We aim to make the C# API reflection-free, mainly for concerns about
performance, and to be able to target NativeAOT in refletion-free mode,
which reduces the binary size.
One of the main usages of reflection still left was the dynamic
invokation of callable delegates, and for some time I wasn't sure
I would find an alternative solution that I'd be happy with.
The new solution uses trampoline functions to invoke the delegates:
```
static void Trampoline(object delegateObj, NativeVariantPtrArgs args, out godot_variant ret)
{
if (args.Count != 1)
throw new ArgumentException($"Callable expected 1 arguments but received {args.Count}.");
string res = ((Func<int, string>)delegateObj)(
VariantConversionCallbacks.GetToManagedCallback<int>()(args[0])
);
ret = VariantConversionCallbacks.GetToVariantCallback<string>()(res);
}
Callable.CreateWithUnsafeTrampoline((int num) => "Foo" + num, &Trampoline);
```
Of course, this is too much boilerplate for user code. To improve this,
the `Callable.From` methods were added. These are overloads that take
`Action` and `Func` delegates, which covers the most common use cases:
lambdas and method groups:
```
// Lambda
Callable.From((int num) => "Foo" + num);
// Method group
string AppendNum(int num) => "Foo" + num;
Callable.From(AppendNum);
```
Unfortunately, due to limitations in the C# language, implicit
conversions from delegates to `Callable` are not supported.
`Callable.From` does not support custom delegates. These should be
uncommon, but the Godot C# API actually uses them for event signals.
As such, the bindings generator was updated to generate trampoline
functions for event signals. It was also optimized to use `Action`
instead of a custom delegate for parameterless signals, which removes
the need for the trampoline functions for those signals.
The change to reflection-free invokation removes one of the last needs
for `ConvertVariantToManagedObjectOfType`. The only remaining usage is
from calling script constructors with parameters from the engine
(`CreateManagedForGodotObjectScriptInstance`). Once that one is made
reflection-free, `ConvertVariantToManagedObjectOfType` can be removed.
If the delegate target is an Object, the connected signal will be registered in that object instead of the middleman. So when that object is destroyed, the signal will be properly disconnected.
Vector4 and Vector4i were implemented incorrectly in godot_variant.
They were also missing their respective Variant conversion callbacks
(used for generic collections).
Took the chance to remove unnecessary native calls for creating
Variant from Vector4, as now it can be done from C# (which is faster).
- Use different syntax for object printing to avoid confusion with arrays.
- Print null as `<null>` to avoid confusion with a string `"null"`.
- Display `<empty>` in editor resource pickers to avoid confusion
with array-based properties.
- Moves interop functions to UnmanagedCallbacks struct that
contains the function pointers and is passed to C#.
- Implements UnmanagedCallbacksGenerator, a C# source generator that
generates the UnmanagedCallbacks struct in C# and the body for the
NativeFuncs methods (their implementation just calls the function
pointer in the UnmanagedCallbacks). The generated methods are needed
because .NET pins byref parameters of native calls, even if they are
'ref struct's, which don't need pinning. The generated methods use
`Unsafe.AsPointer` so that we can benefit from byref parameters
without suffering overhead of pinning.
Co-authored-by: Raul Santos <raulsntos@gmail.com>
Changed the signal declaration signal to:
```
// The following generates a MySignal event
[Signal] public delegate void MySignalEventHandler(int param);
```
Previously, for each scripts class instance that was created from code
rather than by the engine, we were constructing, configuring and
assigning a new CSharpScript.
This has changed now and we make sure there's only one CSharpScript
associated to each type.
The editor no longer needs to create temporary instances to get the
default values. The initializer values of the exported properties are
still evaluated at runtime. For example, in the following example,
`GetInitialValue()` will be called when first looks for default values:
```
[Export] int MyValue = GetInitialValue();
```
Exporting fields with a non-supported type now results in a compiler
error rather than a runtime error when the script is used.
This replaces the way we invoke methods and set/get properties.
This first iteration rids us of runtime type checking in those
cases, as it's now done at compile time.
Later it will also stop needing the use of reflection. After that,
we will only depend on reflection for generic Godot Array and
Dictionary. We're stuck with reflection in generic collections
for now as C# doesn't support generic/template specialization.
This is only the initial implementation. Further iterations are
coming, specially once we switch to the native extension system
which completely changes the way members are accessed/invoked.
For example, with the native extension system we will likely need
to create `UnmanagedCallersOnly` invoke wrapper methods and return
function pointers to the engine.
Other kind of members, like event signals will be receiving the
same treatment in the future.
We're targeting .NET 5 for now to make development easier while
.NET 6 is not yet released.
TEMPORARY REGRESSIONS
---------------------
Assembly unloading is not implemented yet. As such, many Godot
resources are leaked at exit. This will be re-implemented later
together with assembly hot-reloading.
The main focus here was to remove the majority of code that relied on
Mono's embedding APIs, specially the reflection APIs. The embedding
APIs we still use are the bare minimum we need for things to work.
A lot of code was moved to C#. We no longer deal with any managed
objects (`MonoObject*`, and such) in native code, and all marshaling
is done in C#.
The reason for restructuring the code and move away from embedding APIs
is that once we move to .NET Core, we will be limited by the much more
minimal .NET hosting.
PERFORMANCE REGRESSIONS
-----------------------
Some parts of the code were written with little to no concern about
performance. This includes code that calls into script methods and
accesses script fields, properties and events.
The reason for this is that all of that will be moved to source
generators, so any work prior to that would be a waste of time.
DISABLED FEATURES
-----------------
Some code was removed as it no longer makes sense (or won't make sense
in the future).
Other parts were commented out with `#if 0`s and TODO warnings because
it doesn't make much sense to work on them yet as those parts will
change heavily when we switch to .NET Core but also when we start
introducing source generators.
As such, the following features were disabled temporarily:
- Assembly-reloading (will be done with ALCs in .NET Core).
- Properties/fields exports and script method listing (will be
handled by source generators in the future).
- Exception logging in the editor and stack info for errors.
- Exporting games.
- Building of C# projects. We no longer copy the Godot API assemblies
to the project directory, so MSBuild won't be able to find them. The
idea is to turn them into NuGet packages in the future, which could
also be obtained from local NuGet sources during development.
We will be progressively moving most code to C#.
The plan is to only use Mono's embedding APIs to set things at launch.
This will make it much easier to later support CoreCLR too which
doesn't have rich embedding APIs.
Additionally the code in C# is more maintainable and makes it easier
to implement new features, e.g.: runtime codegen which we could use to
avoid using reflection for marshaling everytime a field, property or
method is accessed.
SOME NOTES ON INTEROP
We make the same assumptions as GDNative about the size of the Godot
structures we use. We take it a bit further by also assuming the layout
of fields in some cases, which is riskier but let's us squeeze out some
performance by avoiding unnecessary managed to native calls.
Code that deals with native structs is less safe than before as there's
no RAII and copy constructors in C#. It's like using the GDNative C API
directly. One has to take special care to free values they own.
Perhaps we could use roslyn analyzers to check this, but I don't know
any that uses attributes to determine what's owned or borrowed.
As to why we maily use pointers for native structs instead of ref/out:
- AFAIK (and confirmed with a benchmark) ref/out are pinned
during P/Invoke calls and that has a cost.
- Native struct fields can't be ref/out in the first place.
- A `using` local can't be passed as ref/out, only `in`. Calling a
method or property on an `in` value makes a silent copy, so we want
to avoid `in`.
REGARDING THE BUILD SYSTEM
There's no longer a `mono_glue=yes/no` SCons options. We no longer
need to build with `mono_glue=no`, generate the glue and then build
again with `mono_glue=yes`. We build only once and generate the glue
(which is in C# now).
However, SCons no longer builds the C# projects for us. Instead one
must run `build_assemblies.py`, e.g.:
```sh
%godot_src_root%/modules/mono/build_scripts/build_assemblies.py \
--godot-output-dir=%godot_src_root%/bin \
--godot-target=release_debug`
```
We could turn this into a custom build target, but I don't know how
to do that with SCons (it's possible with Meson).
OTHER NOTES
Most of the moved code doesn't follow the C# naming convention and
still has the word Mono in the names despite no longer dealing with
Mono's embedding APIs. This is just temporary while transitioning,
to make it easier to understand what was moved where.
