Replace gRPC Backend (#10)

**Rationale:**

Having two separate servers and communication methods resulted in additional maintenance & the need to convert often between backend & frontend data types.
By moving the backend communication off of gRPC and to just use websockets it both gives more control & allows for simplification of the implementation.

#8

**Changes:**

- Replaces gRPC backend.
  - New implementation automatically handles reconnect logic
- Implements an api layer
- Migrates examples to the api layer
- Implements a proc macro to make command handling easier
- Implements unit tests for the api layer (90+% coverage)
- Implements integration tests for the proc macro (90+% coverage)

Reviewed-on: #10
Co-authored-by: Sergey Savelyev <sergeysav.nn@gmail.com>
Co-committed-by: Sergey Savelyev <sergeysav.nn@gmail.com>

api-proc-macro/src/into_command_definition.rs

@@ -0,0 +1,123 @@
+use proc_macro_error::abort;
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, parse_quote, Data, DeriveInput, Fields, GenericParam, Generics};
+
+pub fn derive_into_command_definition_impl(
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    let DeriveInput {
+        ident,
+        data,
+        generics,
+        ..
+    }: DeriveInput = parse_macro_input!(item as DeriveInput);
+
+    let data = match data {
+        Data::Struct(data) => data,
+        Data::Enum(data) => abort!(
+            data.enum_token,
+            "IntoCommandDefinition not supported for enum"
+        ),
+        Data::Union(data) => abort!(
+            data.union_token,
+            "IntoCommandDefinition not supported for union"
+        ),
+    };
+
+    let generics = add_trait_bounds(generics);
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let num_fields = data.fields.len();
+
+    let create_param_stream = match &data.fields {
+        Fields::Named(fields) => {
+            let field_entries = fields.named.iter().map(|field| {
+                let name = field.ident.clone().map(|id| id.to_string());
+                let field_type = &field.ty;
+                quote_spanned! { field.span() =>
+                    parameters.push(api::messages::command::CommandParameterDefinition {
+                        name: #name.to_string(),
+                        data_type: <#field_type as api::data_type::ToDataType>::DATA_TYPE,
+                    });
+                }
+            });
+            quote! { #(#field_entries)* }
+        }
+        Fields::Unnamed(fields) => abort!(
+            fields,
+            "IntoCommandDefinition not supported for unnamed structs"
+        ),
+        Fields::Unit => quote! {},
+    };
+    let parse_param_stream = match &data.fields {
+        Fields::Named(fields) => {
+            let field_entries = fields.named.iter().map(|field| {
+                let name = &field.ident;
+                let name_string = field.ident.clone().map(|id| id.to_string());
+                let field_type = &field.ty;
+                quote_spanned! { field.span() =>
+                    let #name: #field_type = (*command
+                        .parameters
+                        .get(#name_string)
+                        .ok_or_else(|| api::messages::command::IntoCommandDefinitionError::ParameterMissing(#name_string.to_string()))?)
+                        .try_into()
+                        .map_err(|_| api::messages::command::IntoCommandDefinitionError::MismatchedType {
+                            parameter: #name_string.to_string(),
+                            expected: <#field_type as api::data_type::ToDataType>::DATA_TYPE,
+                        })?;
+                }
+            });
+            quote! { #(#field_entries)* }
+        }
+        Fields::Unnamed(_) => unreachable!("Already checked this"),
+        Fields::Unit => quote! {},
+    };
+    let param_name_stream = match &data.fields {
+        Fields::Named(fields) => {
+            let field_entries = fields.named.iter().map(|field| {
+                let name = &field.ident;
+                quote_spanned! { field.span() => #name, }
+            });
+            quote! { #(#field_entries)* }
+        }
+        Fields::Unnamed(_) => unreachable!("Already checked this"),
+        Fields::Unit => quote! {},
+    };
+
+    let result = quote! {
+        impl #impl_generics api::messages::command::IntoCommandDefinition for #ident #ty_generics #where_clause {
+            fn create(name: std::string::String) -> api::messages::command::CommandDefinition {
+                let mut parameters = std::vec::Vec::with_capacity( #num_fields );
+                #create_param_stream
+                api::messages::command::CommandDefinition {
+                    name: name,
+                    parameters: parameters,
+                }
+            }
+
+            fn parse(command: api::messages::command::Command) -> core::result::Result<Self, api::messages::command::IntoCommandDefinitionError> {
+                #parse_param_stream
+                Ok(Self {
+                    #param_name_stream
+                })
+            }
+        }
+    };
+
+    result.into()
+}
+
+fn add_trait_bounds(mut generics: Generics) -> Generics {
+    for param in &mut generics.params {
+        if let GenericParam::Type(ref mut type_param) = *param {
+            type_param
+                .bounds
+                .push(parse_quote!(api::data_type::ToDataType));
+            type_param.bounds.push(parse_quote!(
+                core::convert::TryFrom<api::data_value::DataValue>
+            ));
+        }
+    }
+    generics
+}

api-proc-macro/src/lib.rs

@@ -0,0 +1,11 @@
+extern crate proc_macro;
+
+use proc_macro_error::proc_macro_error;
+
+mod into_command_definition;
+
+#[proc_macro_error]
+#[proc_macro_derive(IntoCommandDefinition)]
+pub fn derive_into_command_definition(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    into_command_definition::derive_into_command_definition_impl(item)
+}