UpdateOrder.md

Update order

Now with all the scene stuff out of the way, it's important to know what executes when.

Here's some pseudocode of what happens under the hood:

if (FixedUpdateTimerIsTriggered)
{
	foreach(var scene in scenes)
	{
		foreach(var layer in scene.Layers)
		{
			foreach(var entity in layer.Entities)
				FixedUpdateForEntity();
		}
	}
}

First event to execute is FixedUpdate(). It only executes if its internal timer is up. The code iterates through all scenes, layers, entities and executes their FixedUpdate events. Component's events are evecuted inside the entity's Update().

After FixedUpdate(), regular Update() is executed:

foreach(var scene in scenes)
{
	foreach(var layer in scene.Layers)
	{
		foreach(var entity in layer.Entities)
			UpdateForEntity();
	}
}

It's pretty much the same thing as FixedUpdate, but without the timer.

After Update(), regular Draw() is executed:

foreach(var camera in cameras)
{
	SetCameraMatrixAndSurface();
	foreach(var scene in scenes)
	{
		foreach(var layer in scene.Layers)
		{
			if (layer.IsGUI)
				continue;
		
			foreach(var entity in layer.DepthSortedEntities)
			{
				DrawForEntity();
			}
		}
	}
}

The main difference now is that Draw() executes for each camera separately. So if you have two cameras, each entity will be drawn twice. Also here's where IsGUI flag comes into play. If the layer is considered as GUI, it can't be drawn on camera and is not executed.

After all that, we execute Draw() again, but for layers marked as GUI:

foreach(var scene in scenes)
{
	foreach(var layer in scene.Layers)
	{
		if (!layer.IsGUI)
			continue;
		
		foreach(var entity in layer.DepthSortedEntities)
		{
			DrawForEntity();
		}
	}
}

This time we don't have a camera, so layers are drawn directly on backbuffer and not affected by any camera.

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