A percentage value that states how far the player has progressed on this achievement.
- iOS 4.0+
- macOS 10.8+
- tvOS 9.0+
- watchOS 3.0+
The default value for a newly initialized achievement object is
0. The range of legal values is between
100, inclusive. You decide how that percentage is calculated and when it changes. For example, if the player earns an achievement simply for discovering a location in your game, then you would simply report the achievement as 100 percent complete the first time you report progress to Game Center. On the other hand, for an achievement like “Capture 10 pirates”, your reporting mechanism increments by 10 percent each time the player captures a pirate.