Removes the memory alloc of FocusProvider::GetPointers

[wiwei]

Overview

GetPointers currently returns an IEnumerable backed by a List, which allocs memory for each call to it. While we could have 'saved' on this by caching a result for a particular type T on a per-Update loop basis, this would also end up making things more complex than necessary (i.e. saving cached state, clearing it, making it possible for us to hit bugs there).

Since the function is already an IEnumerable, we can just use yield return typePointer to avoid allocing a return structure (thanks @Alexees for the solution here)

We use this in a bunch of non-test and test locations, so this is verified just with existing tests that pass.

Changes

• Fixes: FocusProvider.GetPointers<T> generates gargabe #8109

[keveleigh]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[RogPodge]

TIL c# has this kind of lazy evaluation

so cool

[hexagonius]

@RogPodge from C# 7 you can even do this:

/// <inheritdoc />
        public IEnumerable<T> GetPointers<T>() where T : class, IMixedRealityPointer
        {
            foreach (var pointer in pointers.Values)
            {
                if (pointer.Pointer as T typePointer)
                {
                    yield return typePointer;
                }
            }
        }

[wiwei]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[keveleigh]

This is an older article (2016) and I've been meaning to retest their method, but it seems like this change might trade garbage for a perf cost: https://jacksondunstan.com/articles/3598

[david-c-kline]

seems like this change might trade garbage for a perf cost

Garbage will eventually result in a performance cost, but not as predictable. This would be good to test in an app with a typical
resource load.

[wiwei]

Unfortunately I haven't had time to do the profiling for this (given @keveleigh 's previous comment - I'll close this out until then)