การเรียกกลับ JavaScript แบบกำหนดเอง

Sometimes, your WebAssembly (WASM) module needs to communicate back to the JavaScript environment it's running in. This is where custom JavaScript callbacks become essential!

They allow your WASM code to trigger actions or pass data back to your web page, making your applications more interactive.

Imagine your WASM module performs complex calculations or processes data. Once it's done, it often needs to:

• Update the UI: Notify JavaScript to refresh a chart or display a result.
• Access Browser APIs: Request JavaScript to use APIs WASM can't directly access, like `localStorage` or `fetch`.
• Log Information: Send debugging messages or status updates to the browser console.
• Handle Events: Respond to user interactions or system events managed by JavaScript.

From the JavaScript side, setting up a callback is straightforward. You simply define a regular JavaScript function. This function will then be made available to your WebAssembly module during its initialization.

Think of it as giving your WASM module a direct line to call back into your JavaScript application when needed.

On the WebAssembly side (e.g., in Rust or C/C++), you need to declare that your module expects to import a function from its host environment (JavaScript).

• In Rust, you use the #[wasm_bindgen] attribute with an extern "C" block.
• This declaration specifies the function's name and its expected signature (parameters and return type).
• It tells the WASM compiler that this function call will be resolved externally by JavaScript.

Let's look at a Rust example. We'll import a JavaScript function named logMessageFromWasm. When Rust calls this, the corresponding JS function will execute.

To run the Rust code from the previous scene, we compile it to a .wasm module and use wasm-bindgen's glue code. Then, we define our JavaScript callback and load the WASM.

Check your browser's developer console after running!

Callbacks are not just for simple notifications; they can also pass data from your WebAssembly module back to JavaScript. This is crucial for returning results or providing context.

• Primitive Types: Numbers (integers, floats) and booleans are passed directly.
• Strings: With wasm-bindgen, Rust strings (`&str`) are efficiently converted to JavaScript strings.
• Complex Objects: For more intricate data structures, `wasm-bindgen` provides mechanisms for seamless serialization and deserialization.

Let's enhance our Rust example to pass a number back to JavaScript. The `log_number_from_wasm` function is imported, expecting an integer argument.

Our Rust function `calculate_and_log` will perform a sum and then pass the result to this JS callback.

Here's the JavaScript setup for the previous Rust example. Notice how the `window.logNumberFromWasm` function now accepts a numeric argument, which it then prints to the console.

Custom callbacks are incredibly versatile and enable WASM to integrate smoothly with the browser's environment. Some common use cases include:

• User Interface Updates: Notifying JS to update elements after a WASM computation is complete.
• Browser API Access: Requesting JS to use `localStorage`, `fetch`, or directly manipulate the DOM.
• Event Handling: Allowing WASM to react to user input or browser events processed by JS.
• Logging and Debugging: Sending detailed messages to the browser console from within WASM for monitoring.

Understanding how to declare and use imported functions is key to effective WASM-JS communication.

You've learned how to enable your WebAssembly modules to call back into JavaScript!

• WASM modules can import and call JavaScript functions.
• wasm-bindgen simplifies this process in Rust, allowing type-safe data exchange.
• Callbacks are crucial for UI updates, accessing browser APIs, and logging from WASM.

This powerful pattern makes WASM applications more dynamic and integrated with the web environment. Keep experimenting!