如何解决这个变异错误？

Question

我收到了这个错误：

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with replaceCharactersInRange:withString:'

但我无法弄清楚我正在变异的不可变对象。

        NSRange subRange = [self.label.text rangeOfString: @"="];
        int numA = 5;
        int numB = 3;

        NSMutableString *mixed = [NSString stringWithFormat: @"%i %i", numA, numB];

        NSMutableString *string = [NSString stringWithString: self.label.text];

        subRange = [string rangeOfString: @"="];

        if (subRange.location != NSNotFound)
            [string replaceCharactersInRange:subRange withString:mixed];

Answer 1

您的NSMutableString创建调用未正确平衡。您承诺编译器正在创建NSMutableString，但您要求NSString创建实例。

例如：

NSMutableString *mixed = [NSString stringWithFormat: @"%i %i", numA, numB];

需要：

NSMutableString *mixed = [NSMutableString stringWithFormat: @"%i %i", numA, numB];

Answer 2

您的NSMutableString实际上是NSString的实例。这是运行时细节，但至少应该对这些行发出警告。改为：

NSMutableString *string = [self.label.text mutableCopy];

Answer 3

您需要像这样创建NSMutableString：

[NSMutableString stringWithString: self.label.text];