Generating Binding Code Wasn't Enough: Moving Unity UI Composition to Compile Time

Source generators are often introduced as a way to remove boilerplate. That is useful, but it was not the main architectural reason FUI moved more of its Unity UI pipeline into Roslyn. The harder question begins after binding code has already been generated: does the runtime still need to scan assemblies, inspect attributes, resolve types, and reconstruct the relationship between a View, ViewModel, BindingContext, and Presenter? FUI's answer is to move that composition step to compile time. The generator does not stop at property notifications and binding callbacks. It also emits binding factories and strongly typed routes, so the Player runtime executes an already-validated object graph instead of rediscovering it. This article explains why that distinction matters, how the design evolved, and what the final architecture gains beyond the vague promise of “less reflection.” The original problem was repetitive protocol code Consider a settings screen with a title, a volume slider, a vibration toggle, and a close button. The ViewModel is small, but connecting it to the UI requires a surprisingly large protocol: propagate property changes to UI elements; propagate control changes back to the ViewModel; connect UI events to commands; perform initial synchronization; unsubscribe every handler during unbinding; construct the matching BindingContext and Presenter. None of these steps is individually difficult. The risk comes from repetition. A missing unsubscribe, an incompatible target member, or an incorrect string may remain invisible until that specific screen opens. The earliest code-generation experiment preserved in FUI's repository was an external FUICompiler executable. It targeted .NET 6, was published as a self-contained win-x64 tool, walked Roslyn syntax nodes, extracted binding attributes, and emitted BindingContext source. The central idea was already present: var classDeclarations = root . DescendantNodes () . OfType < ClassDeclarationSyntax >(); foreach ( v