问题
I made a mistake while creating a TableView class, and accidentally kept my @property as copy when I defined it:
@property (copy, nonatomic) NSMutableArray *words;
I initialised the array "correctly": (note this is the third attempt, so please ignore the fact that I'm not using mutableCopy and other better ways of doing this)
NSArray *fixedWords = @[@"Eeny", @"Meeny", @"Miny", @"Moe", @"Catch", @"A", @"Tiger", @"By", @"His", @"Toe"];
NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords];
self.words = mutWords;
However when I later came to reorder the array, it crashed on the removeObjectAtIndex line:
id object = [self.words objectAtIndex:fromIndexPath.row];
NSUInteger from = fromIndexPath.row;
NSUInteger to = toIndexPath.row;
[self.words removeObjectAtIndex:from];
With the error message
unrecognized selector sent to instance
Took a lot of digging to figure out that this is because the copy means that assigning the NSMutableArray results in creation of a standard (nonmutable) NSArray. Can anyone explain why this is the correct behaviour?
回答1:
-copy, as implemented by mutable Cocoa classes, always returns their immutable counterparts. Thus, when an NSMutableArray is sent -copy, it returns an NSArray containing the same objects.
Because words has the memory qualifier copy, this line:
NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords];
self.words = mutWords;
Expands out to:
NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords];
self.words = [mutWords copy];
Given that NSMutableArray is a subclass of NSArray, the compiler doesn't complain, and you now have a ticking time bomb on your hands because NSArray does not recognize it's mutable subclass' methods (because it cannot mutate it's contents).
回答2:
Properties aren't magical, they're just shorthand. Declaring a @property on your object tells the compiler to create a backing instance variable and accessor methods for it. The actual generated code depends on the attributes you set on your property.
It's important to remember that setting a property using dot syntax is also shorthand. When you call…
self.words = mutWords;
…you're actually invoking the generated accessor method behind the scenes, like this:
[self setWords:mutWords];
Since you specified the copy attribute on your property, you've told the compiler to generate that -setWords: accessor method with code that looks something like this:
- (void)setWords:(NSMutableArray *)words
{
_words = [words copy];
}
Knowing all that, you can see what's happening: the generated setter method will call -copy on the input argument, and assign the result to the backing instance variable. Because the -copy method is always implemented to return a non-mutable object (performing [aMutableString copy] will return an NSString, and so on) setting that property will always store a non-mutable copy.
来源：https://stackoverflow.com/questions/14856681/why-does-a-copy-nonatomic-nsmutablearray-property-create-nsarrays