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Rust calls Dart

Without this feature, the bridge would be a one-way road; but now we have two-way road ;) In short, Rust can call arbitrary Dart functions ("closures" indeed).

Simple example​

Suppose we have a Rust function that accepts a function (closure):

pub fn rust_function(dart_callback: Fn(String) -> String) {
    dart_callback("Tom"); // Will get `Hello, Tom!`
}

Then we can provide a Dart closure:

rustFunction(dartCallback: (name) => 'Hello, $name!');

Make it compile​

To make it compile, we need a bit of boilerplate, and here is the real code. No worries, they are just syntax noise and does not carry anything special.

pub async fn rust_function(dart_callback: impl Fn(String) -> DartFnFuture<String>) {
    dart_callback("Tom".to_owned()).await; // Will get `Hello, Tom!`
}
await rustFunction(dartCallback: (name) => 'Hello, $name!');

More complicated​

Features mentioned in other sections are supported here as well. For example, you can:

  • Use arbitrary non-encodable / non-transferable Dart objects as argument / return values of the Dart closure (via DartOpaque).
  • Let Rust call Dart which calls Rust which calls Dart which calls Rust ;)
  • ...