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title
Router

ReasonReact comes with a router! We've leveraged the language and library features in order to create a router that's:

  • The simplest, thinnest possible.
  • Easily pluggable anywhere into your existing code.
  • Performant and tiny.

Here's the documented public interface, repeated here:

  • ReasonReactRouter.push(string): takes a new path and update the URL.
  • ReasonReactRouter.replace(string): like push, but replaces the current URL.
  • ReasonReactRouter.watchUrl(f): start watching for URL changes. Returns a subscription token. Upon url change, calls the callback and passes it the ReasonReactRouter.url record.
  • ReasonReactRouter.unwatchUrl(watcherID): stop watching for url changes.
  • ReasonReactRouter.dangerouslyGetInitialUrl(): get url record outside of watchUrl. Described later.
  • ReasonReactRouter.useUrl(~serverUrl): get access to the current url as a hook in your React component. No need to setup watch, unwatch, and dangerouslyGetInitialUrl yourself.

Match a Route

There's no API! watchUrl gives you back a url record of the following shape:

type url = {
  /* path takes window.location.pathname, like "/book/title/edit" and turns it into `["book", "title", "edit"]` */
  path: list(string),
  /* the url's hash, if any. The # symbol is stripped out for you */
  hash: string,
  /* the url's query params, if any. The ? symbol is stripped out for you */
  search: string
};

So the url www.hello.com/book/10/edit?name=Jane#author is given back as:

{
  path: ["book", "10", "edit"],
  hash: "author",
  search: "name=Jane"
}

At this point, you can simply pattern match your way to glory!

let url = ReasonReactRouter.useUrl();
switch (url.path) {
| ["book", id, "edit"] => handleBookEdit(id)
| ["book", id] => getBook(id)
| ["book", id, _] => noSuchBookOperation()
| [] => showMainPage()
| ["shop"] | ["shop", "index"] => showShoppingPage()
| ["shop", ...rest] =>
    /* e.g. "shop/cart/10", but let "cart/10" be handled by another function */
  nestedMatch(rest)
| _ => showNotFoundPage()
};

So you can match a path, match a subset of a path, defer part of a matching to a nested logic, etc.

Tips & Tricks

Notice that this is just normal pattern matching:

[@react.component]
let make = () => {
  let url = ReasonReactRouter.useUrl();

  let nowShowing =
    switch (url.hash, MyAppStatus.isUserLoggedIn) {
    | ("active", _) => Active
    | ("completed", _) => Completed
    | ("shared", true) => Shared
    | ("shared", false) when isSpecialUser => /* handle this state please */
    | ("shared", false) => /* handle this state please */
    | _ => All
    };
  /* ... */
}

Push a New Route

From anywhere in your app, just call e.g. ReasonReactRouter.push("/books/10/edit#validated"). This will trigger a URL change (without a page refresh) and components with useUrls will be re-rendered.

We might provide better facilities for typed routing + payload carrying in the future!

Note: because of browser limitations, changing the URL through JavaScript (aka pushState) cannot be detected. The solution is to change the URL then fire a "popState" event. This is what Router.push does, and what the event useUrl listens to. So if, for whatever reason (e.g. incremental migration), you want to update the URL outside of Router.push, just do window.dispatchEvent(new Event('popState')).

Design Decisions

We always strive to lower the performance and learning overhead in ReasonReact, and our router design's no different. The entire implementation, barring browser features detection, is around 20 lines. The design might seem obvious in retrospect, but to arrive here, we had to dig back into ReactJS internals & future proposals to make sure we understood the state update mechanisms, the future context proposal, lifecycle ordering, etc. and reject some bad API designs along the way. It's nice to arrive at such an obvious solution!

The API also doesn't dictate whether matching on a route should return a component, a state update, or a side-effect. Flexible enough to slip into existing apps.

Performance-wise, a JavaScript-like API tends to use a JS object of route string -> callback. We eschewed that in favor of pattern-matching, since the latter in Reason does not allocate memory, and is compiled to a fast jump table in C++ (through the JS JIT). In fact, the only allocation in the router matching is the creation of the url record!

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