小胖说事35-----Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CALayer posi

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，多见于自己定义的UITableView自己定义UITableViewCell高度的地方。如：tableView:heightForHeaderInSection:或tableView:heightForFooterInSection:。这两个delegate方法都没有地方须要除的。在细致看一下文章，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，须要用到sizeWithFont方法计算宽高的，在NSString为nil的时候，sizeWithFont会除以0了。

PS:在使用sizeWithFont计算UItabelView中label中字的长度时。因为表视图的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。