在AppleHealthKit中，用户需要授权应用程序才能访问其健康数据。因此，只有授权应用程序才能访问用户的HealthData，其他应用程序无法访问。

以下是一个示例，展示如何使用Swift5和HealthKit API获取特定用户的身体质量指数（Body Mass Index）：

import HealthKit

// 获取HealthKit权限
let healthStore = HKHealthStore()
let bmiQuantity = HKQuantityType.quantityType(forIdentifier: .bodyMassIndex)!

healthStore.requestAuthorization(toShare: nil, read: [bmiQuantity]) { (success, error) in
    if let error = error {
        print("Error requesting health store authorization: \(error.localizedDescription)")
        return
    }
    
    if success {
        // 获取当前用户的身体质量指数
        let calendar = Calendar.current
        let now = Date()
        let components = calendar.dateComponents([.year, .month, .day], from: now)
        
        guard let startDate = calendar.date(from: components) else {
            print("Error creating start date")
            return
        }
        
        guard let endDate = calendar.date(byAdding: .day, value: 1, to: startDate) else {
            print("Error creating end date")
            return
        }
        
        let predicate = HKQuery.predicateForSamples(withStart: startDate, end: endDate, options: [])
        let sortDescriptor = NSSortDescriptor(key: HKSampleSortIdentifierEndDate, ascending: false)
        
        let query = HKSampleQuery(sampleType: bmiQuantity, predicate: predicate, limit: HKObjectQueryNoLimit, sortDescriptors: [sortDescriptor]) { (query, results, error) in
            if let error = error {
                print("Error fetching body mass index: \(error.localizedDescription)")
                return
            }
            
            if let bmiSamples = results as? [HKQuantitySample] {
                if let mostRecentBmi = bmiSamples.first {
                    let bmiUnit = HKUnit.count().unitDivided(by: HKUnit.meter().unitSquared())
                    let bmi = most