,7 @@ def SolaraViz(
                     model_parameters=reactive_model_parameters,
                     play_interval=reactive_play_interval,
                     render_interval=reactive_render_interval,
+                    use_threads=reactive_use_threads,
                 )
         with solara.Card("Model Parameters"):
             ModelCreator(
@@ -211,6 +231,7 @@ def ModelController(
     model_parameters: dict | solara.Reactive[dict] = None,
     play_interval: int | solara.Reactive[int] = 100,
     render_interval: int | solara.Reactive[int] = 1,
+    use_threads: bool | solara.Reactive[bool] = False,
 ):
     """Create controls for model execution (step, play, pause, reset).
 
@@ -219,37 +240,70 @@ def ModelController(
         model_parameters: Reactive parameters for (re-)instantiating a model.
         play_interval: Interval for playing the model steps in milliseconds.
         render_interval: Controls how often the plots are updated during simulation steps.Higher value reduce update frequency.
+        use_threads: Flag for indicating whether to utilize multi-threading for model execution.
     """
     playing = solara.use_reactive(False)
     running = solara.use_reactive(True)
+
     if model_parameters is None:
         model_parameters = {}
     model_parameters = solara.use_reactive(model_parameters)
-
-    async def step():
-        while playing.value and running.value:
-            await asyncio.sleep(play_interval.value / 1000)
-            do_step()
+    visualization_pause_event = solara.use_memo(lambda: threading.Event(), [])
+
+    def step():
+        try:
+            while running.value and playing.value:
+                time.sleep(play_interval.value / 1000)
+                do_step()
+                if use_threads.value:
+                    visualization_pause_event.set()
+        except Exception as e:
+            print(f"Error in step: {e}")
+            return
+
+    def visualization_task():
+        if use_threads.value:
+            try:
+                while playing.value and running.value:
+                    visualization_pause_event.wait()
+                    visualization_pause_event.clear()
+                    force_update()
+            except Exception as e:
+                print(f"Error in visualization_task: {e}")
 
     solara.lab.use_task(
-        step, dependencies=[playing.value, running.value], prefer_threaded=False
+        step, dependencies=[playing.value, running.value], prefer_threaded=True
+    )
+
+    solara.use_thread(
+        visualization_task,
+        dependencies=[playing.value, running.value],
     )
 
     @function_logger(__name__)
     def do_step():
         """Advance the model by the number of steps specified by the render_interval slider."""
-        for _ in range(render_interval.value):
-            model.value.step()
+        if playing.value:
+            for _ in range(render_interval.value):
+                model.value.step()
+                running.value = model.value.running
+                if not playing.value:
+                    break
+            if not use_threads.value:
+                force_update()
 
-        running.value = model.value.running
-
-        force_update()
+        else:
+            for _ in range(render_interval.value):
+                model.value.step()
+                running.value = model.value.running
+            force_update()
 
     @function_logger(__name__)
     def do_reset():
         """Reset the model to its initial state."""
         playing.value = False
         running.value = True
+        visualization_pause_event.clear()
         _mesa_logger.log(
             10,
             f"creating new {model.value.__class__} instance with {model_parameters.value}",
@@ -285,6 +339,7 @@ def SimulatorController(
     model_parameters: dict | solara.Reactive[dict] = None,
     play_interval: int | solara.Reactive[int] = 100,
     render_interval: int | solara.Reactive[int] = 1,
+    use_threads: bool | solara.Reactive[bool] = False,
 ):
     """Create controls for model execution (step, play, pause, reset).
 
@@ -294,6 +349,7 @@ def SimulatorController(
         model_parameters: Reactive parameters for (re-)instantiating a model.
         play_interval: Interval for playing the model steps in milliseconds.
         render_interval: Controls how often the plots are updated during simulation steps.Higher values reduce update frequency.
+        use_threads: Flag for indicating whether to utilize multi-threading for model execution.
 
     Notes:
         The `step button` increments the step by the value specified in the `render_interval` slider.
@@ -304,27 +360,66 @@ def SimulatorController(
     if model_parameters is None:
         model_parameters = {}
     model_parameters = solara.use_reactive(model_parameters)
-
-    async def step():
-        while playing.value and running.value:
-            await asyncio.sleep(play_interval.value / 1000)
-            do_step()
+    visualization_pause_event = solara.use_memo(lambda: threading.Event(), [])
+    pause_step_event = solara.use_memo(lambda: threading.Event(), [])
+
+    def step():
+        try:
+            while running.value and playing.value:
+                time.sleep(play_interval.value / 1000)
+                if use_threads.value:
+                    pause_step_event.wait()
+                    pause_step_event.clear()
+                do_step()
+                if use_threads.value:
+                    visualization_pause_event.set()
+        except Exception as e:
+            print(f"Error in step: {e}")
+
+    def visualization_task():
+        if use_threads.value:
+            try:
+                loop = asyncio.new_event_loop()
+                asyncio.set_event_loop(loop)
+                pause_step_event.set()
+                while playing.value and running.value:
+                    visualization_pause_event.wait()
+                    visualization_pause_event.clear()
+                    force_update()
+                    pause_step_event.set()
+            except Exception as e:
+                print(f"Error in visualization_task: {e}")
+                return
 
     solara.lab.use_task(
         step, dependencies=[playing.value, running.value], prefer_threaded=False
     )
+    solara.lab.use_task(visualization_task, dependencies=[playing.value])
 
     def do_step():
         """Advance the model by the number of steps specified by the render_interval slider."""
-        simulator.run_for(render_interval.value)
-        running.value = model.value.running
-        force_update()
+        if playing.value:
+            for _ in range(render_interval.value):
+                simulator.run_for(1)
+                running.value = model.value.running
+                if not playing.value:
+                    break
+            if not use_threads.value:
+                force_update()
+
+        else:
+            for _ in range(render_interval.value):
+                simulator.run_for(1)
+                running.value = model.value.running
+            force_update()
 
     def do_reset():
         """Reset the model to its initial state."""
         playing.value = False
         running.value = True
         simulator.reset()
+        visualization_pause_event.clear()
+        pause_step_event.clear()
         model.value = model.value = model.value.__class__(
             simulator=simulator, **model_parameters.value
         )