ios - NSInvalidArgumentException -[NSNull 长度] : unrecognized selector sent to instance

Question

我有以下崩溃报告，但无法找到导致崩溃的地方。因为我在任何地方都使用了长度字段。 你知道这次崩溃的原因吗:

Thread : Fatal Exception: NSInvalidArgumentException
0  CoreFoundation                 0x0000000182b78f5c __exceptionPreprocess
1  libobjc.A.dylib                0x000000019776bf80 objc_exception_throw
2  CoreFoundation                 0x0000000182b7fc6c __methodDescriptionForSelector
3  CoreFoundation                 0x0000000182b7cc14 ___forwarding___
4  CoreFoundation                 0x0000000182a80dcc _CF_forwarding_prep_0
5  UIKit                          0x00000001880da13c -[UILabel _textRectForBounds:limitedToNumberOfLines:includingShadow:]
6  UIKit                          0x00000001880da030 -[UILabel _intrinsicSizeWithinSize:]
7  UIKit                          0x00000001881d5328 -[UILabel intrinsicContentSize]
8  UIKit                          0x00000001881d5188 -[UIView(UIConstraintBasedLayout) _generateContentSizeConstraints]
9  UIKit                          0x00000001881d4b9c -[UIView(UIConstraintBasedLayout) _updateContentSizeConstraints]
10 UIKit                          0x00000001881d0c3c -[UIView(AdditionalLayoutSupport) updateConstraints]
11 UIKit                          0x00000001881d4940 -[UILabel updateConstraints]
12 UIKit                          0x00000001888f9e8c -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
13 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
14 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
15 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
16 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
17 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
18 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
19 Foundation                     0x0000000183a14aa0 -[NSISEngine withBehaviors:performModifications:]
20 UIKit                          0x00000001881d3958 -[UIView(AdditionalLayoutSupport) _withAutomaticEngineOptimizationDisabledIfEngineExists:]
21 UIKit                          0x00000001888fa0c8 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
22 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
23 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
24 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
25 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
26 UIKit                          0x00000001888f9dbc -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
27 UIKit                          0x00000001888fa0f4 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:forSecondPass:]
28 UIKit                          0x00000001881d483c __60-[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded]_block_invoke
29 UIKit                          0x00000001881d3968 -[UIView(AdditionalLayoutSupport) _withAutomaticEngineOptimizationDisabledIfEngineExists:]
30 UIKit                          0x00000001881d4470 -[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded]
31 UIKit                          0x00000001888fa798 -[UIView(AdditionalLayoutSupport) _updateConstraintsAtEngineLevelIfNeeded]
32 UIKit                          0x00000001883d1fcc -[UIView(Hierarchy) _updateConstraintsAsNecessaryAndApplyLayoutFromEngine]
33 UIKit                          0x00000001880cb038 -[UIView(CALayerDelegate) layoutSublayersOfLayer:]
34 QuartzCore                     0x00000001878d1f14 -[CALayer layoutSublayers]
35 QuartzCore                     0x00000001878ccb20 CA::Layer::layout_if_needed(CA::Transaction*)
36 QuartzCore                     0x00000001878cc9e0 CA::Layer::layout_and_display_if_needed(CA::Transaction*)
37 QuartzCore                     0x00000001878cc07c CA::Context::commit_transaction(CA::Transaction*)
38 QuartzCore                     0x00000001878cbdd0 CA::Transaction::commit()
39 UIKit                          0x00000001880ce0dc _UIApplicationHandleEventQueue
40 CoreFoundation                 0x0000000182b305a4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
41 CoreFoundation                 0x0000000182b30038 __CFRunLoopDoSources0
42 CoreFoundation                 0x0000000182b2dd38 __CFRunLoopRun
43 CoreFoundation                 0x0000000182a5cdc0 CFRunLoopRunSpecific
44 GraphicsServices               0x000000018dbb0088 GSEventRunModal
45 UIKit                          0x0000000188136f60 UIApplicationMain
46 Olgera                         0x000000010016c938 main (main.m:15)
47 libdyld.dylib                  0x0000000197f968b8 start

另外我有疑问是 UILabel 的 length 属性导致了这个还是 NSNull 对象是另一种类型的对象？

Answer 1

[NSNull null] 是一个特殊的单例值，用于表示不允许 nil 的集合类型中的 null 值，例如 NSArray 和 NSDictionary。

如果您知道某个值(例如来自服务器)可以是 null，则您需要在尝试将其视为其他值之前检查该值

NSDictionary *data = ...
NSString *name = [data objectForKey:@"name"];
// This will cause a crash if the value of the
// `name` key was `null` since objectForKey:
// will return [NSNull null] 
if ([name length] > 0) { 
 // Do something
}

您要做的是在以任何方式使用它之前检查该值是否不是 null

if (name != [NSNull null]) {
  if ([name length] > 0) {
    // Do something
  }
}