我在尝试通过将闭包传递给MainActor.run.
为了说明这个问题，考虑一个计算文件夹及其子目录中文件数量的函数。它在Task.detached闭包中运行，因为我不希望它阻塞主线程。每 10,000 个文件更新一个Published属性fileCount，通过将闭包传递给MainThread.run.
但是，用户界面无法更新，甚至给了我一个旋转的沙滩球。阻止这种情况的唯一方法是await Task.sleep(1_000_000_000)在调用之前插入MainThread.run. 这是代码：
final class NewFileCounter: ObservableObject {
@Published var fileCount = 0
func findImagesInFolder(_ folderURL: URL) {
let fileManager = FileManager.default
Task.detached {
var foundFileCount = 0
let options = FileManager.DirectoryEnumerationOptions(arrayLiteral: [.skipsHiddenFiles, .skipsPackageDescendants])
if let enumerator = fileManager.enumerator(at: folderURL, includingPropertiesForKeys: [], options: options) {
while let _ = enumerator.nextObject() as? URL {
foundFileCount += 1
if foundFileCount % 10_000 == 0 {
let fileCount = foundFileCount
await Task.sleep(1_000_000_000) // <-- Only works with this in...comment out to see failure
await MainActor.run { self.fileCount = fileCount }
}
}
let fileCount = foundFileCount
await MainActor.run { self.fileCount = fileCount }
}
}
}
}
如果我恢复到实现此目的的旧方法，该代码将起作用：
final class OldFileCounter: ObservableObject {
@Published var fileCount = 0
func findImagesInFolder(_ folderURL: URL) {
let fileManager = FileManager.default
DispatchQueue.global(qos: .userInitiated).async {
let options = FileManager.DirectoryEnumerationOptions(arrayLiteral: [.skipsHiddenFiles, .skipsPackageDescendants])
var foundFileCount = 0
if let enumerator = fileManager.enumerator(at: folderURL, includingPropertiesForKeys: [], options: options) {
while let _ = enumerator.nextObject() as? URL {
foundFileCount += 1
if foundFileCount % 10_000 == 0 {
let fileCount = foundFileCount
DispatchQueue.main.async { self.fileCount = fileCount }
}
}
let fileCount = foundFileCount
DispatchQueue.main.async { self.fileCount = fileCount }
}
}
}
}
我究竟做错了什么？
顺便说一句 - 如果你想尝试这段代码，这里有一个测试工具。请务必选择一个包含大量文件的文件夹及其子文件夹。
import SwiftUI
@main
struct TestFileCounterApp: App {
var body: some Scene {
WindowGroup {
ContentView()
}
}
}
struct ContentView: View {
@State private var showPickerOld = false
@StateObject private var fileListerOld = OldFileCounter()
@State private var showPickerNew = false
@StateObject private var fileListerNew = NewFileCounter()
var body: some View {
VStack {
Button("Select folder to count files using DispatchQueue...") { showPickerOld = true }
Text("\(fileListerOld.fileCount)").foregroundColor(.green)
.fileImporter(isPresented: $showPickerOld, allowedContentTypes: [.folder], onCompletion: processOldSelectedURL )
Divider()
Button("Select folder to count files using Swift 5.5 concurrency...") { showPickerNew = true }
Text("\(fileListerNew.fileCount)").foregroundColor(.green)
.fileImporter(isPresented: $showPickerNew, allowedContentTypes: [.folder], onCompletion: processNewSelectedURL )
}
.frame(width: 400, height: 130)
}
private func processOldSelectedURL(_ result: Result<URL, Error>) {
switch result {
case .success(let url): fileListerOld.findImagesInFolder(url)
case .failure: return
}
}
private func processNewSelectedURL(_ result: Result<URL, Error>) {
switch result {
case .success(let url): fileListerNew.findImagesInFolder(url)
case .failure: return
}
}
}