即使Swift被设置为Double，Swift也会给出无效的返回错误？

Question

我收到此错误：

/Users/xxxx/Desktop/xxx/xxx/ViewController.swift:43:38: Unexpected non-void return value in void function 

和其他类似这样即使我的功能设置为双倍返还。

这是每个返回中出现此错误的函数之一。

func readWeight() -> Double { 
    let quantityType = HKQuantityType.quantityType(forIdentifier: HKQuantityTypeIdentifier.bodyMass) 

    let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { 

     query, results, error in 

     if (error != nil) { 
      print(error!) 
      return 0.0 
     } 

     guard let results = results else { 
      print("No results of query") 
      return 0.0 
     } 

     if (results.count == 0) { 
      print("Zero samples") 
      return 0.0 
     } 

     guard let bodymass = results[0] as? HKQuantitySample else { 
      print("Type problem with weight") 
      return 0.0 
     } 
     return bodymass.quantity.doubleValue(for: HKUnit.pound()) 
    } 

    healthKitStore.execute(weightQuery) 
} 

的是如何用一个实例：

print(readWeight()) 

谢谢！

Answer 1

必须在函数结束时返回Double值。

func readWeight() -> Double { 
let quantityType = HKQuantityType.quantityType(forIdentifier:HKQuantityTypeIdentifier.bodyMass) 

let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { 

    query, results, error in 

    if (error != nil) { 
     print(error!) 
     return 0.0 
    } 

    guard let results = results else { 
     print("No results of query") 
     return 0.0 
    } 

    if (results.count == 0) { 
     print("Zero samples") 
     return 0.0 
    } 

    guard let bodymass = results[0] as? HKQuantitySample else { 
     print("Type problem with weight") 
     return 0.0 
    } 
    return bodymass.quantity.doubleValue(for: HKUnit.pound()) 
} 

healthKitStore.execute(weightQuery) 
return some double value here 
} 

Answer 2

我想你想达到什么是类似的东西：

func readWeight(result: @escaping (Double) -> Void) { 
    let quantityType = HKQuantityType.quantityType(forIdentifier: HKQuantityTypeIdentifier.bodyMass) 

    let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { 

    query, results, error in 

    if (error != nil) { 
     print(error!) 
     result(0.0) 
    } 

    guard let results = results else { 
     print("No results of query") 
     result(0.0) 
    } 

    if (results.count == 0) { 
     print("Zero samples") 
     result(0.0) 
    } 

    guard let bodymass = results[0] as? HKQuantitySample else { 
     print("Type problem with weight") 
     result(0.0) 
    } 
    result(bodymass.quantity.doubleValue(for: HKUnit.pound())) 
} 

    healthKitStore.execute(weightQuery) 
} 

然后你可以使用readWeight方式类似：

readWeight(result: { myResult in 
    let comparisonResult = myResult == 0.0 
}) 

所以这里myResult是价值你正在寻找。

这一切都是由于HKSampleQuery异步执行造成的，因此您无法立即知道返回值。你必须等待它的结果，然后把结果作为方法的参数给出一个闭包。在结束

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

Swift: How to pass in a closure as a function argument

Answer 3

的括号内的线路：

也许你应该多看一些关于关闭

let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { ... } 

实际发送另一个关闭工作的一部分作为HKSampleQuery的初始值设定项的参数。把它们想象成一个独立的功能，当你拨打healthKitStore.execute(weightQuery)时，它将被执行。如Apple's documentation中所述，该封闭的签名是：(HKSampleQuery, [HKSample]?, Error?) -> Void。这意味着你不能从该闭包中的语句中返回任何东西。

牢记这一点，你可以修改你的方法如下：

func printWeight() { 
    let quantityType = HKQuantityType.quantityType(forIdentifier: HKQuantityTypeIdentifier.bodyMass) 

    let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { 

     query, results, error in 

     if (error != nil) { 
      print(error!) 
     } 

     guard let results = results else { 
      print("No results of query") 
     } 

     if (results.count == 0) { 
      print("Zero samples") 
     } 

     guard let bodymass = results[0] as? HKQuantitySample else { 
      print("Type problem with weight") 
     } 

     print("Bodymass:", bodymass.quantity.doubleValue(for: HKUnit.pound())) 
    } 

    healthKitStore.execute(weightQuery) 
} 

，然后只需要调用printWeight()得到的结果。

Answer 4

您需要使用块。所以函数本身将返回void。当HKSampleQuery开始时，它会得到执行并等待结果，而您的readWeight函数继续执行，然后结束返回void。此时，您的HKSampleQuery仍在执行。完成后，它会通过完成处理程序发布结果。所以如果你想对结果Double做任何事情，你需要在完成处理程序中完成。所以，你的功能将

func readWeight(completion: @escaping (Double) -> Void) { 
    let quantityType = HKQuantityType.quantityType(forIdentifier: HKQuantityTypeIdentifier.bodyMass) 

    let weightQuery = HKSampleQuery(sampleType: quantityType!, predicate: nil, limit: 1, sortDescriptors: nil) { 

    query, results, error in 

    if (error != nil) { 
     print(error!) 
     completion(0.0) 
    } 

    guard let results = results else { 
     print("No results of query") 
     completion(0.0) 
    } 

    if (results.count == 0) { 
     print("Zero samples") 
     completion(0.0) 
    } 

    guard let bodymass = results[0] as? HKQuantitySample else { 
     print("Type problem with weight") 
     completion(0.0) 
    } 
    completion(bodymass.quantity.doubleValue(for: HKUnit.pound())) 
} 

    healthKitStore.execute(weightQuery) 
} 

要使用结果：

self.readWeight(){ (result) in 
    //This result is a double value that is returned from HKSampleQuery 
}