Add a simple formatters library

We can add common formats here so you don't need to repeat them all the
time.

plugins/gain/src/lib.rs

@@ -18,6 +18,7 @@
 extern crate nih_plug;
 
 use nih_plug::{
+    formatters,
     params::{FloatParam, Params, Range},
     plugin::{BufferConfig, BusConfig, Plugin, ProcessStatus, Vst3Plugin},
     util,
@@ -53,7 +54,7 @@ impl Default for GainParams {
                 },
                 name: "Gain",
                 unit: " dB",
-                value_to_string: None,
+                value_to_string: formatters::f32_rounded(2),
                 string_to_value: None,
             },
         }

src/formatters.rs

@@ -0,0 +1,22 @@
+// nih-plug: plugins, but rewritten in Rust
+// Copyright (C) 2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+//! Convenience functions for formatting and parsing parameter values in common formats.
+
+/// Round an `f32` value to always have a specific number of decimal digits.
+pub fn f32_rounded(digits: usize) -> Option<Box<dyn Send + Sync + Fn(f32) -> String>> {
+    Some(Box::new(move |x| format!("{:.digits$}", x)))
+}

src/lib.rs

@@ -14,8 +14,11 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+// TODO: Once everything is more fleshed out, document the basic usage of this library
+
 #[macro_use]
 pub mod debug;
+pub mod formatters;
 pub mod params;
 pub mod plugin;
 pub mod util;