C# progress report: iOS and signals as events

By: Ignacio Roldán Etcheverry Apr 06, 2020

Initially and for quite some time C# was only supported in Godot on desktop platforms. In the last year we made good progress extending support to Android and WebAssembly, and now it's time to add iOS to that list as well.

In this progress report I'm also introducing a new way of working with Godot signals in C#.

This work is kindly sponsored by Microsoft.

iOS support

DodgeTheCreepsCS on the iOS Simulator _The Dodge The Creeps C# demo running on the iOS Simulator_

Godot 4.0 will be getting C# support for iOS. We also plan to include it in a 3.2.x release, which could be 3.2.2 if everything goes well.

Similarly to WebAssembly, the Mono JIT compiler is not available on iOS devices. Instead, Godot will perform Ahead-of-Time (AOT) compilation when exporting a game and the resulting libraries will be added to the generated Xcode project.

Other than the additional time that is spent doing AOT compilation of assemblies, the export process should be the same as it is with games that don't use C#.

For those interested in building Godot from source, the code is available in the 3.2-mono-ios branch (soon to be merged in the 3.2 branch). Currently the master branch doesn't support a lot of platforms (including iOS) due to the ongoing work with the renderer. The Compiling with Mono page includes information about building the Mono runtime, the Base Class Library and the AOT cross-compiler.

Signals as events

Signals are the Godot version of the observer pattern. Engine classes use signals to notify listeners when an event occurs. User scripts can also create their own signals.

This is what a signal declaration looks like in GDScript:

signal text_changed(text)

Code can interact with signals via the connect (listen), disconnect (stop listening) and emit_signal (notify/raise) methods. Signals are also accessible from the Godot editor where new connections can be created.

The equivalent of a signal declaration in C# is a delegate declaration with a Signal attribute. They can be interacted with them using the aforementioned methods. While this did the job well and it's pretty much the same as GDScript, I was never fully satisfied with it.

In .NET, the common way to implement the observer pattern is using events. Compared to events, C# code that uses Godot signals looks foreign.

Last month, motivated by the new Callable type, I decided to re-think the implementation of signals in C#. The goal this time is to expose them as events. It's not finished, but the initial results are looking great.

Code completion for signals events Code-completion of signals events for the Godot.CheckBox class

In the following code example we can see how it compares to the old style:

// Old style
class TextField : Node {
    [Signal] delegate void TextChanged(string text);

    void Foo() {
        Connect(nameof(TextChanged), this, nameof(TextChangedCallback));
        EmitSignal(nameof(TextChanged), "bar");

    void TextChangedCallback(string text) { /* ... */ }

// New style
class TextField : Node {
    delegate void TextChangedHandler(string text);
    [Signal] event TextChangedHandler TextChanged;

    void Foo() {
        TextChanged += TextChangedCallback;
        // TextChanged?.Invoke("bar"); // Nope, not yet :(
        EmitSignal(nameof(TextChanged), "bar");

    void TextChangedCallback(string text) { /* ... */ }

As seen in the example, the most important part that's missing is support for raising the event. This right now would only invoke the event delegates, but it won't emit the signal, which is important to notify engine and GDScript listeners. As such, we still need to use the classic EmitSignal call, which will emit the signal as well as raise the event. More work will be needed in order to support event raising.

What comes next

This progress report came really delayed. You can expect a new one in the next weeks, in which I'll be announcing Godot extensions for both Visual Studio and VS Code!

After that there are two more tasks in my current roadmap. One of them is separating the Godot API into namespaces (right now we have everything in the same bloated namespace).

The other task is writing a C# API to wrap the Godot file system. This new API is meant to replace the auto-generated Directory and File classes (which will still be available for compatibility). The new API is meant to be very similar to what developers are used to in .NET.

C# is now working on almost every platform Godot supports, with the exception of the Universal Windows Platform (UWP). While UWP is not part of my current roadmap, support for it is likely to happen before 4.0 is released. Also WebAssembly, while already working, is still lacking AOT compilation from Godot which is of high priority.