我刚刚下载最新的XCode(7.1 beta),并开始玩iOS9.
我有一个应用程序在iOS8中完美无缺,但是现在我在UITableViewCell类中覆盖drawRect方法得到以下错误：
“这个应用程序正在从后台线程中修改autolayout引擎,这可能会导致引擎损坏和奇怪的崩溃,这将在以后的版本中引发异常.
这里是回溯：
Stack:( 0 CoreFoundation 0x000000010a749f65 __exceptionPreprocess + 165 1 libobjc.A.dylib 0x0000000109dcfdeb objc_exception_throw + 48 2 CoreFoundation 0x000000010a749e9d +[NSException raise:format:] + 205 3 Foundation 0x0000000109b442e5 _AssertAutolayoutOnMainThreadOnly + 79 4 Foundation 0x00000001099a4ece -[NSISEngine withBehaviors:performModifications:] + 31 5 UIKit 0x000000010b9d425b -[UIView(AdditionalLayoutSupport) _withAutomaticEngineOptimizationdisabledIfEngineExists:] + 58 6 UIKit 0x000000010b9d4d9e -[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded] + 254 7 UIKit 0x000000010b702760 -[UITableViewCellContentView updateConstraintsIfNeeded] + 185 8 UIKit 0x000000010b9d5ab3 -[UIView(AdditionalLayoutSupport) _updateConstraintsAtEngineLevelIfNeeded] + 272 9 UIKit 0x000000010b1e6274 -[UIView(Hierarchy) _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 159 10 UIKit 0x000000010b1f5d84 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 710 11 QuartzCore 0x000000010ae1059a -[CALayer layoutSublayers] + 146 12 QuartzCore 0x000000010ae04e70 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 366 13 QuartzCore 0x000000010ae04cee _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24 14 QuartzCore 0x000000010adf9475 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 277 15 QuartzCore 0x000000010ae26c0a _ZN2CA11Transaction6commitEv + 486 16 QuartzCore 0x000000010ae2737c _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 92 17 CoreFoundation 0x000000010a675967 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23 18 CoreFoundation 0x000000010a6758d7 __CFRunLoopDoObservers + 391 19 CoreFoundation 0x000000010a66ae4c CFRunLoopRunSpecific + 524 20 CoreFoundation 0x000000010a71e011 CFRunLoopRun + 97 21 SDWebImage 0x000000010971773c -[SDWebImageDownloaderOperation start] + 1868 22 Foundation 0x0000000109961e47 __NSOQSchedule_f + 194 23 libdispatch.dylib 0x000000010d93849b _dispatch_client_callout + 8 24 libdispatch.dylib 0x000000010d91e8ec _dispatch_queue_drain + 2215 25 libdispatch.dylib 0x000000010d91de0d _dispatch_queue_invoke + 601 26 libdispatch.dylib 0x000000010d920a56 _dispatch_root_queue_drain + 1420 27 libdispatch.dylib 0x000000010d9204c5 _dispatch_worker_thread3 + 111 28 libsystem_pthread.dylib 0x000000010dc80a9d _pthread_wqthread + 729 29 libsystem_pthread.dylib 0x000000010dc7e3dd start_wqthread + 13 )
这是drawRect方法：
override func drawRect(rect: CGRect) {
super.drawRect(rect)
let cellRect = rect
// Values
let buttonBoxX = CELL_MARGIN + CELL_MARGIN/2
let buttonBoxY = cellRect.height - buttonBoxHeight
let buttonBoxWidth = rect.width - CELL_MARGIN * 3
// Set Button Box
let buttonBoxRect = CGRectMake(buttonBoxX,buttonBoxY,buttonBoxWidth,buttonBoxHeight )
let buttonBox = UIBezierPath(roundedRect: buttonBoxRect,byRoundingCorners: [.Bottomright,.BottomLeft],cornerRadii: CGSize(width: CORNER_RADIUS,height: CORNER_RADIUS)) // Create the path
UIColor.whiteColor().setFill() // Set the Fill to be white
buttonBox.fill()
}
我的理解(参见this question)是,复杂的计算等应该在后台线程上完成,并且在主线程上更新UI,因为UI不是线程安全的.
但是,如果我使用以下内容：
override func drawRect(rect: CGRect) {
super.drawRect(rect)
let cellRect = rect
// Values
let buttonBoxX = CELL_MARGIN + CELL_MARGIN/2
let buttonBoxY = cellRect.height - buttonBoxHeight
let buttonBoxWidth = rect.width - CELL_MARGIN * 3
// Set Button Box
let buttonBoxRect = CGRectMake(buttonBoxX,height: CORNER_RADIUS)) // Create the path
UIColor.whiteColor().setFill() // Set the Fill to be white
dispatch_async(dispatch_get_main_queue(),{
buttonBox.fill()
})
}
我现在得到一个CGContext错误…
<Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable. <Error>: CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable. <Error>: CGContextAddpath: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable. <Error>: CGContextDrawPath: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable. <Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable. <Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace,please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
有什么建议么 ？
解决方案：
好.这是交易.我正在将图像异步加载到UIImageView中,但不是在主线程的UI中“绘制”它,而是在后台线程上.此外;在将图像添加到UI之后,我在UITableViewCell上调用setNeedsdisplay,从而再次调用drawRect方法,但这一次在后台线程上.
解决方法
你注意到的第一个错误看起来不像你的drawRect：代码.你的drawRect：看起来不错没有做任何特别复杂的事情.如果您的路径更复杂,您可能希望缓存它,但可能原样很好.更有可能您正在尝试修改背景线程上其他位置的UI.当您重写drawRect时,它可能只会弹起来,因为这会改变UI更新的发生方式(没有自定义的drawRect：系统可能可以应用简单的转换).您需要查找在后台线程上进行UIKit调用的位置.
如有疑问,如果以UI开头,您可能无法在后台线程上使用它.这不完全是真的(您可以在后台线程上创建UIBezierPath,甚至可以将其绘制到非屏幕上下文中),但作为第一个近似值,这是一件好事.
这段代码是不正确的：
dispatch_async(dispatch_get_main_queue(),{
buttonBox.fill()
})
对drawRect的调用：最好已经在主队列中(所以调度到主队列是没有帮助的).如果不是,见上文.在该块执行时,绘制周期结束,所以没有任何上下文可以进行绘制.