信號量

DispatchSemaphore 是GCD提供的API，通過設(shè)置一個閾值，允許競爭許可信號最大的線程數(shù)

• 詳細(xì)機制

1. wait方法判斷和獲取信號量，當(dāng)前信號量>0,開始執(zhí)行任務(wù)(線程)，獲取信號量(-1)
   任務(wù)(線程)執(zhí)行結(jié)束，調(diào)用signal方法，釋放信號量(+1)

2.wait方法判斷和獲取信號量，當(dāng)前信號量<=0，線程進入阻塞狀態(tài)

import Foundation

let maxConcurrentDownloads = 3
let semaphore = DispatchSemaphore(value: maxConcurrentDownloads)

func downloadFile(url: URL, completion: @escaping (Data?, Error?) -> Void) {
    semaphore.wait()  // 獲取信號量-1
    print("Starting download for \(url)")

    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        if let error = error {
            print("Download failed: \(error)")
            completion(nil, error)
        } else {
            print("Download completed for \(url)")
            completion(data, nil)
        }
        
        semaphore.signal()  // 釋放信號量+1
    }
    task.resume()//啟動任務(wù)
}

let urls = [
    URL(string: "https://example.com/file1")!,
    URL(string: "https://example.com/file2")!,
    URL(string: "https://example.com/file3")!,
    URL(string: "https://example.com/file4")!
]

for url in urls {
    downloadFile(url: url) { data, error in
        if let data = data {
            // 處理下載的數(shù)據(jù)
            print("Downloaded data: \(data.count) bytes")
        } else if let error = error {
            // 處理錯誤
            print("Error: \(error)")
        }
    }
}

// 保持主線程運行，以便所有下載任務(wù)完成
RunLoop.main.run()