小编Alv*_*ivi的帖子

我正在尝试将列表中的元素打印到新行上,但我无法将其工作;

printElements :: [String] -> IO()
printElements (x:xs) =  print x (some kind of newline, then loop?) printElements xs

所以这:

["1","2","2","4"]

会给:

1
2
3
4

以下代码在线给我一个错误return p.foo(self).错误说:'P' does not have a member named 'foo'.

protocol P {
  typealias T
  func foo(c: C<T>) -> T
  func foo2() -> T
}

class C<T> {
  var p: P
  init (p: P) {
    self.p = p
  }
  func bar() -> T {
    return p.foo(self);
  }
}

该协议P定义了一个相关的类型,它应该与任何专门的C类型正确匹配.我错过了什么吗？或不？

最近，我注意到滚动的性能正在降低。我找到了问题所在，发现原因是使用了UIFont（descriptor：size :)构造函数创建的字体。我更改了UIFont（name：size :)的构造函数，并解决了我的问题。

我在一个项目中隔离了问题。代码是：

import UIKit

class ViewController: UIViewController {
  @IBOutlet weak var firstLabel: UILabel!
  @IBOutlet weak var secondLabel: UILabel!

  @IBAction func onStartTest(sender: AnyObject) {
    startMeasurement()
    let firstFont = UIFont(name: "Marker Felt", size: 16)
    firstLabel.font = firstFont
    finishMeasurement("UIFont(name)")

    startMeasurement()
    let secondFontDescriptor = UIFontDescriptor(name: "Marker Felt", size: 16)
    let secondFont = UIFont(descriptor: secondFontDescriptor, size: 16)
    secondLabel.font = secondFont
    finishMeasurement("UIFont(descriptor)")
  }
}

private var time: UInt64 = 0

public func startMeasurement() {
  time = mach_absolute_time()
} …