Module yew::platform

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Yew’s compatibility between JavaScript Runtime and Native Runtimes.

This module is also published under the name prokio on


When designing components and libraries that works on both WebAssembly targets backed by JavaScript Runtime and non-WebAssembly targets with Native Runtimes. Developers usually face challenges that requires applying multiple feature flags throughout their application:

  1. Select I/O and timers that works with the target runtime.
  2. Native Runtimes usually require Send futures and WebAssembly types are usually !Send.


To alleviate these issues, Yew implements a single-threaded runtime that executes ?Send (Send or !Send) futures.

On platforms with multi-threading support, Yew spawns multiple independent runtimes proportional to the CPU core number. When tasks are spawned with a runtime handle, it will randomly select a worker thread from the internal pool. All tasks spawned with spawn_local will run on the same thread as the thread the task was running. When the runtime runs in a WebAssembly target, all tasks will be scheduled on the main thread.

This runtime is designed in favour of IO-bounded workload with similar runtime cost. When running I/O workloads, it would produce a slightly better performance as tasks are never moved to another thread. However, If a worker thread is busy, other threads will not be able to steal tasks scheduled on the busy thread. When you have a CPU-bounded task where CPU time is significantly more expensive, it should be spawned with a dedicated thread (or Web Worker) and communicates with the application using channels.

Yew platform provides the following components:

  1. A Task Scheduler that is capable of running non-Send tasks.
  2. A Timer that is compatible with the scheduler backend.
  3. Task Synchronisation Mechanisms.

Runtime Backend

The Yew runtime is implemented with different runtimes depending on the target platform and can use all features (timers / IO / task synchronisation) from the selected native runtime:

  • wasm-bindgen-futures (WebAssembly targets)
  • tokio (non-WebAssembly targets)


  • Asynchronous utilities to work with Strings.
  • Task synchronisation primitives for pinned tasks.
  • Utilities for bridging time and tasks.