The name 'drying_paint' comes from the expression "watching paint dry".
This module provides a system to "watch" some values for changes and run
code whenever they change.
The typical usage is as follows: you first define a structure to hold
data, including some "watched" data.
```rust
struct HelloData {
name: Watched<String>,
greeting: String,
}
```
Implementing the trait WatcherInit for that structure gives you an place
to set-up the code that should run when a watched value changes.
```rust
impl WatcherInit for HelloData {
fn init(watcher: &mut WatcherMeta<Self>) {
watcher.watch(|root| {
root.greeting = format!("Hello, {}!", root.name);
});
}
}
```
Normally you need to wrap the data struct in a Watcher, so it's common
to alias the watcher type to cleanup the syntax a bit:
```rust
type Hello = Watcher<HelloData>;
```
Creating watchers and setting watched data needs to happen within a
WatchContext. WatchContext::update_current() will cause all the pending
watcher code to run.
```rust
fn main() {
let mut ctx = WatchContext::new();
ctx.with(|| {
let mut obj = Hello::new();
*obj.data_mut().name = "Rust".to_string();
WatchContext::update_current();
assert_eq!(obj.data().greeting, "Hello, Rust!");
});
}
```