acalism

extension UIPageViewController {
    var bk_scrollView: UIScrollView? {
        if let v = view as? UIScrollView { 
            return v // 事实上，view不是UIScrollView类型
        }
        for v in view.subviews where v is UIScrollView {
            return v as? UIScrollView // view.subviews 只有一个元素，其类型是 _UIQueuingScrollView，是 UIScrollView 的子类
        }
        return nil

acalism

// 如果 UIViewController.view.addSubView(scrollView)，会导致屏幕左边缘的返回手势失效。
// 解决办法如下：

if let ges = navigationController?.interactivePopGestureRecognizer {
    scrollView.panGestureRecognizer.require(toFail: ges) // 允许左边缘手势返回
}

// 对于 UIPageViewController，可以先找到其 scrollView，再如法炮制

acalism

// 1. file url 与 path 转换

let tmpURL = URL(fileURLWithPath: NSTemporaryDirectory())
let path = tmpURL.path // 
// 事实上，tmpURL.absoluteString得到的是 file:// 格式，而path是没有协议（scheme）部分的，应以slash开头。




// 2. 检查文件存在性

acalism

// Let's declare two structs that with different variables and different boolean values:

struct A {
    let x = true
}

struct B {
    let y = false
}

acalism

UIDebuggingInformationOverlay is private class created by Apple.
http://ryanipete.com/blog/ios/swift/objective-c/uidebugginginformationoverlay/
https://www.raywenderlich.com/177890/swizzling-in-ios-11-with-uidebugginginformationoverlay


FLEX is preferred, 
https://github.com/Flipboard/FLEX

acalism

po view.recersiveDescription 可以输出整个view层次
po Thread.callStackSymbols   输出栈信息

acalism

// textField included by a cell, and the cell included by a scrollView

override func willMove(toWindow newWindow: UIWindow?) {
    super.willMove(toWindow: newWindow)
    if newWindow == nil {
        NotificationCenter.default.removeObserver(self, name: .UIKeyboardWillChangeFrame, object: nil)
    } else {
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardFrameWillChange(_:)), name: .UIKeyboardWillChangeFrame, object: nil)
    }
}

acalism

extension WKWebView {
    /// sync version
    ///
    /// - Parameters:
    ///   - js: 要执行的javascript代码
    ///   - timeout: 限时返回（单位秒），0表示不限时
    /// - Returns: 执行结果
    func evaluatingJavaScript(_ js: String, timeout: TimeInterval = 0) -> Any? {
        var res: Any? = nil
        var finish = false

acalism

// 这两个方法有冲突（调用时无法区分），除非其中一个去掉IndexPath后的问号

/// 不能有默认参数 nil，因为inout不能操作常量
func shouldChange(indexPath: inout IndexPath?) -> Bool {
    indexPath = IndexPath(item: 0, section: 1)
    return true
}

/// 可以有默认参数 nil，
func shouldChange(indexPath: UnsafeMutablePointer<IndexPath?>? = nil) -> Bool {

acalism

// 1. URL 转为 URLComponents 时，什么时候会变成 nil
// 2. URLComponents 转为 URL 时，什么时候会变成 nil
// 3. URL 协议后不带双斜杠时，为什么 host 会变成 path（如下例）


let str = "http:im.qq.com" // "mailto:someone@example.com"
if let url = URL(string: str), let uc = URLComponents(url: url, resolvingAgainstBaseURL: true) {
    print(url)
    print(uc, uc.url!)
    print(uc.scheme!, uc.path)