Save Weight to HealthKit

SaveWeight.m

double weight = _txtWeight.text.doubleValue;
HKUnit *poundUnit = [HKUnit poundUnit];
HKQuantity *weightQuantity = [HKQuantity quantityWithUnit:poundUnit doubleValue:weight];

HKQuantityType *weightType = [HKQuantityType quantityTypeForIdentifier:HKQuantityTypeIdentifierBodyMass];
NSDate *now = [NSDate date];

HKQuantitySample *weightSample = [HKQuantitySample quantitySampleWithType:weightType quantity:weightQuantity startDate:now endDate:now];

[self.healthStore saveObject:weightSample withCompletion:^(BOOL success, NSError *error) {
    [self retrieveWeight];
}];