Reduce allocations

[TimLariviere]

It's a concurrent PR to #802 because I'm testing a different approach.
Ultimately only one PR will be merged, containing the best of the 2.

I have been profiling FabulousContacts (slightly modified version) on Windows, in an Android emulator.

Here's the list of changes I made

• Use struct tuples instead of tuples in the ViewBuilders.Update... when comparing old/new values with match ... with
• Use struct union for Collections.Operation
• Remove printf in ViewUpdaters.updateNavigationPages (should use Program.withConsoleTrace instead in a future version)
• Replaced Array.tryFind (fun x -> ...) by a simple recursive function to avoid lambda context capture

Here's the result as of today (left: Fabulous 0.57, right: this PR)

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[TimLariviere]

/azp run full build

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[vshapenko]

for updateIncremental would be worth take a look at #780 to reduce unneccessary allocations, i think

[TimLariviere]

@vshapenko For now, I'm currently looking at reducing allocations in Collections.diff, as this is where most allocations come from.

[TimLariviere]

Ok, I managed to reduce allocations in Collections.diff with a dirty trick.
The problem was essentially the fact that Collections.diff and Collections.adaptForObservableCollection were returning an F# list with Operation to apply to a list to go from State A to State B. Those functions are called a lot of times (~20.000 times just for adding 20 people to the contact list in FabulousContacts), so any allocation quickly become a lot of allocations...

So to avoid having those returned F# lists, I preallocated an array from the ArrayPool for the maximum size the algorithm will need and made diff/adaptForObservableCollection directly work with it.

Also made sure that all local functions were not implicitly capturing variables to avoid allocations.

I will need to clean up this algorithm before merging.

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[vshapenko]

Looks very promising. Looking forward to try this merge in real action

[TimLariviere]

@vshapenko Yes! I will surely publish a preview release.

[TimLariviere]

Fabulous has to box struct to store them in ViewElement because it stores any kind of value as obj.
So, to reduce allocations on structs (Xamarin.Forms.Color, Xamarin.Forms.Thickness and Xamarin.Forms.LayoutOptions), I put memoization mechanism by storing the boxed values and reusing those instead of reboxing each time.

Also I've been bit by dotnet/fsharp#526 when trying to reduce struct boxing.
So to avoid implicit boxing of structs like ValueOption, I replaced the = operator with a custom function.

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[TimLariviere]

This time, I changed the way we update Attached Properties to avoid passing an overload dictionary each time we update a ViewElement.
Instead the overloaded updater is a function stored in ViewElement just like update.

This removed instantiations of the 5th and 7th most instantiated types, but with apparently little effect on the global memory allocation.

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[vshapenko]

@TimLariviere , preview is ready to try?

[TimLariviere]

It's almost ready. Just need to fix the few comments I made and then I'll merge this PR and push the preview package to NuGet.
Should be done by the end of day.

I'll continue working on allocations in a next PR.

[TimLariviere]

/azp run full build

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[TimLariviere]

/azp run full build

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[TimLariviere]

/azp run full build

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Azure Pipelines successfully started running 1 pipeline(s).

[TimLariviere]

/azp run full build

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Azure Pipelines successfully started running 1 pipeline(s).

[TimLariviere]

/azp run full build

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Azure Pipelines successfully started running 1 pipeline(s).