From Xojo Documentation
| This class is supported on Mobile (iOS).
Use ＃If...＃Endif with the Target... constants to ensure you only use this class where it is supported.
Used when callings declares that require Obj-C blocks. An iOSBlock is created from a Delegate and in turn creates an Objective-C block that will call into the delegate. This Objective-C block is retrieved from the Handle property and passed into declares.