ios - XCode 8 奇怪的 Playground 错误，无法查找符号

Question

我试图在 Xcode 8 的 Playground 中使用自动布局。但是，一旦我添加了这一行，我就开始收到此错误:

Playground execution failed: error: Couldn't lookup symbols:
  __TWPCSo6UIView17PlaygroundSupport22PlaygroundLiveViewableS0_
  __swift_FORCE_LOAD_$_swiftCoreGraphics
  __swift_FORCE_LOAD_$_swiftDarwin
  __TMaC17PlaygroundSupport14PlaygroundPage
  __swift_FORCE_LOAD_$_swiftObjectiveC
  __swift_FORCE_LOAD_$_swiftUIKit
  _playground_log_hidden
  _playground_logger_initialize
  __swift_FORCE_LOAD_$_swiftCoreImage
  __swift_FORCE_LOAD_$_swiftDispatch
  __TFC17PlaygroundSupport14PlaygroundPageau7currentS0_
  __swift_FORCE_LOAD_$_swiftFoundation
  __TFC17PlaygroundSupport14PlaygroundPages8liveViewGSqPS_22PlaygroundLiveViewable__

* thread #1: tid = 0x14b6b4, 0x000000010f2f43c0 MyPlayground`executePlayground, queue = 'com.apple.main-thread', stop reason = breakpoint 1.2
  * frame #0: 0x000000010f2f43c0 MyPlayground`executePlayground
    frame #1: 0x000000010f2f39c0 MyPlayground`__37-[XCPAppDelegate enqueueRunLoopBlock]_block_invoke + 32
    frame #2: 0x000000010fe0c89c CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #3: 0x000000010fdf1944 CoreFoundation`__CFRunLoopDoBlocks + 356
    frame #4: 0x000000010fdf10b5 CoreFoundation`__CFRunLoopRun + 901
    frame #5: 0x000000010fdf0ad4 CoreFoundation`CFRunLoopRunSpecific + 420
    frame #6: 0x000000011518aa61 GraphicsServices`GSEventRunModal + 161
    frame #7: 0x0000000110996de4 UIKit`UIApplicationMain + 159
    frame #8: 0x000000010f2f36e9 MyPlayground`main + 201
    frame #9: 0x00000001132e068d libdyld.dylib`start + 1

我不知道这是什么意思。我的意思是，我的解释是 Xcode 无法找到像 UIKit 这样的库，但我知道情况并非如此，因为我使用 UIKit 并且一直很好，直到那行自动布局使聚会崩溃。源代码真的很小，所以我完整地包含了它:

import UIKit  
import PlaygroundSupport  
let view = UIView(frame: CGRect(x: 0, y: 0, width: 500, height: 500))  
view.translatesAutoresizingMaskIntoConstraints = false  
view.backgroundColor = UIColor.white  
PlaygroundPage.current.liveView = view  
let field = UITextField(frame: CGRect(x: 0, y: 0, width: 200, height: 50))  
field.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1).isActive = true  
field.backgroundColor = UIColor.red  
view.addSubview(field)

源代码底部的第三行似乎是导致问题的原因。这是我搞砸了还是测试版错误？我在这里迷路了。

Answer 1

将 field 添加为 view 的 subview ，然后设置约束。为不在同一 View 层次结构中的 View 添加约束时会引发异常。

例如:

import UIKit
import PlaygroundSupport

let view = UIView(frame: CGRect(x: 0, y: 0, width: 500, height: 500))
view.backgroundColor = UIColor.white
PlaygroundPage.current.liveView = view
let field = UITextField(frame: CGRect(x: 0, y: 0, width: 200, height: 50))

view.addSubview(field)

field.translatesAutoresizingMaskIntoConstraints = false
field.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1).isActive = true
// add the rest of your constraints 
field.backgroundColor = UIColor.red

您通常还需要调用 view.layoutIfNeeded()