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Re-do interrupted and aborted navigations
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Closes #38 by making it explicit that the stop button is meant to trigger the navigate event's AbortSignal.

Closes #10 by getting rid of queued-up navigations, their associated event, and their associated callback variants to update() and push(). Instead, navigations during other navigations interrupt the ongoing one. As the new example tries to show, this seems to work pretty well.

Closes #13 by canceling all pending navigations when another one starts, of which the case discussed there is one example.
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@domenic
domenic committed Mar 11, 2021
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148 changes: 88 additions & 60 deletions README.md
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Expand Up @@ -77,10 +77,10 @@ appHistory.addEventListener("currentchange", e => {
- [Example: handling failed navigations](#example-handling-failed-navigations)
- [Example: single-page app "redirects"](#example-single-page-app-redirects)
- [Example: cross-origin affiliate links](#example-cross-origin-affiliate-links)
- [Aborted navigations](#aborted-navigations)
- [New navigation APIs](#new-navigation-apis)
- [Example: using `navigateInfo`](#example-using-navigateinfo)
- [Navigations while a navigation is ongoing](#navigations-while-a-navigation-is-ongoing)
- [Queued up single-page navigations](#queued-up-single-page-navigations)
- [Example: next/previous buttons](#example-nextprevious-buttons)
- [Per-entry events](#per-entry-events)
- [Current entry change monitoring](#current-entry-change-monitoring)
- [Complete event sequence](#complete-event-sequence)
Expand Down Expand Up @@ -477,6 +477,33 @@ appHistory.addEventListener("navigate", e => {

_TODO: it feels like this should be less disruptive than a cancel-and-perform-new-navigation; it's just a tweak to the outgoing navigation. Using the same code as the previous example feels wrong. See discussion in [#5](https://github.com/WICG/app-history/issues/5)._
#### Aborted navigations
As shown in [the example above](#example-replacing-navigations-with-single-page-app-navigations), the `navigate` event come with an `event.signal` property that is an [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal). This signal will transition to the aborted state if any of the following occur before the promise passed to `respondWith()` settles:
- The user presses their browser's stop button (or similar UI, such as the <kbd>Esc</kbd> key).
- Another navigation is started, either by the user or programmatically. This includes back/forward navigations, e.g. the user pressing their browser's back button.
The signal will not transition to the aborted state if `respondWith()` is not called. This means it cannot be used to observe the interruption of a [cross-document](#appendix-types-of-navigations) navigation, if that cross-document navigation was left alone and not converted into a same-document navigation by using `respondWith()`. Similarly, `window.stop()` will not impact `respondWith()`-derived same-document navigations.
Whether and how the application responds to this abort is up to the web developer. In many cases, such as in [the example above](#example-replacing-navigations-with-single-page-app-navigations), this will automatically work: by passing the `event.signal` through to any `AbortSignal`-consuming APIs like `fetch()`, those APIs will get aborted, and the resulting `"AbortError"` `DOMException` propagated to be the rejection reason for the promise passed to `respondWith()`. But it's possible to ignore it completely, as in the following example:

```js
appHistory.addEventListener("navigate", event => {
event.respondWith((async () => {
await new Promise(r => setTimeout(r, 10_000));
document.body.innerHTML = `Navigated to ${event.destination.url}`;
}());
});
```

In this case:

- The user pressing the stop button will have no effect, and after ten seconds `document.body` will get updated anyway with the destination URL of the original navigation.
- Navigation to another URL will not prevent the fact that in ten seconds `document.body.innerHTML` will be updated to show the original destination URL.

See [the companion document](./interception-details.md#trying-to-interrupt-a-slow-navigation-but-the-navigate-handler-doesnt-care) for full details on exactly what happens in such scenarios.

### New navigation APIs

In a single-page app using `window.history`, the typical flow is:
Expand Down Expand Up @@ -588,85 +615,84 @@ appHistory.addEventListener("navigate", e => {
Note that in addition to `appHistory.push()` and `appHistory.replace()`, the [previously-discussed](#navigation-through-the-app-history-list) `appHistory.back()`, `appHistory.forward()`, and `appHistory.navigateTo()` methods can also take a `navigateInfo` option.
#### Navigations while a navigation is ongoing
#### Example: next/previous buttons
**This section is under heavy construction. There are several open issues on it and it's not fully integrated with the latest thinking on the [detailed navigation interception lifecycle](./interception-details.md).**
Consider trying to code next/previous buttons that perform single-page navigations, for example in a photo gallery application. This has some interesting properties:
Because this proposal makes the web-developer-facing concept of a navigation always asynchronous, i.e. from the start of the `navigate` event through to the end of any promise passed to `respondWith()` settling, it's possible for navigations to happen while an existing navigation is ongoing. Even very simple code like the following would trigger this:
- If the user presses next five times quickly, you want to be sure to skip them ahead five photos, and not to waste work on the intermediate four.
- If the user presses next five times and then previous twice, you want to load the third photo, not wasting work on any others.
- If the user presses previous/next and the previous/next item in their app history is the previous/next photo, then you want to just navigate them through the app history list, like their browser back and forward buttons.
- You'll want to make sure any "permalink" or "share" UI is updated ASAP after such button presses, even if the photo is still loading.

```js
appHistory.push("/first"); // intentionally no `await`
appHistory.push("/second");
```

In this proposal, any [interceptable](#navigation-monitoring-and-interception) navigations are queued up, one after the other: thus, in the above code example, first one complete navigation (including the `navigate` event and any of its work) finishes for `/first`, and only after that's done does the navigation to `/second` go through. This is true regardless of how the navigation is triggered: i.e., the `location.href` setter, `<a>` clicks, and `history.pushState()` all result in such queued-up navigations.
Note that non-interceptable navigations, such as user-initiated navigations via the URL bar or back button, jump the queue and interrupt any ongoing or queued-up navigations. This prevents a deep queue from being used to trap the user on a page.
To give visibility into this queuing process, and allow applications and frameworks to manage the queue, there's an `upcomingnavigate` event. Inside the event handler, you can inspect both the upcoming (queued) navigation's `AppHistoryEntry`, and the `AppHistoryEntry` of the ongoing navigation. You can also discard the upcoming entry. It might be used as follows:
All of this basically "just works" with the `navigate` event and other app history APIs. A large part of this is because `navigate`-event created single-page navigations [synchronously update the current URL](#navigation-monitoring-and-interception), and [abort any ongoing navigations](#aborted-navigations). The code to handle it would look like the following:

```js
appHistory.addEventListener("upcomingnavigate", e => {
if (isNotImportant(e.upcoming.url) && isImportant(e.ongoing.url)) {
e.discardUpcoming();
const appState = {
currentPhoto: 0,
totalPhotos: 10
};
const next = document.querySelector("button#next");
const previous = document.querySelector("button#previous");
const permalink = document.querySelector("span#permalink");
next.onclick = () => {
const nextPhotoInHistory = photoNumberFromURL(appHistory.entries[appHistory.current.index + 1]?.url);
if (nextPhotoInHistory === appState.currentPhoto + 1) {
appHistory.forward();
} else {
appHistory.push(`/photos/${appState.currentPhoto + 1}`);
}
});
```
#### Queued up single-page navigations
**This section is under heavy construction. There are several open issues on it and it's not fully integrated with the latest thinking on the [detailed navigation interception lifecycle](./interception-details.md).**

Consider trying to code a "next" button that performs a single-page navigation. This can be prone to race conditions, since with the app history API, single-page navigations are asynchronous. For example, if you're on `/photos/1` and click the next button twice, the intended behavior is to end up at `photos/3`, even if `photos/2` takes a long time to load and the click handler executes while the URL bar still reads `/photos/1`.
Concretely, code such as the following is buggy:
```js
let currentPhoto = 1;
};
document.querySelector("#next").onclick = async () => {
await appHistory.push(`/photos/${currentPhoto + 1}`);
previous.onclick = () => {
const prevPhotoInHistory = photoNumberFromURL(appHistory.entries[appHistory.current.index - 1]?.url);
if (nextPhotoInHistory === appState.currentPhoto - 1) {
appHistory.back();
} else {
appHistory.push(`/photos/${appState.currentPhoto - 1}`);
}
};
appHistory.addEventListener("navigate", e => {
appHistory.addEventListener("navigate", event => {
const photoNumber = photoNumberFromURL(e.destination.url);
if (photoNumber) {
if (photoNumber && e.canRespond) {
e.respondWith((async () => {
const blob = await (await fetch(`/raw-photos/${photoNumber}.jpg`)).blob();
// Synchronously update app state and next/previous/permalink UI:
appState.currentPhoto = photoNumber;
previous.disabled = appState.currentPhoto === 0;
next.disabled = appState.currentPhoto === appState.totalPhotos - 1;
permalink.textContent = e.destination.url;
// Asynchronously update the photo, passing along the signal so that
// it all gets aborted if another navigation interrupts us:
const blob = await (await fetch(`/raw-photos/${photoNumber}.jpg`, { signal: e.signal })).blob();
const url = URL.createObjectURL(blob);
document.querySelector("#current-photo").src = url;
currentPhoto = photoNumber;
})());
}());
}
});
function photoNumberFromURL(url) {
if (!url) {
return null;
}
const result = /\/photos/(\d+)/.exec((new URL(url)).pathname);
if (result) {
return Number(result[1]);
}
return null;
}
```

To fix this, the `appHistory.push()` and `appHistory.update()` APIs have callback variants. The callback will only be called after all ongoing navigations have finished. This allows non-buggy code such as the following:
Let's look at our scenarios again:
```js
document.querySelector("#next").onclick = async () => {
await appHistory.push(() => {
const photoNumber = photoNumberFromURL(appHistory.current.url);
return { url: `/photos/${photoNumber + 1}` };
});
};
```
Although not shown in the above example, the callback could also return a `state` value.
_TODO: should the callback be able to say "nevermind, I don't care anymore, please don't navigate"? We could let the *caller* do that by passing an `AbortSignal` after the callback... And the general *app* can do it using `upcomingnavigate`... Maybe throwing an exception??_
In general, the idea of these callback variants is that there are cases where the new URL or state is not determined synchronously, and is a function of the current state of the world at the time the navigation is ready to be performed.
- If the user presses next five times quickly, you want to be sure to skip them ahead five photos, and not to waste work on the intermediate four: this works as intended, as the first four fetches (for photos 1, 2, 3, 4) get aborted via their `e.signal`, while the last one (for photo 5) completes.
- If the user presses next five times and then previous twice, you want to load the third photo, not wasting work on any others: this works as intended, as the first six fetches (for photos 1, 2, 3, 4, 5, 4 again) get aborted via their `e.signal`, while the last one (for photo 3 again) completes.
- If the user presses previous/next and the previous/next item in their app history is the previous/next photo, then you want to just navigate them through the app history list, like their browser back and forward buttons: this works as intended, due to the if statements inside the `click` handlers for the next and previous buttons.
- You'll want to make sure any "permalink" or "share" UI is updated ASAP after such button presses, even if the photo is still loading: this works as intended, since we can do this work synchronously before loading the photo.

### Per-entry events

Expand Down Expand Up @@ -776,9 +802,14 @@ Between the per-`AppHistoryEntry` events and the `window.appHistory` events, as
1. Alternately, if the promise passed to `event.respondWith()` rejects:
1. `appHistory.current.finished` changes to `true`.
1. `appHistory.current` fires `finish`.
1. `navigateerror` fires on `window.appHistory`.
1. `navigateerror` fires on `window.appHistory` with the rejection reason as its `error` property.
1. Any loading spinner UI stops.
1. If the process was initiated by a call to an `appHistory` API that returns a promise, then that promise gets rejected with the same rejection reason.
1. Alternately, if the navigation gets [aborted](#aborted-navigations) before either of those two things occur:
1. `appHistory.current.finished` stays `false`, and `appHistory.current` never fires the `finish` event.
1. `navigateerror` fires on `window.appHistory` with an `"AbortError"` `DOMException` as its `error` property.
1. Any loading spinner UI stops. (But potentially restarts, or maybe doesn't stop at all, if the navigation was aborted due to a second navigation starting.)
1. If the process was initiated by a call to an `appHistory` API that returns a promise, then that promise gets rejected with the same with an `"AbortError"` `DOMException`.

For more detailed analysis, including specific code examples, see [this dedicated document](./interception-details.md).

Expand Down Expand Up @@ -1003,7 +1034,7 @@ Note that as currently planned, any such programmatic navigations, including one
### Integration with navigation
To understand when navigation interception and queuing interacts with the existing navigation spec, see [the navigation types appendix](#appendix-types-of-navigations). In cases where interception is allowed and takes place, it is essentially equivalent to preventing the normal navigation and instead performing the [URL and history update steps](https://html.spec.whatwg.org/#url-and-history-update-steps). See more detail in the [dedicated document](./interception-details.md).
To understand when navigation interception interacts with the existing navigation spec, see [the navigation types appendix](#appendix-types-of-navigations). In cases where interception is allowed and takes place, it is essentially equivalent to preventing the normal navigation and instead synchronously performing the [URL and history update steps](https://html.spec.whatwg.org/#url-and-history-update-steps). See more detail in the [dedicated document](./interception-details.md).
The way in which navigation interacts with session history entries generally is not meant to change; the correspondence of a session history entry to an `AppHistoryEntry` does not introduce anything novel there.
Expand Down Expand Up @@ -1056,6 +1087,7 @@ Thanks also to
[@MelSumner](https://github.com/MelSumner),
[@mmocny](https://github.com/mmocny),
[@natechapin](https://github.com/natechapin),
[@pshrmn](https://github.com/pshrmn),
[@SetTrend](https://github.com/SetTrend),
[@slightlyoff](https://github.com/slightlyoff), and
[@Yay295](https://github.com/Yay295)
Expand Down Expand Up @@ -1140,11 +1172,9 @@ interface AppHistory : EventTarget {
Promise<undefined> update(USVString url, optional AppHistoryPushOrUpdateOptions options = {});
Promise<undefined> update(optional AppHistoryPushOrUpdateFullOptions options = {}); // one member required: see issue #52
Promise<undefined> update(AppHistoryNavigationCallback);
Promise<undefined> push(USVString url, optional AppHistoryPushOrUpdateOptions options = {});
Promise<undefined> push(optional AppHistoryPushOrUpdateFullOptions options = {});
Promise<undefined> push(AppHistoryNavigationCallback callback);
Promise<undefined> navigateTo(DOMString key, optional AppHistoryNavigationOptions = {});
Promise<undefined> back(optional AppHistoryNavigationOptions = {});
Expand Down Expand Up @@ -1185,8 +1215,6 @@ dictionary AppHistoryPushOrUpdateFullOptions : AppHistoryPushOrUpdateOptions {
USVString url;
};
callback AppHistoryNavigationCallback = AppHistoryEntryFullOptions ();
[Exposed=Window]
interface AppHistoryNavigateEvent : Event {
constructor(DOMString type, optional AppHistoryNavigateEventInit eventInit = {});
Expand Down
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