Async watch


This section uses parts of the code created in previous tutorial parts.

Use selectors instead, where possible

This feature can be usually replaced with async selectors and it is recommended to do so.

Each watch reaction can also be async function.

Let's say, each time user opens new todo, we want to check all attachments and warn user if they contain viruses.

We could write such watch like:

// Selector from our previous example
const todoAttachments = selectorFamily(async todoId => {
const attachments = await todoApi.getAttachments(todoId);
return attachments;
watch(async () => {
const { openedTodoId } = todos;
// Note we're using await this time as there is no to suspend inside async function!
const attachments = await todoAttachments(openedTodoId).promise;
const warnings = await scanningApi.scanAttachments(attachments);
if (warnings) {

Few notes here.

  • Again, after each await phase - reaction will check if any previously used values changed. If this is the case - reaction will instantly cancel next steps and start over
  • We're using selector.promise instead of selector.value. This is because we're in async function, so there is no need to suspend current reaction on each async read attempt.

In async functions always use const value = await selector.promise instead of const value = selector.value.

Using inside async functions will throw an error.