swift - iOS 中的 Collection View

Question

我试图使用 swift 4 和 openWeather API 构建一个天气应用程序，但我被困在这个函数中:

func fetchWeather(){
    let weatherGetter = GetWeather()
    self.cityWeather = [Temperature]()
            for city in cities {
                    weatherGetter.getWeather(city: city) { (temp) in
                        guard let temp = temp else {return}
                        print(temp)
                        self.cityWeather?.append(temp)
                }
            }
    print(self.cityWeather?.count)
    self.collectionView?.reloadData()
}

所以这个函数在控制台中给了我这个

Optional(0) Temperature(city: "London", cityTemperature: "294.06", tempIcon: "01d") Temperature(city: "Tokyo", cityTemperature: "297.33", tempIcon: "09n") Temperature(city: "Paris", cityTemperature: "298.0", tempIcon: "01d") Temperature(city: "Moscow", cityTemperature: "291.41", tempIcon: "01n")

这对我来说真的很奇怪，因为 print(self.cityWeather?.count) 在 for 循环之后使用，但在控制台中 Optional(0) 排在第一位。

所以如果我使用这个方法

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return cityWeather?.count ?? 0
}

我的 Collection View 单元格不会显示，因为它返回 0。

但是如果在 self.cityWeather?.append(temp) 之后使用 print(self.cityWeather) 我明白了! (我删除的所有其他打印语句):

Optional([WhatsWeather.Temperature(city: "Moscow", cityTemperature: "291.41", tempIcon: "01n")]) Optional([WhatsWeather.Temperature(city: "Moscow", cityTemperature: "291.41", tempIcon: "01n"), WhatsWeather.Temperature(city: "London", cityTemperature: "294.05", tempIcon: "01d")]) Optional([WhatsWeather.Temperature(city: "Moscow", cityTemperature: "291.41", tempIcon: "01n"), WhatsWeather.Temperature(city: "London", cityTemperature: "294.05", tempIcon: "01d"), WhatsWeather.Temperature(city: "Paris", cityTemperature: "298.0", tempIcon: "01d")]) Optional([WhatsWeather.Temperature(city: "Moscow", cityTemperature: "291.41", tempIcon: "01n"), WhatsWeather.Temperature(city: "London", cityTemperature: "294.05", tempIcon: "01d"), WhatsWeather.Temperature(city: "Paris", cityTemperature: "298.0", tempIcon: "01d"), WhatsWeather.Temperature(city: "Tokyo", cityTemperature: "297.38", tempIcon: "09n")])

Answer 1

只有在所有城市数据都存储在您的数据源中后，您才应该重新加载 collectionView，因为这是一个 API 调用，接收每个响应都需要时间。因此，您应该做的是在完成内部检查是否已收到所有数据，然后重新加载 collectionView。

func fetchWeather(){
    let weatherGetter = GetWeather()
    self.cityWeather = [Temperature]()
    var networkCallsDone = 0
    for city in cities {
        weatherGetter.getWeather(city: city) { (temp) in
            networkCallsDone += 1
            guard let temp = temp else {
                if networkCallsDone == cities.count {
                    DispatchQueue.main.async {
                        collectionView.reloadData()    //In case the last network call to finish does not return valid data
                    }        
                }
                return
            }
            print(temp)
            self.cityWeather?.append(temp)
            if networkCallsDone == cities.count {
                 DispatchQueue.main.async {
                     collectionView.reloadData()
                 }
            }
        }
    }
}

编辑:感谢 matt 在检查 count 和线程问题时指出逻辑错误。