什么是 Swift 中的桥接转换，如以下警告所示：来自“数据”的条件向下转换？ 到 'CKRecordValue 是一个桥接转换

Question

What is a bridging conversion in Swift? What does "bridging" mean?

I get a warning in the following code where I marked with a comment saying "// warning":

import UIKit
import CloudKit

let int: UInt8 = 1
let data: Data? = Data([int])
let record: CKRecord = CKRecord(recordType: "record_type")
record.setObject(data as? CKRecordValue, forKey: "field") // warning

The warning says:

Conditional downcast from 'Data?'to 'CKRecordValue' (aka '__CKRecordObjCValue') is a bridging conversion; did you mean to use 'as'?

I also have code that uses a bridging conversion:

import Foundation
import CoreData


extension Vision {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Vision> {
        return NSFetchRequest<Vision>(entityName: "Vision")
    }

    @NSManaged public var media: NSObject?

}

private var privateEntityInstance: Vision
private var privateMedia: Data? = nil

privateEntityInstance.media = privateMedia as NSObject?

where privateEntityInstance.media is an optional and privateMedia is also an optional. Would that code work, so that CoreData will save the appropriate value of the media attribute whether it be an NSObject or a nil?

Answer 1

• as? CKRecordValue as? CKRecordValue is conditional downcast (from a less specific to a more specific type)
• as CKRecordValue? is a bridging conversion (for example from a concrete type to a protocol or from a Swift type to its Objective-C counterpart) . This is the syntax the compiler expects.

However in Swift 5 Data conforms to CKRecordValueProtocol so you can write

let int: UInt8 = 1
let data = Data([int])
let record = CKRecord(recordType: "record_type")
record["field"] = data

It's recommended to prefer always Key Subscription record["field"] over setObject:forKey: because the latter requires the bridge cast to an object eg

let data = Data([int]) as NSData // or as CKRecordValue
...
record.setObject(data, forKey: "field")

And don't annotate a worse type (optional) Data? than the actual type (non-optional) Data .