问题
I'm trying to understand the way ARC works, and as far as I know, I should be doing something wrong here. This is the code I'm using:
Interface:
@interface ViewController : UIViewController{
}
@property (strong, nonatomic) NSString * myString ;
@property (weak, nonatomic) NSString * myPointer ;
Implementation:
- (void)viewDidLoad{
[super viewDidLoad];
self.myString = @"Hello world!" ; // myString is strong
self.myPointer = self.myString ; // myPointer var is weak
[self performSelector:@selector(makeNilMyValue) withObject:nil afterDelay:1];
[self performSelector:@selector(printValues) withObject:nil afterDelay:2];
}
- (void) makeNilMyValue{
self.myString = nil ;
}
- (void) printValues{
NSLog(@"myString: %@", self.myString) ;
NSLog(@"myPointer: %@", self.myPointer) ;
}
After executing this, I get:
2012-02-26 11:40:41.652 test1[933:207] myString: (null)
2012-02-26 11:40:41.653 test1[933:207] myPointer: Hello world!
If I'm not wrong, due to myPointer is weak, it shouldn't retain the content of the object. So, it should show nil instead of "Hello World!".
What am I doing wrong?
Following Caleb's answer, I have created another weak pointer, see code below:
- (void)viewDidLoad{
[super viewDidLoad];
self.myString = @"Hello world!" ; // myString is strong
self.myPointer = self.myString ; // myPointer var is weak
self.myPointer2 = self.myString ; // myPointer2 var is weak
[self performSelector:@selector(makeNilMyValue) withObject:nil afterDelay:1];
[self performSelector:@selector(printValues) withObject:nil afterDelay:2];
}
- (void) makeNilMyValue{
self.myPointer2 = @"value changed!" ;
self.myString = nil ;
}
- (void) printValues{
NSLog(@"myString: %@", self.myString) ;
NSLog(@"myPointer: %@", self.myPointer) ;
}
The point is that I still got the same answer I used to have:
2012-02-26 12:08:13.426 test1[1333:207] myString: (null)
2012-02-26 12:08:13.427 test1[1333:207] myPointer: Hello world!
回答1:
As Caleb pointed out, using a constant NSString for this example is not a good idea.
The simplest way to create a string object in source code is to use the Objective-C @"..." construct:
NSString *temp = @"/tmp/scratch"; Note that, when creating a string constant in this fashion, you should avoid using anything but 7-bit ASCII characters. Such an object is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated, though you can retain and release them as you do any other object. You can also send messages directly to a string constant as you do any other string:
BOOL same = [@"comparison" isEqualToString:myString];
The documentation explains that constant strings will never disappear.
Try using something else for your experiment. I tried NSObject and it produced expected results.
Interface:
@interface ViewController : UIViewController
@property (strong, nonatomic) NSObject * myString;
@property (weak, nonatomic) NSObject * myPointer;
@end
Implementation:
@implementation ViewController
@synthesize myString = _myString;
@synthesize myPointer = _myPointer;
- (void)viewDidLoad{
[super viewDidLoad];
self.myString = [[NSObject alloc] init];
self.myPointer = self.myString;
self.myString = nil;
NSLog(@"myString: %@", self.myString);
NSLog(@"myPointer: %@", self.myPointer);
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}
@end
Weak pointers are set to nil when there are no strong pointers to the memory as explained in the documentation - Apple Developer or llvm.
__weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.
回答2:
So, it should show nil instead of "Hello World!".
Constant strings are never deallocated, so your `@"Hello World!" never goes away. This is why your weak reference is never set to nil.
来源：https://stackoverflow.com/questions/9452822/ios-arc-weak-and-strong-properties