How to support low data mode networking using allowsConstrainedNetworkAccess

Swift version: 5.6

Paul Hudson @twostraws October 19th 2019

iOS lets users enable Low Data Mode for any cellular or WiFi connection, which signals to apps that they should be careful how much data they use. This might mean downloading lower-resolution images, it might mean disabling prefetching, or some other way of cutting down on bandwidth use.

By default your app does not honor the user’s low data mode setting, but you can change that by setting the allowsConstrainedNetworkAccess property to false for a given URLRequest. For example:

var request = URLRequest(url: someURL)
request.allowsConstrainedNetworkAccess = false

When that request executes iOS will immediately return an error if low data mode is enabled, which might be your cue to do another request for less data or lower-resolution images, for example. You can detect this error by typecasting it to a URLError, then checking if the networkUnavailableReason property is set to .constrained:

if let error = error as? URLError, error.networkUnavailableReason == .constrained {
    // user has activated low data mode so this request could not be satisfied
}

Tip: There is a similarly named URLSession property called allowsExpensiveNetworkAccess, which determines whether network requests can be made over a personal hotspot. It’s considered expensive because often users on cellular networks have lower data caps, but broadly speaking you should prefer working with low data mode because it gives users control.