Swift中viewDidLoad内数组count始终返回0的问题求助

Hey there! I see exactly what's going on here—this is a classic asynchronous operation timing issue. Let's break it down and fix it step by step.

问题原因 #

Your initialization function uses URLSession.dataTask, which runs asynchronously in the background. That means when your viewDidLoad runs:

• You call initialization, but the network request starts immediately in the background and doesn't block the rest of the code.
• The line print(days.count) runs right away, before the network request finishes and self.days = downloadDays.list gets executed. So days is still an empty array at that point.
• The completed closure you pass in only runs after the network request succeeds and days is populated, which is why print(self.days.count) inside it works correctly.

修复方案 #

Here's how to adjust your code to handle this properly:

1. Move table view updates to the completion closure: You need to reload your table view only after the data is ready, so it can display the new content.
2. Fix duplicate delegate setup: You accidentally set tableView.delegate = self twice—we'll clean that up.
3. Set the table view data source: You're missing tableView.dataSource = self (you'll also need to conform to UITableViewDataSource to implement required methods).

修改后的完整代码 #

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource { // Add UITableViewDataSource conformance
    // Object array for weather
    var objectArray = [WeaObject]()
    var days = [List]()
    @IBOutlet weak var tableView: UITableView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Setup table view delegates once
        tableView.delegate = self
        tableView.dataSource = self
        
        initialization { [weak self] in // Use [weak self] to avoid retain cycles
            print("success")
            print(self?.days.count ?? 0)
            // Reload table view now that data is ready
            self?.tableView.reloadData()
        }
        
        // This print will still show 0, and that's expected!
        print("Early days count (before network request finishes): \(days.count)")
    }
    
    func initialization(completed:@escaping () -> ()){
        let jsonUrlString="http://api.openweathermap.org/data/2.5/forecast?zip=22003&appid=c632597b1892b4d415c64ca0e9fca1f1"
        guard let url = URL(string: jsonUrlString) else { return }
        
        URLSession.shared.dataTask(with: url) { [weak self] (data, response, err) in // Add [weak self] here too
            guard let data = data else { return }
            
            do {
                let downloadDays = try JSONDecoder().decode(TestingClass.self, from: data)
                self?.days = downloadDays.list
                
                DispatchQueue.main.async {
                    completed()
                }
            } catch let jsonErr {
                print("Error serializing json:", jsonErr)
            }
        }.resume()
    }
    
    // MARK: - UITableViewDataSource Required Methods
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return days.count // Now this will return the correct count after data loads
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "YourCellIdentifier", for: indexPath)
        // Configure your cell using days[indexPath.row]
        return cell
    }
}

额外注意事项 #

• Retain Cycles: I added [weak self] in both the dataTask closure and the completed closure to prevent memory leaks. Always use weak references when self is captured in asynchronous closures.
• Error Handling: Right now, if the network request fails, your completed closure never runs. You might want to add a failure case to handle errors gracefully (e.g., show an alert to the user).

内容的提问来源于stack exchange，提问作者Benjamin So