workbox-precaching routes are not compatible with other packages

[philipwalton]

Library Affected:
workbox-precaching

The addRoute() (and by extension precacheAndRoute()) method in workbox-precaching adds its own fetch event listener + cache-first response logic rather than depending on the Router and Route modules from workbox-routing and the CacheFirst module from workbox-strategies.

This is not ideal for a few reasons:

• Code is duplicated and needs to be tested twice
• Logging for precached routes is different from logging for all other routes
• Precache route logic (the configuration options that addRoute() and precacheAndRoute() offer) is not available to runtime routes.
• Precache route handlers are not exposed to the developer, so they can't be reused with workbox-streams code (instead, developers have to create their own CacheFirst strategy instances anyway).

I'm sure there's historical reasons for (from the sw-precache/sw-toolbox merge), but I think we should explore whether it would be possible to consolidate these features in the future.

IMO, ideally, workbox-precaching would depend on workbox-routing and workbox-strategies, and the configuration options for customizing preaching routes could be added to the workbox-routing package and thus available to all routes.

[jeffposnick]

Historically, there were members of the core team who felt that minimizing cross-module dependencies was a priority, so that using workbox-precaching wouldn't automatically incur the overheard of the module size of workbox-routing and workbox-strategies.

I am... less concerned about that, and in particular, if we assume that more folks are going to create custom Workbox bundles in the future, evaluating costs based on the full bundle size of a given module isn't necessarily as meaningful.

[jeffposnick]

For some more historical context, the v2 codebase had all of this logic inside of the common workbox-sw module, kind of mixed together:

workbox/packages/workbox-sw/src/lib/workbox-sw.js

Lines 131 to 146 in 5b97922

	// Create a Router instance that's used by the `Route` for precached assets.
	// See https://github.com/GoogleChrome/workbox/issues/839
	this._precacheRouter = new Router(
	this._revisionedCacheManager.getCacheName(),
	);
	this._router = new Router(
	this._revisionedCacheManager.getCacheName(),
	);
	if (handleFetch) {
	// Give precedence to the _precacheRouter by registering its `fetch`
	// handler first.
	this._precacheRouter.addFetchListener();
	this._router.addFetchListener();
	}

(I'm not arguing that we should return to that catch-all workbox-sw module; just sharing it for context.)

With v3, we started separating things out into self-contained modules with minimal overlap or cross-module code sharing.

[divyeshsachan]

I do vote for @philipwalton for this, since it'll expose all options of workbox-strategies to precache eventually resolving issue #1760 and add great flexibility. By default it should depend on CacheFirst / CacheFirst strategy.
After this developers can use other strategies for precache like staleWhileRevalidate, etc, refer https://stackoverflow.com/questions/54651722/how-can-i-set-a-default-setcatchhandler-with-google-workbox-v4 and #1897.