feat: workbox-window method to trigger SW update check

[jaulz]

R: @jeffposnick @philipwalton

Fixes #2130

This PR introduces an update method on the Workbox instance in order to check for updates after the page has loaded. This is useful for long running applications that are seldom closed (e.g. mail applications) and where you need to check for updates on regular time interval.

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[coveralls]

Coverage increased (+0.08%) to 78.731% when pulling 364c488 on jaulz:feat/window-update into b0825d7 on GoogleChrome:master.

[philipwalton]

I'm not sure resetting these here is what we want to do. Based on how I describe the intention of the "external" events in the workbox-window guide, an update found via calling update() is (for all intents and purposes) the same as an update found via the app being updated in a new tab.

Do you disagree? Based on what you said in this comment I thought you wanted to treat them the same way: #2130 (comment)

[jaulz]

I changed it now but I don't understand why an update via update() is the same as an update via a new tab? Isn't the update eventually the same as a register() which is called after the page has been loaded?

[philipwalton]

Isn't the update eventually the same as a register() which is called after the page has been loaded?

Not quite. The difference is register() is always called first, and after the register() call you either have an update or you don't. But when you have an update from the register() call (in the same tab), it's always for a SW that is older (or the same age) as the code running on the window.

But if an update is found when calling update() (or .register() in another tab in the future), it means the updated SW is newer than the code running on the page, which means there could potentially be conflicts, depending on how old the code running on the page is.

[philipwalton]

Thanks, this looks good now. Do you want to try adding a test to https://github.com/GoogleChrome/workbox/blob/ba31cb87236ac4d3b1cd9c1b6580d518a6d47b73/test/workbox-window/window/test-Workbox.mjs?

You can debug the window tests by running gulp test-server in your terminal, and then visiting http://localhost:3004/test/workbox-window/window/ in your browser to see the results.

If you're not sure how to do that (or have problems getting the tests to run), I can merge your changes into another branch and add the tests myself when I get a chance.

[jaulz]

Okay, I added some tests... though, I couldn't find out how to actually replicate an update (i.e. a file change). Any idea or do you think these tests are enough?

[philipwalton]

Thanks! (Sorry for the relayed reply)