Swift中Operation间如何传递数据？我的实现遇异常

问题描述 #

我正在学习Swift的Operation相关内容，能将一系列操作加入队列运行，但操作间传递数据时遇到问题：getVOOCSVData操作能正确获取数据（可通过其completionBlock中的打印语句确认），但后续的convertStringToArray操作运行时传入的数据为空。

原代码 #

操作配置代码 #

// operation setup
struct VOOModel {
    init() {
        let operationQueue = OperationQueue()
        let checkForUpdatedVOOData = IsUpdatedVOODataAvailable()
        checkForUpdatedVOOData.completionBlock = {
            print("Updated CSV Data = \(checkForUpdatedVOOData.updatedCSVData)")
            print("Latest Date = \(checkForUpdatedVOOData.latestCSVDate)")
        }
        let getVOOCSVData = GetVOOCSVData()
        getVOOCSVData.addDependency(checkForUpdatedVOOData)
        getVOOCSVData.completionBlock = {
            print("completion block get voo csv data")
            print("\(getVOOCSVData.csvData)") // prints out all of the data correctly
        }
        let convertStringToArray = ConvertStringToArray2(csvData: getVOOCSVData.csvData)
        convertStringToArray.addDependency(getVOOCSVData)
        convertStringToArray.completionBlock = {
            print("completion block convert string to array")
        }
        operationQueue.addOperations([checkForUpdatedVOOData, getVOOCSVData, convertStringToArray], waitUntilFinished: true)
    } // end init
} // end structure

ConvertStringToArray2类实现 #

// operation class
final class ConvertStringToArray2: Operation {
    var localData: String
    var sortedDataArray : [IndexFund] = []
    init(csvData: String) {
        localData = csvData
    }
    override func main() {
        var dataArray = [IndexFund]()
        var rows = localData.components(separatedBy: "\n")  // gives rows a value of 1
        rows.removeFirst()
        for row in rows {
            let columns = row.components(separatedBy: ",")
            if columns.count == 6 {
                let theDate = columns[0].replacingOccurrences(of: "-", with: "")
                let timeStamp = stringDateFormatter.date(from: theDate)!
                let open = Double(columns[1])!
                let high = Double(columns[2])!
                let low = Double(columns[3])!
                let close = Double(columns[4])!
                let theVolume = columns[5].trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)
                let volume = Int64(theVolume)
                let fundValuesByDay = IndexFund(
                    timeStamp: timeStamp,
                    open: open,
                    high: high,
                    low: low,
                    close: close,
                    volume: volume!)
                dataArray.append(fundValuesByDay)
            }
            sortedDataArray = dataArray.sorted {
                $0.timeStamp < $1.timeStamp
            }
        }
    }
}

问题原因 #

初始化ConvertStringToArray2时直接传入getVOOCSVData.csvData，此时getVOOCSVData还未执行，csvData是空值。虽然通过addDependency保证了操作执行顺序，但初始化是同步完成的，依赖机制不影响初始化阶段的参数传递。

解决方案 #

方案一：修改操作类，持有前序操作引用 #

修改ConvertStringToArray2，让它持有getVOOCSVData的弱引用，在main方法中获取已赋值的数据：

final class ConvertStringToArray2: Operation {
    weak var csvDataOperation: GetVOOCSVData?
    var sortedDataArray : [IndexFund] = []
    
    init(operation: GetVOOCSVData) {
        csvDataOperation = operation
        super.init()
    }
    
    override func main() {
        guard let localData = csvDataOperation?.csvData, !localData.isEmpty else {
            print("CSV数据为空，无法转换")
            return
        }
        
        var dataArray = [IndexFund]()
        let rows = localData.components(separatedBy: "\n").dropFirst()
        
        for row in rows {
            let columns = row.components(separatedBy: ",")
            guard columns.count == 6,
                  let timeStamp = stringDateFormatter.date(from: columns[0].replacingOccurrences(of: "-", with: "")),
                  let open = Double(columns[1]),
                  let high = Double(columns[2]),
                  let low = Double(columns[3]),
                  let close = Double(columns[4]),
                  let volume = Int64(columns[5].trimmingCharacters(in: .whitespacesAndNewlines)) else {
                continue
            }
            
            let fundValuesByDay = IndexFund(
                timeStamp: timeStamp,
                open: open,
                high: high,
                low: low,
                close: close,
                volume: volume
            )
            dataArray.append(fundValuesByDay)
        }
        
        sortedDataArray = dataArray.sorted { $0.timeStamp < $1.timeStamp }
    }
}

同时修改操作初始化代码：

let convertStringToArray = ConvertStringToArray2(operation: getVOOCSVData)
convertStringToArray.addDependency(getVOOCSVData)

方案二：在前序操作完成后创建后续操作 #

修改getVOOCSVData的completionBlock，在数据准备完成后再创建并添加convertStringToArray操作：

struct VOOModel {
    init() {
        let operationQueue = OperationQueue()
        let checkForUpdatedVOOData = IsUpdatedVOODataAvailable()
        checkForUpdatedVOOData.completionBlock = {
            print("Updated CSV Data = \(checkForUpdatedVOOData.updatedCSVData)")
            print("Latest Date = \(checkForUpdatedVOOData.latestCSVDate)")
        }
        
        let getVOOCSVData = GetVOOCSVData()
        getVOOCSVData.addDependency(checkForUpdatedVOOData)
        
        getVOOCSVData.completionBlock = {
            print("completion block get voo csv data")
            print("\(getVOOCSVData.csvData)")
            
            guard !getVOOCSVData.csvData.isEmpty else { return }
            
            let convertStringToArray = ConvertStringToArray2(csvData: getVOOCSVData.csvData)
            convertStringToArray.completionBlock = {
                print("completion block convert string to array")
            }
            operationQueue.addOperation(convertStringToArray)
        }
        
        operationQueue.addOperations([checkForUpdatedVOOData, getVOOCSVData], waitUntilFinished: true)
    }
}

注意事项 #

• 操作间传递数据时，避免在初始化阶段直接依赖未执行操作的属性，因为初始化是同步的，此时属性值还未被赋值
• 方案一中使用weak引用前序操作，避免循环引用导致内存泄漏
• 方案二更灵活，确保只有前序操作完成且数据有效时才创建后续操作

内容的提问来源于stack exchange，提问作者Chris