Swift4 'substring(to:)' is deprecated: Please use String slicing subscript with a 'partial range upt
解决Swift4substringdeprecated:PleaseuseStringslicingsubscriptwitha‘partialrangeupto’operator.extensionString{funcsubstring(from:Int?,to:Int?)->String{ifletstart=from{guardstart=0else{return""}}ifletstar