Use builder pattern for widgets (#941)

* Use builder pattern for widgets

* Arguments to new function

proc-macros/src/widget_builder.rs

@@ -0,0 +1,162 @@
+use proc_macro2::{Ident, Literal, TokenStream as TokenStream2};
+use quote::ToTokens;
+use syn::spanned::Spanned;
+use syn::{Attribute, Data, DeriveInput, Field, PathArguments, Type};
+
+/// Check if a specified `#[widget_builder target]` attribute can be found in the list
+fn has_attribute(attrs: &[Attribute], target: &str) -> bool {
+	attrs
+		.iter()
+		.filter(|attr| attr.path.to_token_stream().to_string() == "widget_builder")
+		.any(|attr| attr.tokens.to_token_stream().to_string() == target)
+}
+
+/// Make setting strings easier by allowing all types that `impl Into<String>`
+///
+/// Returns the new input type and a conversion to the origional.
+fn easier_string_assignment(field_ty: &Type, field_ident: &Ident) -> (TokenStream2, TokenStream2) {
+	if let Type::Path(type_path) = field_ty {
+		if let Some(last_segement) = type_path.path.segments.last() {
+			// Check if this type is a `String`
+			// Based on https://stackoverflow.com/questions/66906261/rust-proc-macro-derive-how-do-i-check-if-a-field-is-of-a-primitive-type-like-b
+			if last_segement.ident == Ident::new("String", last_segement.ident.span()) {
+				return (
+					quote::quote_spanned!(type_path.span() => impl Into<String>),
+					quote::quote_spanned!(field_ident.span() => #field_ident.into()),
+				);
+			}
+		}
+	}
+	(quote::quote_spanned!(field_ty.span() => #field_ty), quote::quote_spanned!(field_ident.span() => #field_ident))
+}
+
+/// Extract the identifier of the field (which should always be present)
+fn extract_ident(field: &Field) -> syn::Result<&Ident> {
+	field
+		.ident
+		.as_ref()
+		.ok_or_else(|| syn::Error::new_spanned(field, "Constructing a builder not supported for unnamed fields"))
+}
+
+/// Find the type passed into the builder and the right hand side of the assignment.
+///
+/// Applies special behaviour for easier String and WidgetCallback assignment.
+fn find_type_and_assignment(field: &Field) -> syn::Result<(TokenStream2, TokenStream2)> {
+	let field_ty = &field.ty;
+	let field_ident = extract_ident(field)?;
+
+	let (mut function_input_ty, mut assignment) = easier_string_assignment(field_ty, field_ident);
+
+	// Check if type is `WidgetCallback`
+	if let Type::Path(type_path) = field_ty {
+		if let Some(last_segement) = type_path.path.segments.last() {
+			if let PathArguments::AngleBracketed(generic_args) = &last_segement.arguments {
+				if let Some(first_generic) = generic_args.args.first() {
+					if last_segement.ident == Ident::new("WidgetCallback", last_segement.ident.span()) {
+						// Assign builder pattern to assign the closure directly
+						function_input_ty = quote::quote_spanned!(field_ty.span() => impl Fn(&#first_generic) -> crate::messages::message::Message + 'static + Send + Sync);
+						assignment = quote::quote_spanned!(field_ident.span() => crate::messages::layout::utility_types::layout_widget::WidgetCallback::new(#field_ident));
+					}
+				}
+			}
+		}
+	}
+	Ok((function_input_ty, assignment))
+}
+
+// Construct a builder function for a specific field in the struct
+fn construct_builder(field: &Field) -> syn::Result<TokenStream2> {
+	// Check if this field should be skipped with `#[widget_builder(skip)]`
+	if has_attribute(&field.attrs, "(skip)") {
+		return Ok(Default::default());
+	}
+	let field_ident = extract_ident(field)?;
+
+	// Create a doc comment literal describing the behaviour of the function
+	let doc_comment = Literal::string(&format!("Set the `{field_ident}` field using a builder pattern."));
+
+	let (function_input_ty, assignment) = find_type_and_assignment(field)?;
+
+	// Create builder function
+	Ok(quote::quote_spanned!(field.span() =>
+		#[doc = #doc_comment]
+		pub fn #field_ident(mut self, #field_ident: #function_input_ty) -> Self{
+			self.#field_ident = #assignment;
+			self
+		}
+	))
+}
+
+pub fn derive_widget_builder_impl(input_item: TokenStream2) -> syn::Result<TokenStream2> {
+	let input = syn::parse2::<DeriveInput>(input_item)?;
+
+	let struct_name_ident = input.ident;
+
+	// Extract the struct fields
+	let fields = match &input.data {
+		Data::Enum(enum_data) => return Err(syn::Error::new_spanned(enum_data.enum_token, "Derive widget builder is not supported for enums")),
+		Data::Union(union_data) => return Err(syn::Error::new_spanned(union_data.union_token, "Derive widget builder is not supported for unions")),
+		Data::Struct(struct_data) => &struct_data.fields,
+	};
+
+	// Create functions based on each field
+	let builder_functions = fields.iter().map(construct_builder).collect::<Result<Vec<_>, _>>()?;
+
+	// Check if this should not have the `widget_holder()` function due to a `#[widget_builder(not_widget_holder)]` attribute
+	let widget_holder_fn = if !has_attribute(&input.attrs, "(not_widget_holder)") {
+		// A doc comment for the widget_holder function
+		let widget_holder_doc_comment = Literal::string(&format!("Wrap {struct_name_ident} as a WidgetHolder."));
+
+		// Construct the `widget_holder` function
+		quote::quote! {
+			#[doc = #widget_holder_doc_comment]
+			pub fn widget_holder(self) -> crate::messages::layout::utility_types::layout_widget::WidgetHolder{
+				crate::messages::layout::utility_types::layout_widget::WidgetHolder::new( crate::messages::layout::utility_types::layout_widget::Widget::#struct_name_ident(self))
+			}
+		}
+	} else {
+		quote::quote!()
+	};
+
+	// The new function takes any fields tagged with `#[widget_builder(constructor)]` as arguments.
+	let new_fn = {
+		// A doc comment for the new function
+		let new_doc_comment = Literal::string(&format!("Create a new {struct_name_ident}, based on default values."));
+
+		let is_constructor = |field: &Field| has_attribute(&field.attrs, "(constructor)");
+
+		let idents = fields.iter().filter(|field| is_constructor(field)).map(extract_ident).collect::<Result<Vec<_>, _>>()?;
+		let types_and_assignments = fields.iter().filter(|field| is_constructor(field)).map(find_type_and_assignment).collect::<Result<Vec<_>, _>>()?;
+		let (types, assignments): (Vec<_>, Vec<_>) = types_and_assignments.into_iter().unzip();
+
+		let construction = if idents.is_empty() {
+			quote::quote!(Default::default())
+		} else {
+			let default = (idents.len() != fields.len()).then_some(quote::quote!(..Default::default())).unwrap_or_default();
+			quote::quote! {
+				Self {
+					#(#idents: #assignments,)*
+					#default
+				}
+			}
+		};
+
+		quote::quote! {
+			#[doc = #new_doc_comment]
+			pub fn new(#(#idents: #types),*) -> Self {
+				#construction
+			}
+		}
+	};
+
+	// Construct the code block
+	Ok(quote::quote! {
+		impl #struct_name_ident {
+			#new_fn
+
+			#(#builder_functions)*
+
+			#widget_holder_fn
+		}
+	})
+}