Why does PlayerData have a Length ?

[unity-technologies]

Why does PlayerData have a Length ?

[Prastiwar]

struct Group
{
    public int Length;
    public EntityArray Entities;
    public ComponentDataArray<Foo> Foos;
    public ComponentDataArray<Bar> Bars;
}

Group is group of all Entities in active world that have Foo and Bar component
It's like array of entities with references to specific given components.
So - Group is like EntityArray but with references to components, EntityArray itself is just array of Entities (and entity is just an index used in map)
Length is length of EntityArray, so also Length of Foos and Bars, because each entity has Foo and Bar component

It's more obvious in this case

for(int i; i < m_group.Length; i++) // iterate over all entities
{
    // It's safe to iterate like that, because every array in group has the same length
    // and indexing is also injected in that way to use it like this:
    var actualEntity = m_group.Entities[i];  // Actual iterating Entity
    var actualFoo    = m_group.Foos[i];      // Foo component "attached" to actualEntity
    var actualBar    = m_group.Bards[i];     // Bar component "attached" to actualEntity
}

Using m_group.Foos.Length or m_group.Bar.Length would be a bit confusing in this case.

Summary - every array in injected group would have same Length, because it's length of Entities and each entity has every given component

[darbotron]

I'm 99% sure this is because the [Inject] attribute expects whatever struct it's injecting to be a struct of arrays (SoA) where all the contained ComponentDataArray have the same length.

The generalised usage of the [Inject] idiom (e.g. see the struct used by other classes derived from ComponentSystem) makes more obvious sense since the injected structs typically have multiple ComponentDataArray with different component types.

[NVentimiglia]

@starikcetin

struct PlayerData
        {            
            // SEE THIS LINE
            public int Length;

            public ComponentDataArray<PlayerInput> Input;
        }

I assume PlayerData.Length and PlayerData.Input.Length are the same value. They both describe the number of PlayerInput's, no ? Competing and confusing. Would suggest removing PlayerData.Length.

[NVentimiglia]

Not sure I like the competing values. Will confuse others.

Maybe an interface defining why we need this? Will exclusion break things?

[starikcetin]

@NVentimiglia
What do you mean by competing values?

[starikcetin]

@NVentimiglia
I still don't understand why you think they are competing.

When you are injecting multiple arrays in the same struct, they are all injected in parallel in terms of entities. Therefore they all have the same length. Instead of checking the length of every single array (which would be the same for all of them), I, personally, very much prefer injecting a single length variable.

[NVentimiglia]

@starikcetin

Maybe I am confused. So it is possible for these values to be different if we have multiple arrays? Are the rules defining what this 'Length' value means documented somewhere?

[starikcetin]

@NVentimiglia

Say you have this:

struct Group
{
    public int Length;

    public ComponentDataArray<Foo> Foos;
    public ComponentDataArray<Bar> Bars;
}

The Length is the amount of entities that matched.

When you do Foos[2] it will, say, give you the Foo on the entity # 2. When you do Bars[2] it will give you the Bar on the entity # 2.

Therefore the indexes of those arrays are each a different matched entity, and you get different components of the same entity when you apply the same index to different arrays.

There is not much of a documentation yet, but you may watch the talks from Unite Austin, they delve deep into how the ECS operates.

[NVentimiglia]

Gotcha, so if Foos is 3 and Bars is 2, Length will be 3? Is that possible with how groups work? I was assuming that If I requested a group with Foo and Bar, I would get only entities with both (both 3).

[starikcetin]

@NVentimiglia

Your assumption is correct. You only get entities that has both of the components. Hence why the arrays are parallel, and that they have the same length.

So no, the scenario you proposed is not possible, Length of Foos cannot be 2 while Bars is 3. They both will either be 3 or 2.

[NVentimiglia]

@Prastiwar Super helpful