我想屯远，作為一個iOS開發(fā)人員悲幅，你應(yīng)該知道DateFormatter的實例創(chuàng)建操作览妖，是多么的昂貴。
在這篇文章中挥下，我想看一下創(chuàng)建DateFormatter實例的成本以及如何有效的緩存他們揍魂。

實驗

1.DateFormatter為每個日期轉(zhuǎn)換創(chuàng)建一個新的實例。
2.DateFormatter對所有日期轉(zhuǎn)換重復(fù)使用相同的實例棚瘟。

class DateConverter {
    
    let dateFormat = "y/MM/dd @ HH:mm"
    
    func convertDatesWithUniqueFormatter(_ dates: [Date]) {
        
        for date in dates {
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = dateFormat
            _ = dateFormatter.string(from: date)
        }
    }
    
    func convertDatesWithReusedFormatter(_ dates: [Date]) {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = dateFormat
        
        for date in dates {
            _ = dateFormatter.string(from: date)
        }
        
    }
    
}

convertDatesWithUniqueFormatter代表第一個場景,convertDatesWithReusedFormatter第二個場景现斋。兩種方法都遵循類似的結(jié)構(gòu) - 循環(huán)遍歷日期數(shù)組并將每個日期格式化為字符串表示形式，唯一的區(qū)別在于如何DateFormatter使用偎蘸。

我們用單元測試來測試一下性能：

import XCTest
@testable import Practice

class PracticeTests: XCTestCase {

    var sut:DateConverter!
    let dates = Array(repeating: Date(), count: 100)
    
    
    override func setUp() {
        // Put setup code here. This method is called before the invocation of each test method in the class.
   
        sut = DateConverter()
    }

    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
   
        sut = nil
    }
    
    func test_convertDatesWithUniqueFormatter_performance() {
        
        measure {
            sut.convertDatesWithUniqueFormatter(dates)
        }
    }
    
    func test_convertDatesWithReusedFormatter_performance() {
        
        measure {
            sut.convertDatesWithReusedFormatter(dates)
        }
    }

    func testExample() {
        // This is an example of a functional test case.
        // Use XCTAssert and related functions to verify your tests produce the correct results.
    
    }

    func testPerformanceExample() {
        // This is an example of a performance test case.
        self.measure {
            // Put the code you want to measure the time of here.
        }
    }

}

場景一：

image.png

場景二：

image.png

如何使用高性能的DateFormatter

既然已經(jīng)確定重用DateFormatter實例可以提高性能，并且我們已經(jīng)確定這種性能改進(jìn)將帶來更好的用戶體驗耻矮，那么問題是：
“我們?nèi)绾沃赜盟厍俊?br> 很簡單，可以將DateFormatter實例提取到局部變量或私有屬性中裆装，因此可以重用它踱承。

class DateFormattingHelper {
    
    static let shared = DateFormattingHelper()
    
    static let dobDateFormatter: DateFormatter = {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "y/MM/dd @ HH:mm"
        return dateFormatter
    }()
    
    func formatDOB(_ date:Date, with dateFormatter: DateFormatter) -> String {
        let formattedDate = dateFormatter.string(from: date)
        return "Date of birth:\(formattedDate)"
    }
}

調(diào)用：

let dateFormatter = DateFormattingHelper.shared.dobDateFormatter
        let dobFormattedString = DateFormattingHelper.shared.formatDOB(Date(), with: dateFormatter)
        print(dobFormattedString)

只要我們在使用的時候，傳入DateFormatter:就可以了哨免，但是茎活，這有個弊端，如果我們多個format格式怎么辦呢琢唾？創(chuàng)建多個這個的static 屬性载荔？？采桃？

更優(yōu)雅的辦法

class CachedDateFormattingHelper {
    // MARK: - Shared
    
    static let shared = CachedDateFormattingHelper()
    
    // MARK: - Queue
    
    let cachedDateFormattersQueue = DispatchQueue(label: "com.boles.date.formatter.queue")
    
    // MARK: - Cached Formatters
    
    private var cachedDateFormatters = [String : DateFormatter]()
    
    private func cachedDateFormatter(withFormat format: String) -> DateFormatter {
        return cachedDateFormattersQueue.sync {
            let key = format
            if let cachedFormatter = cachedDateFormatters[key] {
                return cachedFormatter
            }
            
            let dateFormatter = DateFormatter()
            dateFormatter.locale = Locale(identifier: "en_US_POSIX")
            dateFormatter.dateFormat = format
            
            cachedDateFormatters[key] = dateFormatter
            
            return dateFormatter
        }
    }
    
    // MARK: - DOB
    
    func formatDOBDate(_ date: Date) -> String {
        let dateFormatter = cachedDateFormatter(withFormat: "y/MM/dd @ HH:mm")
        let formattedDate = dateFormatter.string(from: date)
        return ("Date of birth: \(formattedDate)")
    }
    
    // MARK: - Account
    
    func formatLastActiveDate(_ date: Date, now: Date = Date()) -> String {
        let yesterday = Calendar.current.date(byAdding: .day, value: -1, to: now)!
        
        var dateFormatter = cachedDateFormatter(withFormat: "dd MMM @ HH:mm")
        if date > yesterday {
            dateFormatter = cachedDateFormatter(withFormat: "HH:mm")
        }
        
        let formattedDate = dateFormatter.string(from: date)
        return ("Last active: \(formattedDate)")
    }
    
    // MARK: - Post
    
    func formatPostCreatedDate(_ date: Date) -> String {
        let dateFormatter = cachedDateFormatter(withFormat: "d MMM 'of' y")
        let formattedDate = dateFormatter.string(from: date)
        return formattedDate
    }
    
    // MARK: - Commenting
    
    func formatCommentedDate(_ date: Date) -> String {
        let dateFormatter = cachedDateFormatter(withFormat: "dd MMM @ HH:mm")
        let formattedDate = dateFormatter.string(from: date)
        return ("Comment posted: \(formattedDate)")
    }
}

我們用一個字典懒熙，把對應(yīng)的創(chuàng)建過的格式的formatter存起來，每次普办，只要去查一下工扎，有沒有生成過這種格式的formatter，有就拿來用衔蹲，沒有就創(chuàng)建保存肢娘，并且返回出去，這樣下次同樣的格式就有了舆驶。這樣是不是很智能橱健，很酷??????

來吧，改造你的DateFormatter吧Ｉ沉＞械础！撬陵！