如何在swift中同步循环
我正在尝试编写一个列表排序算法,我使用一个网络调用(API请求)到GoogleMapsAPI获取列表中两点之间距离的信息 我使用while循环,并在列表上迭代,直到列表的大小为0 在每次迭代中,我进行一次网络调用,在它响应之后,我从列表中删除一些内容 我已经尝试在下面的代码中使用信号量,但它并没有像预期的那样工作如何在swift中同步循环,swift,Swift,我正在尝试编写一个列表排序算法,我使用一个网络调用(API请求)到GoogleMapsAPI获取列表中两点之间距离的信息 我使用while循环,并在列表上迭代,直到列表的大小为0 在每次迭代中,我进行一次网络调用,在它响应之后,我从列表中删除一些内容 我已经尝试在下面的代码中使用信号量,但它并没有像预期的那样工作 let semaphore = DispatchSemaphore(value: 1) let dispatchQueue = DispatchQueue(label: "taskQu
let semaphore = DispatchSemaphore(value: 1)
let dispatchQueue = DispatchQueue(label: "taskQueue")
dispatchQueue.async {
while unvistedPoints.count > 0{
print("The size of the list is ", unvisited.count)
self.findNextVistablePoint(visited: visitedPoints, unvisted: unvistedPoints, completion: { (pointToVisit) in
let indexofPointToVisit = unvistedPoints.firstIndex(where: {$0 === pointToVisit})
unvistedPoints.remove(at: indexofPointToVisit!)
visitedPoints.append(pointToVisit)
semaphore.signal()
})
semaphore.wait()
}
print语句应该打印6,5,4,3,2,1。等待应该在网络请求之前进行。此外,这里确实没有理由使用dispatchQueue asynch,因为循环所做的工作非常少(网络请求已经是异步的),而且我看不到在这里使用while循环和在可以使用for循环时对数组进行变异的价值。以下是一个例子:
import PlaygroundSupport
import UIKit
struct StarwarsCharacter: Codable {
let name: String
}
enum APIResult<T> {
case failure(Error), success(T)
}
func getCharactersSerially(completion: @escaping (APIResult<StarwarsCharacter>) -> ()) {
var characters: [StarwarsCharacter] = []
let semaphore = DispatchSemaphore(value: 1)
let urls = (1...9).map {"https://swapi.co/api/people/\($0)"}.compactMap(URL.init(string:))
urls.forEach { url in
semaphore.wait()
print("starting request for \(url) at \(Date())")
URLSession.shared.dataTask(with: url) { data, response, error in
print("completed request for \(url) at \(Date())")
defer {
semaphore.signal()
}
guard error == nil,
let data = data,
let character = try? JSONDecoder().decode(StarwarsCharacter.self, from: data) else {
completion(.failure(error ?? NSError()))
return
}
completion(.success(character))
}.resume()
}
}
PlaygroundPage.current.needsIndefiniteExecution = true
getCharactersSerially() { result in
switch result {
case .failure(let error):
print(error.localizedDescription)
case .success(let character):
print(character.name)
}
}
导入PlaygroundSupport
导入UIKit
结构starwarschracter:Codable{
let name:String
}
枚举结果{
案例失败(错误),成功(T)
}
func getCharactersSeries(完成:@escaping(apireult)->()){
变量字符:[starwarschracter]=[]
让信号量=分派信号量(值:1)
让URL=(1…9).map{”https://swapi.co/api/people/\($0)“}.compactMap(URL.init(字符串:))
url.forEach{url在
信号量。等待()
打印(“在\(日期())开始请求\(url))
URLSession.shared.dataTask(with:url){data,response,中的错误
打印(“在\(日期())完成\(url)请求”)
推迟{
信号量
}
保护错误==零,
让数据=数据,
让character=try?JSONDecoder().decode(starwarschracter.self,from:data)else{
完成(.failure(错误??NSError()))
返回
}
完成(.success(字符))
}1.简历()
}
}
PlaygroundPage.current.NeedsDefiniteExecution=true
GetCharactersSerial(){结果为
切换结果{
案例。失败(let错误):
打印(错误。本地化描述)
案例。成功(让角色):
打印(字符.名称)
}
}
等待应该在网络请求之前进行。此外,这里确实没有理由使用dispatchQueue asynch,因为循环所做的工作非常少(网络请求已经是异步的),而且我看不到在这里使用while循环和在可以使用for循环时对数组进行变异的价值。以下是一个例子:
import PlaygroundSupport
import UIKit
struct StarwarsCharacter: Codable {
let name: String
}
enum APIResult<T> {
case failure(Error), success(T)
}
func getCharactersSerially(completion: @escaping (APIResult<StarwarsCharacter>) -> ()) {
var characters: [StarwarsCharacter] = []
let semaphore = DispatchSemaphore(value: 1)
let urls = (1...9).map {"https://swapi.co/api/people/\($0)"}.compactMap(URL.init(string:))
urls.forEach { url in
semaphore.wait()
print("starting request for \(url) at \(Date())")
URLSession.shared.dataTask(with: url) { data, response, error in
print("completed request for \(url) at \(Date())")
defer {
semaphore.signal()
}
guard error == nil,
let data = data,
let character = try? JSONDecoder().decode(StarwarsCharacter.self, from: data) else {
completion(.failure(error ?? NSError()))
return
}
completion(.success(character))
}.resume()
}
}
PlaygroundPage.current.needsIndefiniteExecution = true
getCharactersSerially() { result in
switch result {
case .failure(let error):
print(error.localizedDescription)
case .success(let character):
print(character.name)
}
}
导入PlaygroundSupport
导入UIKit
结构starwarschracter:Codable{
let name:String
}
枚举结果{
案例失败(错误),成功(T)
}
func getCharactersSeries(完成:@escaping(apireult)->()){
变量字符:[starwarschracter]=[]
让信号量=分派信号量(值:1)
让URL=(1…9).map{”https://swapi.co/api/people/\($0)“}.compactMap(URL.init(字符串:))
url.forEach{url在
信号量。等待()
打印(“在\(日期())开始请求\(url))
URLSession.shared.dataTask(with:url){data,response,中的错误
打印(“在\(日期())完成\(url)请求”)
推迟{
信号量
}
保护错误==零,
让数据=数据,
让character=try?JSONDecoder().decode(starwarschracter.self,from:data)else{
完成(.failure(错误??NSError()))
返回
}
完成(.success(字符))
}1.简历()
}
}
PlaygroundPage.current.NeedsDefiniteExecution=true
GetCharactersSerial(){结果为
切换结果{
案例。失败(let错误):
打印(错误。本地化描述)
案例。成功(让角色):
打印(字符.名称)
}
}
下面是一些简化的操场代码,演示如何使用信号量来确保您的请求以串行方式执行:
import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
class SomeAsyncClass {
var unvistedPoints = [ 6,5,4,3,2,1 ]
let dispatchQueue = DispatchQueue(label: "taskQueue") // serial queue
let semaphore = DispatchSemaphore(value: 1)
public func doAsyncStuff() {
for point in self.unvistedPoints {
print("Queuing point \(point)")
dispatchQueue.async {
// block before sending the network request
self.semaphore.wait()
self.makeFakeNetworkRequest(point, completion: {
// request complete
print("Completed \(point)")
self.semaphore.signal()
})
}
}
}
func makeFakeNetworkRequest(_ point:Int, completion:()->()) {
let interval = TimeInterval(exactly: (arc4random() % 3) + 1)!
print("Point \(point): Sleeping for: \(interval)")
Thread.sleep(forTimeInterval: interval)
print("Point \(point): Awoken after: \(interval)")
completion()
}
}
var c = SomeAsyncClass()
c.doAsyncStuff()
以下是输出:
Queuing point 6
Queuing point 5
Queuing point 4
Point 6: Sleeping for: 3.0
Queuing point 3
Queuing point 2
Queuing point 1
Point 6: Awoken after: 3.0
Completed 6
Point 5: Sleeping for: 3.0
Point 5: Awoken after: 3.0
Completed 5
Point 4: Sleeping for: 3.0
Point 4: Awoken after: 3.0
Completed 4
Point 3: Sleeping for: 3.0
Point 3: Awoken after: 3.0
Completed 3
Point 2: Sleeping for: 3.0
Point 2: Awoken after: 3.0
Completed 2
Point 1: Sleeping for: 3.0
Point 1: Awoken after: 3.0
Completed 1
尽管如此,这并不是最好的方法。您最好使用为此目的而设计的iOS结构,即OperationQueue——它具有细粒度并发控制(
maxConcurrentOperationCount
),并且可以用作URLSession(delegateQueue
)的基础。如果适合您的需要,我建议您使用该结构。下面是一些简化的操场代码,演示如何使用信号量来确保您的请求以串行方式执行:
import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
class SomeAsyncClass {
var unvistedPoints = [ 6,5,4,3,2,1 ]
let dispatchQueue = DispatchQueue(label: "taskQueue") // serial queue
let semaphore = DispatchSemaphore(value: 1)
public func doAsyncStuff() {
for point in self.unvistedPoints {
print("Queuing point \(point)")
dispatchQueue.async {
// block before sending the network request
self.semaphore.wait()
self.makeFakeNetworkRequest(point, completion: {
// request complete
print("Completed \(point)")
self.semaphore.signal()
})
}
}
}
func makeFakeNetworkRequest(_ point:Int, completion:()->()) {
let interval = TimeInterval(exactly: (arc4random() % 3) + 1)!
print("Point \(point): Sleeping for: \(interval)")
Thread.sleep(forTimeInterval: interval)
print("Point \(point): Awoken after: \(interval)")
completion()
}
}
var c = SomeAsyncClass()
c.doAsyncStuff()
以下是输出:
Queuing point 6
Queuing point 5
Queuing point 4
Point 6: Sleeping for: 3.0
Queuing point 3
Queuing point 2
Queuing point 1
Point 6: Awoken after: 3.0
Completed 6
Point 5: Sleeping for: 3.0
Point 5: Awoken after: 3.0
Completed 5
Point 4: Sleeping for: 3.0
Point 4: Awoken after: 3.0
Completed 4
Point 3: Sleeping for: 3.0
Point 3: Awoken after: 3.0
Completed 3
Point 2: Sleeping for: 3.0
Point 2: Awoken after: 3.0
Completed 2
Point 1: Sleeping for: 3.0
Point 1: Awoken after: 3.0
Completed 1
尽管如此,这并不是最好的方法。您最好使用为此目的而设计的iOS结构,即OperationQueue——它具有细粒度并发控制(maxConcurrentOperationCount
),并且可以用作URLSession(delegateQueue
)的基础。如果它适合您的需要,我建议您使用该结构