Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CAL

2011-06-11 15:19:17.167 ***[930:707] *** Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CALayer position contains NaN: [160 nan]'  
*** Call stack at first throw:  
(  
    0   CoreFoundation                      0x3365d64f __exceptionPreprocess + 114  
    1   libobjc.A.dylib                     0x33aa0c5d objc_exception_throw + 24  
    2   CoreFoundation                      0x3365d491 +[NSException raise:format:arguments:] + 68  
    3   CoreFoundation                      0x3365d4cb +[NSException raise:format:] + 34  
    4   QuartzCore                          0x361db61d _ZL18CALayerSetPositionP7CALayerRKN2CA4Vec2IdEEb + 140  
    5   QuartzCore                          0x361db58b -[CALayer setPosition:] + 38  
    6   QuartzCore                          0x361db4d7 -[CALayer setFrame:] + 390  
    7   UIKit                               0x330eb455 -[UIView(Geometry) setFrame:] + 188  
    8   UIKit                               0x3314854f -[UITableViewCell setFrame:] + 98  
    9   UIKit                               0x33147b3b -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:withIndexPath:] + 850  
    10  UIKit                               0x3314776b -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:] + 34  
    11  UIKit                               0x331400cd -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow:] + 936  
    12  UIKit                               0x3313f27d -[UITableView layoutSubviews] + 140  
    13  UIKit                               0x330eb5fb -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 26  
    14  CoreFoundation                      0x335caf03 -[NSObject(NSObject) performSelector:withObject:] + 22  
    15  QuartzCore                          0x361dcbb5 -[CALayer layoutSublayers] + 120  
    16  QuartzCore                          0x361dc96d CALayerLayoutIfNeeded + 184  
    17  QuartzCore                          0x361e21c5 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 212  
    18  QuartzCore                          0x361e1fd7 _ZN2CA11Transaction6commitEv + 190  
    19  QuartzCore                          0x361db055 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 56  
    20  CoreFoundation                      0x33634a35 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 16  
    21  CoreFoundation                      0x33636465 __CFRunLoopDoObservers + 412  
    22  CoreFoundation                      0x3363775b __CFRunLoopRun + 854  
    23  CoreFoundation                      0x335c7ec3 CFRunLoopRunSpecific + 230  
    24  CoreFoundation                      0x335c7dcb CFRunLoopRunInMode + 58  
    25  GraphicsServices                    0x3413041f GSEventRunModal + 114  
    26  GraphicsServices                    0x341304cb GSEventRun + 62  
    27  UIKit                               0x33114d69 -[UIApplication _run] + 404  
    28  UIKit                               0x33112807 UIApplicationMain + 670  
    29  PXiPhone                            0x000028df main + 70  
    30  PXiPhone                            0x00002894 start + 40  
)  
terminate called after throwing an instance of 'NSException 

 某天，加完一个功能，在iPhone4上报以上异常，然后crash掉，iPhone3上没事。百思不得其解，于是google之。看了一篇stackoverflow.com 的文章，说是某些地方代码除以0了，多见于自定义的UITableView自定义UITableViewCell高度的地方，如：tableView:heightForHeaderInSection:或tableView:heightForFooterInSection:。这两个delegate方法都没有地方需要除的。在仔细看一下文章，说多见于适用sizeWithFont方法计算UILabel宽高的地方。恍然大悟，小弟有一个地方使用了UILabel来显示文本，需要用到sizeWithFont方法计算宽高的，在NSString为nil的时候，sizeWithFont会除以0了。

原文：http://blog.csdn.net/hanspy/article/details/6538156

PS:在使用sizeWithFont计算UItabelView中label中字的长度时，由于表视图的cell是复用的，所以在计算label中的NSString的长度时，NSString可能为nil，此时可能会除以0，便会报错。