UILabel category block property crash when main thread checked

user_12449179 0
Viewed 53
@interface UILabel (Chainable)
@property (nonatomic, readonly) UILabel *(^color)(NSString *value);
@end

@implementation UILabel (Chainable)
- (UILabel *(^)(NSString *value))color {
  return ^(NSString *value) {
    self.textColor = UIColor.whiteColor; // 
    return self;
  };
}
@end

UILabel *label = [UILabel new].color(@"0xffffff");

Why coding like this cause crash:

Thread 1: EXCBADACCESS (code=1, address=0x0) when xcode -> edit scheme -> Diagnostics -> main thread checker is selected

Only crash when using debug mode.

crash ios
2 Answers

The problem is the name. -color is an existing extension on UILabel. I believe it's been deprecated for longer than iOS has been public (textColor is from iOS 2), but it still exists:

dsdump /System/iOSSupport/System/Library/PrivateFrameworks/UIKitCore.framework | grep color\]
0x00000e95243 -[UILabel(UILabelDeprecatedMethods) color]

It just forwards to -textColor:

                     -[UILabel color]:
0000000000e95243         push       rbp                                         ; Objective C Implementation defined at 0x1629e80 (instance method), DATA XREF=0x1629e80
0000000000e95244         mov        rbp, rsp
0000000000e95247         mov        rsi, qword [0x1675228]                      ; argument "selector" for method _objc_msgSend, @selector(textColor)
0000000000e9524e         pop        rbp
0000000000e9524f         jmp        qword [_objc_msgSend_12b96d8]               ; _objc_msgSend, _objc_msgSend_12b96d8,_objc_msgSend

Making two extensions with the same method name is undefined behavior in ObjC, and you're getting the wrong one.

Without the main thread checker, or in release mode, I expect you're seeing that this is quietly doing nothing. When the main thread checker injects check-code, I expect things don't quite line up, which leads to the crash. (But this is all undefined behavior. Lots of things could happen. I haven't explored the compiler output to see exactly what does happen.)

You need to change the name of this extension.

user_973370

Don't use the name UILabel you can use UILabelCustom interface and implementations then you can call it like

UILabelCustom *label = [UILabelCustom new].color(@"0xffffff");

or then you can also inherit it to the native one UILabel

user_57314790
Related