Optimistic Updates

Angular Query provides two ways to optimistically update your UI before a mutation has completed. You can either use the onMutate option to update your cache directly, or leverage the returned variables to update your UI from the injectMutation result.

Via the UI

This is the simpler variant, as it doesn't interact with the cache directly.

addTodo = injectMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  // make sure to _return_ the Promise from the query invalidation
  // so that the mutation stays in `pending` state until the refetch is finished
  onSettled: async () => {
    return await queryClient.invalidateQueries({ queryKey: ['todos'] })
  },
}))

addTodo = injectMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  // make sure to _return_ the Promise from the query invalidation
  // so that the mutation stays in `pending` state until the refetch is finished
  onSettled: async () => {
    return await queryClient.invalidateQueries({ queryKey: ['todos'] })
  },
}))

you will then have access to addTodo.variables, which contain the added todo. In your UI list, where the query is rendered, you can append another item to the list while the mutation is pending:

@Component({
  template: `
    @for (todo of todos.data(); track todo.id) {
      <li>{{ todo.title }}</li>
    }
    @if (addTodo.isPending()) {
      <li style="opacity: 0.5">{{ addTodo.variables() }}</li>
    }
  `,
})
class TodosComponent {}

@Component({
  template: `
    @for (todo of todos.data(); track todo.id) {
      <li>{{ todo.title }}</li>
    }
    @if (addTodo.isPending()) {
      <li style="opacity: 0.5">{{ addTodo.variables() }}</li>
    }
  `,
})
class TodosComponent {}

We're rendering a temporary item with a different opacity as long as the mutation is pending. Once it completes, the item will automatically no longer be rendered. Given that the refetch succeeded, we should see the item as a "normal item" in our list.

If the mutation errors, the item will also disappear. But we could continue to show it, if we want, by checking for the isError state of the mutation. variables are not cleared when the mutation errors, so we can still access them, maybe even show a retry button:

@Component({
  template: `
    @if (addTodo.isError()) {
      <li style="color: red">
        {{ addTodo.variables() }}
        <button (click)="addTodo.mutate(addTodo.variables())">Retry</button>
      </li>
    }
  `,
})
class TodosComponent {}

@Component({
  template: `
    @if (addTodo.isError()) {
      <li style="color: red">
        {{ addTodo.variables() }}
        <button (click)="addTodo.mutate(addTodo.variables())">Retry</button>
      </li>
    }
  `,
})
class TodosComponent {}

If the mutation and the query don't live in the same component

This approach works very well if the mutation and the query live in the same component, However, you also get access to all mutations in other components via the dedicated injectMutationState function. It is best combined with a mutationKey:

// somewhere in your app
addTodo = injectMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  onSettled: () => queryClient.invalidateQueries({ queryKey: ['todos'] }),
  mutationKey: ['addTodo'],
}))

// access variables somewhere else

mutationState = injectMutationState<string>(() => ({
  filters: { mutationKey: ['addTodo'], status: 'pending' },
  select: (mutation) => mutation.state.variables,
}))

// somewhere in your app
addTodo = injectMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  onSettled: () => queryClient.invalidateQueries({ queryKey: ['todos'] }),
  mutationKey: ['addTodo'],
}))

// access variables somewhere else

mutationState = injectMutationState<string>(() => ({
  filters: { mutationKey: ['addTodo'], status: 'pending' },
  select: (mutation) => mutation.state.variables,
}))

variables will be an Array, because there might be multiple mutations running at the same time. If we need a unique key for the items, we can also select mutation.state.submittedAt. This will even make displaying concurrent optimistic updates a breeze.

Via the cache

When you optimistically update your state before performing a mutation, there is a chance that the mutation will fail. In most of these failure cases, you can just trigger a refetch for your optimistic queries to revert them to their true server state. In some circumstances though, refetching may not work correctly and the mutation error could represent some type of server issue that won't make it possible to refetch. In this event, you can instead choose to roll back your update.

To do this, injectMutation's onMutate handler option allows you to return a value that will later be passed to both onError and onSettled handlers as the last argument. In most cases, it is most useful to pass a rollback function.

Updating a list of todos when adding a new todo

updateTodo = injectMutation((client) => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await client.cancelQueries({ queryKey: ['todos'] })

    // Snapshot the previous value
    const previousTodos = client.getQueryData(['todos'])

    // Optimistically update to the new value
    client.setQueryData(['todos'], (old) => [...old, newTodo])

    // Return a context object with the snapshotted value
    return { previousTodos }
  },
  // If the mutation fails,
  // use the context returned from onMutate to roll back
  onError: (err, newTodo, context) => {
    client.setQueryData(['todos'], context.previousTodos)
  },
  // Always refetch after error or success:
  onSettled: () => {
    client.invalidateQueries({ queryKey: ['todos'] })
  },
}))

updateTodo = injectMutation((client) => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await client.cancelQueries({ queryKey: ['todos'] })

    // Snapshot the previous value
    const previousTodos = client.getQueryData(['todos'])

    // Optimistically update to the new value
    client.setQueryData(['todos'], (old) => [...old, newTodo])

    // Return a context object with the snapshotted value
    return { previousTodos }
  },
  // If the mutation fails,
  // use the context returned from onMutate to roll back
  onError: (err, newTodo, context) => {
    client.setQueryData(['todos'], context.previousTodos)
  },
  // Always refetch after error or success:
  onSettled: () => {
    client.invalidateQueries({ queryKey: ['todos'] })
  },
}))

Updating a single todo

updateTodo = injectMutation((client) => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await client.cancelQueries({ queryKey: ['todos', newTodo.id] })

    // Snapshot the previous value
    const previousTodo = client.getQueryData(['todos', newTodo.id])

    // Optimistically update to the new value
    client.setQueryData(['todos', newTodo.id], newTodo)

    // Return a context with the previous and new todo
    return { previousTodo, newTodo }
  },
  // If the mutation fails, use the context we returned above
  onError: (err, newTodo, context) => {
    client.setQueryData(['todos', context.newTodo.id], context.previousTodo)
  },
  // Always refetch after error or success:
  onSettled: (newTodo) => {
    client.invalidateQueries({ queryKey: ['todos', newTodo.id] })
  },
}))

updateTodo = injectMutation((client) => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await client.cancelQueries({ queryKey: ['todos', newTodo.id] })

    // Snapshot the previous value
    const previousTodo = client.getQueryData(['todos', newTodo.id])

    // Optimistically update to the new value
    client.setQueryData(['todos', newTodo.id], newTodo)

    // Return a context with the previous and new todo
    return { previousTodo, newTodo }
  },
  // If the mutation fails, use the context we returned above
  onError: (err, newTodo, context) => {
    client.setQueryData(['todos', context.newTodo.id], context.previousTodo)
  },
  // Always refetch after error or success:
  onSettled: (newTodo) => {
    client.invalidateQueries({ queryKey: ['todos', newTodo.id] })
  },
}))

You can also use the onSettled function in place of the separate onError and onSuccess handlers if you wish:

injectMutation({
  mutationFn: updateTodo,
  // ...
  onSettled: (newTodo, error, variables, context) => {
    if (error) {
      // do something
    }
  },
})

injectMutation({
  mutationFn: updateTodo,
  // ...
  onSettled: (newTodo, error, variables, context) => {
    if (error) {
      // do something
    }
  },
})

When to use what

If you only have one place where the optimistic result should be shown, using variables and updating the UI directly is the approach that requires less code and is generally easier to reason about. For example, you don't need to handle rollbacks at all.

However, if you have multiple places on the screen that would require to know about the update, manipulating the cache directly will take care of this for you automatically.