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API Render lifecycle hooks

You may hook into specific stages of the rendering process in order to change the result or handle error cases.

The techniques below apply to all functions that render, most notably up.render(), up.follow(), up.submit() and up.reload(). For brevity we only use up.render() in examples. Most callbacks also have an equivalent HTML attribute for use in links or forms, e.g. [up-on-loaded] for { onLoaded }.

Running code after rendering

Render returns a promise that fulfills when fragments successfully were inserted and compiled. To run code after fragments were updated, await that promise:

await up.render({ url: '/path', target: '.target' })
console.log("Updated fragment is", document.querySelector('.target'))

Awaiting postprocessing

After the up.render() promise fulfills the fragments may still change further through animation or revalidation. To run code when animations have concluded and cached content was revalidated, use the up.render().finished promise:

let result = await up.render({ target: '.target', url: '/path' }).finished
console.log("Final fragments: ", result.fragments)

The up.render().finished promise resolves to the last up.RenderResult that updated a fragment. If revalidation re-rendered the fragment, it is the result from the second render pass. If no revalidation was performed, or if revalidation yielded an empty response, it is the result from the initial render pass.

The promise rejects when there is any error during the initial render pass or during revalidation.

Instead of awaiting a promise you may also pass an { onFinished } callback.
In HTML you can set an [up-on-finished] attribute on a link or form.

Running code after each render pass

To run code after every render pass, use the { onRendered } callback. This callback may be called zero, one or two times:

  • When the server rendered an empty response, no fragments are updated. { onRendered } is not called.
  • When the server rendered a matching fragment, it will be updated on the page. { onRendered } is called with the result.
  • When revalidation renders a second time, { onRendered } is called again with the final result.

In HTML you can set an [up-on-rendered] attribute on a link or form:

  up-on-rendered="console.log('Updated fragment is', document.querySelector('.target'))">
  Click me

Inspecting the render result

Both up.render() and up.render().finished promises resolve to an up.RenderResult object. You may query this object for the effective results of each render pass:

let result = await up.render({ url: '/path', target: '.target', failTarget: '.errors' })
console.log("Updated layer: ", result.layer)
console.log("Updated fragments: ", result.fragments)
console.log("Effective option used: ", result.options)

Controlling the render process

Functions like up.render() and up.follow() offer numerous options and events that allow you to control the render process.

Intent Hook Type
Inspect response before rendering { onLoaded } Callback
Inspect response before rendering up:fragment:loaded Event
Modify new elements up.compiler() Component registry
Modify new elements { onRendered } Callback
Modify new elements up:fragment:inserted Event
Preserve elements within a fragment [up-keep] HTML attribute
Control scrolling { scroll } Option
Control focus { focus } Option
Control concurrency { abort } Option
Control concurrency { disable } Option

For a full list of available options see up.render() parameters and the lifecycle diagram below.

Handling errors

The promises returned by up.render() and up.render().finished reject if any error is thrown during rendering, or if the server responds with an HTTP error code.

You may handle the following error cases:

Error case Hook Type
Server responds with non-200 HTTP status fail-prefixed options Options
Server responds with non-200 HTTP status up.RenderResult (thrown) Error
Disconnect or timeout up.Offline Error
Disconnect or timeout { onOffline } Callback
Disconnect or timeout up:fragment:offline Event
Target selector not found up.CannotMatch Error
Compiler throws error error Error
Fragment update was aborted up.AbortError Error
Fragment update was aborted up:fragment:aborted Event
Any error thrown while rendering { onError } Callback
Any error thrown while rendering up.Error Error superclass

Errors in user code

Unpoly functions are generally not interrupted by errors in user code, such as compilers, transitions or callbacks.

When a user-provided function throws an exception, Unpoly instead an emits error event on window. The operation then succeeds successfully:

up.compiler('.element', () => { throw new Error('broken compiler') })
let element = up.element.affix(document.body, '.element')
up.hello(element) // no error is thrown

This behavior is consistent with how the web platform handles errors in event listeners and custom elements.

Debugging and testing

Exceptions in user code are also logged to the browser's error console. This way you can still access the stack trace or detect JavaScript errors in E2E tests.

Some test runners like Jasmine already listen to the error event and fail your test if any uncaught exception is observed. In Jasmine you may use jasmine.spyOnGlobalErrorsAsync() to make assertions on the unhandled error.

Error handling example

To demonstrate control flow in case of error, the code below handles many different error cases:

try {
  window.addEventListener('error', function(event) {
    console.log('Compiler threw error', event.error)
  let result = await up.render({
    url: '/path',
    target: '.target',    // selector to replace for 200 OK status
    failTarget: '.errors' // selector to replace for non-200 status
} catch (error) {
  if (error instanceof up.RenderResult) {
    // Server sent HTML with a non-200 status code
    console.log("Updated .errors with", error.fragments)
  } else if (error instanceof up.CannotMatch) {
    console.log("Could not find .target in current page or response")
  } else if (error instanceof up.Aborted) {
    console.log("Request to aborted")
  } else if (error instanceof up.Offline) {
    console.log("Connection loss or timeout")
  } else {
    console.log("Other error while rendering: ", error)

Note how we use a fail-prefixed render option { failTarget } to update a different fragment in case the server responds with an error code. See handling failed responses for more details on handling server responses with an error code.

Preventing a render pass

The render lifecycle emits many events that you can prevent by calling event.preventDefault(). When these events are prevented, the render process will abort and no elements will be changed. Focus and scroll positions will be kept. The up.render() promise will reject with an up.AbortError.

The most important preventable events are:


The last preventable event is up:fragment:loaded. It is emitted after a response is loaded but before any elements were changed.

Also see skipping unnecessary rendering.

Changing options before rendering

Events like up:link:follow, up:form:submit and up:fragment:loaded allow you to adjust render options by mutating event.renderOptions.

The code below will open all form-contained links in an overlay, as to not lose the user's form data:

up.on('up:link:follow', 'form a', function(event, link) {
  if (link.closest('form')) {
    event.renderOptions.layer = 'new'

If you have compilers that only set default attributes, consider using a single event listener that manipulates event.renderOptions. It's much leaner than a compiler, which needs to be called for every new fragment. ]()

Advanced example

Assume we give a new attribute [require-session] to links that require a signed-in user:

<a href="/projects" require-session>My projects</a>

When clicking the link without a session, a login form should open in a modal overlay. When the user has signed in successfully, the overlay closes and the original link is followed.

We can implement this with the following handler:

up.on('up:link:follow', 'a[require-session]', async function(event) {
  if (!isSignedIn()) {
    // Abort the current render pass

    // Wait until the user has signed in in a modal
    await up.layer.ask('/session/new', { acceptLocation: '/welcome' })

    // Start a new render pass with the original render pass

Lifecycle diagram

The diagram below attempts to visualize the sequence of render steps, edge cases and error states.

Render lifecycle diagram