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WebAssembly (WASM) for High Performance Apps · レッスン

WASMによる非同期処理

promisesとasync/awaitを使い、メインスレッドをブロックせずにWASMで時間のかかるタスクを処理する非同期パターンを実装します

「WASMによる非同期処理」はCoddyKit上の無料WebAssembly (WASM) for High Performance Appsレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはWebAssembly (WASM) for High Performance Apps学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 WebAssembly (WASM) for High Performance Appsコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Why Asynchronous WASM?

When a WebAssembly module performs a complex calculation or a long task, it can freeze your web page. This is because JavaScript is single-threaded, and most direct WASM calls are synchronous.

Asynchronous WASM patterns allow these tasks to run without completely blocking the main browser thread, keeping your UI responsive.

WASM's Synchronous Nature

When JavaScript calls an exported WASM function, that function executes entirely before JavaScript regains control. If the WASM function takes a long time, the browser's main thread will be blocked.

This means no UI updates, no user input processing, making the page appear frozen. True parallel execution for WASM often involves Web Workers, which is covered in another lesson.

Strategy: Breaking into Chunks

To prevent blocking the main thread, we can break a long-running WASM task into smaller, manageable chunks. Instead of one huge, blocking WASM call, we make many small ones.

Between each small WASM call, JavaScript can yield control back to the browser's event loop. This allows the UI to update and remain responsive.

WASM Function for a Chunk

Here's a Rust function that represents one "chunk" of work. It takes a starting value and performs a small number of calculations. In a real application, this could process a part of an image or a segment of data.

pub extern "C" fn process_chunk(start_val: u32, iterations: u32) -> u32 {
    let mut sum = start_val;
    for i in 0..iterations {
        sum = sum.wrapping_add(i); // Simulate work, prevent overflow
    }
    sum
}

Promises in JavaScript

To handle asynchronous operations, JavaScript uses Promises. A Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

  • Pending: Initial state, neither fulfilled nor rejected.
  • Fulfilled: Operation completed successfully.
  • Rejected: Operation failed.

We use .then() for success and .catch() for errors.

Simplifying with async/await

async/await is syntactic sugar built on Promises, making asynchronous code look and behave more like synchronous code, improving readability. It's built on Promises.

  • An async function always returns a Promise.
  • The await keyword can only be used inside an async function. It pauses the execution until the Promise settles, allowing other tasks to run.

This is crucial for orchestrating our WASM chunks.

Making WASM Calls Yield

We can create a JavaScript utility function that wraps a synchronous WASM call in a Promise. By using setTimeout(..., 0), we tell the browser to execute the WASM call after the current event loop cycle finishes.

This effectively "yields" control back to the browser, allowing it to update the UI before the next WASM chunk runs.

function callWasmAsync(wasmExports, funcName, ...args) {
  return new Promise(resolve => {
    setTimeout(() => {
      const result = wasmExports[funcName](...args);
      resolve(result);
    }, 0); // Yield control to the event loop
  });
}

Full Async WASM Example

Here's a complete, runnable example demonstrating how to manage a long-running WASM task asynchronously using chunking and JavaScript's async/await.

Notice how `setTimeout(0)` around the WASM call allows the browser to breathe between chunks.

// main.js
// Assume wasm_module.wasm (containing process_chunk) is loaded.
// For this runnable example, we'll simulate the WASM function.

// This simulates our WASM module's process_chunk function
function simulateWasmProcessChunk(start_val, iterations) {
    let sum = start_val;
    for (let i = 0; i < iterations; i++) {
        sum = (sum + i) % 10000; // Simplified calculation
    }
    return sum;
}

// Utility to call a synchronous task asynchronously
function callSyncTaskAsync(taskFunc, ...args) {
  return new Promise(resolve => {
    setTimeout(() => {
      const result = taskFunc(...args);
      resolve(result);
    }, 0); // Yield control to the event loop
  });
}

async function performLongTask() {
  console.log("Starting async task...");
  let currentSum = 0;
  const totalChunks = 5;
  const iterationsPerChunk = 10_000_000; // Simulate heavy work

  for (let i = 0; i < totalChunks; i++) {
    // Call the simulated WASM chunk asynchronously
    currentSum = await callSyncTaskAsync(
      simulateWasmProcessChunk,
      currentSum,
      iterationsPerChunk
    );
    console.log(`Chunk ${i + 1} done. Current sum: ${currentSum}`);
  }
  console.log("Async task finished. Final sum:", currentSum);
  return currentSum;
}

performLongTask();

Handling Errors Asynchronously

Just like any asynchronous operation, you should handle potential errors. If a WASM chunk call fails (e.g., due to invalid input or memory issues), the Promise will reject.

You can use try...catch with async/await or .catch() with Promises to gracefully handle these situations in your JavaScript code.

Async WASM Check

Consider a WebAssembly function heavy_calc() that takes 5 seconds to run. If you call it directly from JavaScript's main thread, what is the primary consequence?

Recap: Async WASM Patterns

We've learned how to handle long-running WASM tasks asynchronously on the main thread without using Web Workers. Key takeaways:

  • WASM functions are synchronous when called from JavaScript.
  • Break long tasks into smaller "chunks."
  • Use JavaScript Promises and async/await to orchestrate these chunks.
  • Yield control to the browser's event loop (e.g., with setTimeout(0)) between chunk calls to keep the UI responsive.

This pattern is crucial for maintaining a smooth user experience in complex web applications.

よくある質問

「WASMによる非同期処理」レッスンは無料ですか？

はい。「WASMによる非同期処理」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、WebAssembly (WASM) for High Performance Appsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 WebAssembly (WASM) for High Performance Appsコースには全4レッスンが含まれています。

「WASMによる非同期処理」で何を学びますか？

promisesとasync/awaitを使い、メインスレッドをブロックせずにWASMで時間のかかるタスクを処理する非同期パターンを実装します ブラウザで直接実行するハンズオンコードでWebAssembly (WASM) for High Performance Appsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

WebAssembly (WASM) for High Performance Appsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWebAssembly (WASM) for High Performance Appsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。

「WASMによる非同期処理」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWebAssembly (WASM) for High Performance Appsレッスンでコードを書いて実行できますか？

はい。すべてのWebAssembly (WASM) for High Performance Appsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. WASMによる非同期処理
  2. カスタムJavaScriptコールバック
  3. エラー処理と例外
  4. JSとWASMの境界を越えたメモリと型付き配列の共有
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