WiFi Assist: Apple finally explains how iOS9's data-eating feature works

It does use your data while you're connected to WiFi, but not as much as you might have feared

Craig Federighi, Apple senior vice president of Software Engineering, speaks about iOS 9 during an Apple event (Justin Sullivan/Getty)

Following criticism from users over iOS9's new 'WiFi Assist' feature, which many said was eating up their data allowance when they didn't want it to, Apple has clarified how it works.

WiFi Assist works by swapping between WiFi and mobile data to constantly give you the best connection possible.

So if you have a slow WiFi connection and a webpage is taking ages to load, your phone will switch to data to get it up quicker.

It's a good feature if you have unlimited data, but many people who upgraded to iOS9 without knowing about WiFi Assist found it started using up their data allowance, even when they had turned on WiFi specifically to conserve their data as much as they could.

If you've upgraded to Apple's latest mobile OS, WiFi Assist is turned on automatically. The data icon appears in the top status bar on your iPhone when it's turned on, but this is an easy notification to miss.

Now, Apple has clarified precisely how this useful/annoying feature works.

On a recently-added support page on Apple's website, it says:

WiFi Assist will not automatically switch to cellular if you're data roaming.

So it won't start using data if you're connected to WiFi abroad, saving you the problem of returning home to find you've got a £500 phone bill.

WiFi Assist only works when you have apps running in the foreground and doesn't activate with background downloading of content.

If you're only using WhatsApp on your phone but you've also got Facebook, Maps and Twitter running in the background, it's not going to use up your data updating them.

WiFi Assist doesn't activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.

If you're playing music on Spotify and you've got WiFi Assist on, it's not going to use all your data streaming the song.

Apple's spelled out how WiFi Assist works, but it's not clear why it took them so long to explain.

At any rate, WiFi Assist could eat into your data while you're connected to WiFi, but it's probably not using up as much as you feared.

If you ran out of data after upgrading to iOS9 and blamed WiFi Assist, then maybe it's just because you spent too much time on Facebook.