System groups

A system group can have systems and other system groups as its children. A system group updates its children in a sorted order on the main thread.

To create your own system group, create a class that inherits from ComponentSystemGroup. Use the UpdateInGroup attribute on the member systems to specify which systems need to be updated in a given group. Using the UpdateInGroup attribute ensures that Unity creates all systems in an order that respects the CreateAfter and CreateBefore attributes.

Creation order of systems

By default, Unity creates systems in an order that doesn't respect system groups, but does respect CreateAfter and CreateBefore attributes.

OnCreate methods are called in the same order as systems are created.

It's best practice to use the CreateAfter and CreateBefore attributes, and either the default world creation or ICustomBootstrap for advanced use cases. This is the recommended way of creating systems over calling World.GetOrCreateSystem.

This reduces the likelihood of errors such as if the system creation order changes to another one that also satisfies the attribute constraints. If you must refer to another system in your OnCreate method, you should use CreateAfter to ensure that your system is created after the other system you're referring to, and then use World.GetExistingSystem to retrieve it.

More commonly, you might want to access a singleton component or other resource that another system creates in that system's OnCreate method. You can add [CreateAfter(typeof(OtherSystem))] to the system type to ensure that your OnCreate method runs after OtherSystem.OnCreate.

Destruction order of systems

When you call World.Dispose, Unity destroys the systems in reverse order of their actual creation. Unity destroys them in this order even if their creation broke the CreateAfter or CreateBefore constraints (for example if you manually called CreateSystem out of order).

Update order of systems

Every time you add a group to a system group, the group re-sorts the system update order for that group before updating again. To control the update order of a system group, add the UpdateBefore or UpdateAfter attribute to a system to specify which systems it should update before or after. These attributes only apply relative to direct children of the same system group.

You can also use the OrderFirst or OrderLast parameters to customize the update order, and these take precedence over UpdateBefore and UpdateAfter. For example:

// If PeachSystem and DaisySystem are children of the same group, then the
// entity component system puts PeachSystem somewhere before DaisySystem in 
// the sorted order.
[UpdateBefore(typeof(DaisySystem))]
public partial class PeachSystem : SystemBase { }

Because everything in a group's update list updates together, the global order of system updates represents a hierarchical ordering, where all direct children of a group are ordered first by OrderFirst and OrderLast, and then by UpdateBefore and UpdateAfter constraints. Because the children can also be system groups, these children groups might update many grandchild systems before returning to the current group's update list.

In the Editor, you can use the Systems window (Window > Entities > Systems) to see the full list of system groups in your application, and their ordering:

Systems window showing hierarchical ordering of system updates

For more information, see the Systems window documentation.

Default system groups

There is a set of default system groups that you can use to update systems in the correct phase of a frame. You can nest one system group inside another so that all systems in a group update in the correct phase and update according to the order within their group.

The default world contains a hierarchy of ComponentSystemGroup instances. There are three root-level system groups in the Unity player loop:

• InitializationSystemGroup: Updates at the end of the Initialization phase of the player loop.
• SimulationSystemGroup: Updates at the end of the Update phase of the player loop.
• PresentationSystemGroup: Updates at the end of the PreLateUpdate phase of the player loop.

System ordering attributes

You can use the following attributes on a system to determine its update order:

Attribute	Description
UpdateInGroup	Specify a ComponentSystemGroup that this system should be a member of. If you don't set this attribute, it's automatically be added to the default world's SimulationSystemGroup. You can use the optional OrderFirst and OrderLast parameters to sort systems before or after all other systems in the group that don't have the same parameter set.
UpdateBefore UpdateAfter	Order systems relative to other systems within the same group. When you apply this attribute to groups, they implicitly constrain all member systems inside them. For example, if CarSystem is in CarGroup, and TruckSystem is in TruckGroup, and CarGroup and TruckGroup are both members of VehicleGroup, then the ordering of CarGroup and TruckGroup implicitly determines the relative ordering of CarSystem and TruckSystem.
CreateBefore CreateAfter	Order system creation relative to other systems. The same ordering rules for UpdateBefore and UpdateAfter apply here, except that there are no groups, and no OrderFirst/OrderLast. These attributes override the default behavior. System destruction order is defined as the reverse of creation order.
DisableAutoCreation	Prevents Unity from creating the system during the default world initialization. See Manual system creation below.

Manual system creation

If you need to create a system manually, mark the system in question with the DisableAutoCreation, and then you can use World.CreateSystem to create a system. To add a system to a group, use the AddSystemToUpdateList method. You can add also other system groups to existing system groups.

One problem with this approach is that it doesn't guarantee that systems get created in an order that respects the CreateAfter and CreateBefore attributes on each system.

Multiple worlds

You can create multiple worlds and you can instantiate the same component system class in more than one world. You can also update each instance at different rates from different points in the update order. The Netcode package uses this to create separate worlds for client and server running in the same process.

To do this, use the ICustomBootstrap interface to manage systems in multiple worlds. For more information, see the documentation on ICustomBootstrap.