Add an API for persisting arbitrary fields

nih_plug_derive/src/lib.rs

@@ -4,7 +4,7 @@ use proc_macro::TokenStream;
 use quote::quote;
 use syn::spanned::Spanned;
 
-#[proc_macro_derive(Params, attributes(id))]
+#[proc_macro_derive(Params, attributes(id, persist))]
 pub fn derive_params(input: TokenStream) -> TokenStream {
     let ast = syn::parse_macro_input!(input as syn::DeriveInput);
 
@@ -24,19 +24,23 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
         }
     };
 
-    // We only care about fields with an `id` attribute. We'll build a that creates a hashmap
+    // We only care about fields with `id` and `persist` attributes. For the `id` fields we'll build
-    // containing pointers to those parmaeters.
+    // a mapping function that creates a hashmap containing pointers to those parmaeters. For the
-    let mut param_insert_tokens = Vec::new();
+    // `persist` function we'll create functions that serialize and deserialize those fields
+    // individually (so they can be added and removed independently of eachother) using JSON.
+    let mut param_mapping_insert_tokens = Vec::new();
+    let mut field_serialize_tokens = Vec::new();
+    let mut field_deserialize_tokens = Vec::new();
     for field in fields.named {
         let field_name = match &field.ident {
             Some(ident) => ident,
             _ => continue,
         };
 
-        // We'll add another attribute for persistent fields later, and that's going to be mutually
+        // These two attributes are mutually exclusive
-        // exclusive with this id attribute
+        let mut id_attr: Option<String> = None;
-        let mut id_attr = None;
+        let mut persist_attr: Option<String> = None;
-        for attr in field.attrs {
+        for attr in &field.attrs {
             if attr.path.is_ident("id") {
                 match attr.parse_meta() {
                     Ok(syn::Meta::NameValue(syn::MetaNameValue {
@@ -60,15 +64,84 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
                         .into()
                     }
                 };
+            } else if attr.path.is_ident("persist") {
+                match attr.parse_meta() {
+                    Ok(syn::Meta::NameValue(syn::MetaNameValue {
+                        lit: syn::Lit::Str(s),
+                        ..
+                    })) => {
+                        if persist_attr.is_none() {
+                            persist_attr = Some(s.value());
+                        } else {
+                            return syn::Error::new(attr.span(), "Duplicate persist attribute")
+                                .to_compile_error()
+                                .into();
+                        }
+                    }
+                    _ => {
+                        return syn::Error::new(
+                            attr.span(),
+                            "The persist attribute should be a key-value pair with a string argument: #[persist = \"foo_bar\"]",
+                        )
+                        .to_compile_error()
+                        .into()
+                    }
+                };
             }
         }
 
-        if let Some(param_id) = id_attr {
+        match (id_attr, persist_attr) {
+            (Some(param_id), None) => {
                 // The specific parameter types know how to convert themselves into the correct ParamPtr
                 // variant
-            param_insert_tokens
+                param_mapping_insert_tokens
                     .push(quote! { param_map.insert(#param_id, self.#field_name.as_ptr()); });
             }
+            (None, Some(stable_name)) => {
+                // We don't know anything about the field types, but because we can generate this
+                // function we get type erasure for free since we only need to worry about byte
+                // vectors
+                field_serialize_tokens.push(quote! {
+                    match ::nih_plug::params::serialize_field(&self.#field_name) {
+                        Ok(data) => {
+                            serialized.insert(String::from(#stable_name), data);
+                        }
+                        Err(err) => {
+                            ::nih_plug::nih_log!(
+                                "Could not serialize '{}': {}",
+                                #stable_name,
+                                err
+                            )
+                        }
+                    };
+                });
+                field_deserialize_tokens.push(quote! {
+                     #stable_name => {
+                         match ::nih_plug::params::deserialize_field(&data) {
+                             Ok(deserialized) => {
+                                 self.#field_name = deserialized;
+                             }
+                             Err(err) => {
+                                 ::nih_plug::nih_log!(
+                                     "Could not deserialize '{}': {}",
+                                     #stable_name,
+                                     err
+                                 )
+                             }
+                         };
+                     }
+                });
+            }
+            (Some(_), Some(_)) => {
+                return syn::Error::new(
+                    field.span(),
+                    "The id and persist attributes are mutually exclusive",
+                )
+                .to_compile_error()
+                .into();
+            }
+            (None, None) => (),
+        }
     }
 
     quote! {
@@ -78,10 +151,27 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
             ) -> std::collections::HashMap<&'static str, nih_plug::params::ParamPtr> {
                 let mut param_map = std::collections::HashMap::new();
 
-                #(#param_insert_tokens)*
+                #(#param_mapping_insert_tokens)*
 
                 param_map
             }
+
+            fn serialize_fields(&self) -> ::std::collections::HashMap<String, Vec<u8>> {
+                let mut serialized = ::std::collections::HashMap::new();
+
+                #(#field_serialize_tokens)*
+
+                serialized
+            }
+
+            fn deserialize_fields(&mut self, serialized: ::std::collections::HashMap<&str, Vec<u8>>) {
+                for (field_name, data) in serialized {
+                    match field_name {
+                        #(#field_deserialize_tokens)*
+                        _ => nih_log!("Unknown field name: {}", field_name),
+                    }
+                }
+            }
         }
     }
     .into()

src/params.rs

@@ -21,6 +21,11 @@ use std::pin::Pin;
 pub type FloatParam = PlainParam<f32>;
 pub type IntParam = PlainParam<i32>;
 
+/// Re-export for use in the [Params] proc-macro.
+pub use serde_json::from_slice as deserialize_field;
+/// Re-export for use in the [Params] proc-macro.
+pub use serde_json::to_vec as serialize_field;
+
 /// A distribution for a parameter's range. Probably need to add some forms of skewed ranges and
 /// maybe a callback based implementation at some point.
 #[derive(Debug)]
@@ -292,16 +297,20 @@ impl NormalizebleRange<i32> for Range<i32> {
 /// This implementation is safe when using from the wrapper because the plugin object needs to be
 /// pinned, and it can never outlive the wrapper.
 pub trait Params {
-    /// Create a mapping from unique parameter IDs to parameters. Dereferencing the pointers stored
+    /// Create a mapping from unique parameter IDs to parameters. This is done for every parameter
-    /// in the values is only valid as long as this pinned object is valid.
+    /// field marked with `#[id = "stable_name"]`. Dereferencing the pointers stored in the values
+    /// is only valid as long as this pinned object is valid.
     fn param_map(self: Pin<&Self>) -> HashMap<&'static str, ParamPtr>;
 
-    // TODO: Also handle custom state persistence in this state. Instead o having a callback for
+    /// Serialize all fields marked with `#[persist = "stable_name"]` into a hash map containing
-    //       custom state loading and restoring, it will be way nicer to use serde for this. Another
+    /// JSON-representations of those fields so they can be written to the plugin's state and
-    //       attribute `#[persist]` can mark fields that are `Serialize + Deserialize`. They these
+    /// recalled later. This uses [serialize_field] under the hood.
-    //       fields can then be serialized alongside the `param_id: normalized_value` pairs for
+    fn serialize_fields(&self) -> HashMap<String, Vec<u8>>;
-    //       normal parameters.
+
-    // fn persistent_fields(self: Pin<&mut Self>) -> Vec<&mut (dyn Serialize + Deserialize + Send + Sync)>;
+    /// Restore all fields marked with `#[persist = "stable_name"]` from a hashmap created by
+    /// [Self::serialize_fields]. This gets called when the plugin's state is being restored. This
+    /// uses [deserialize_field] under the hood.
+    fn deserialize_fields(&mut self, serialized: HashMap<&str, Vec<u8>>);
 }
 
 /// Internal pointers to parameters. This is an implementation detail used by the wrappers.