Why are Apple and Epic going to court over Fortnite currency?

We look at the issues that led the two firms to pick a fight over content allowed on Apple's App Store

Apple and Epic Games will go head to head in court in front of a US federal judge on Monday, the latest stage in the Fortnite maker's campaign to break open the iPhone's walled garden.

The feud has been growing since last August, when Epic set in motion a plan - known internally as project liberty" - to try to get past the restrictions Apple places on software made for iPhones and iPads. Here is what brought the two companies to this point.