我知道strong和weak是在属性声明中使用的修饰符,而__strong和__weak是在实例变量声明中使用的...strong表示只要我拥有该对象就将其保存在内存中,而weak表示只要其他人具有对它的强引用,就将其保存在内存中...对吗?但我不明白为什么在属性中使用weak,而在实例变量中使用__weak时行为不同?这就是我的疑惑所在...
@interface DemoViewController (){
__weak NSArray *weakArray;
__strong NSArray *strongArray;
__weak NSString *weakString;
__strong NSString *strongString;
}
@property (weak) NSString *weakStringProperty;
@property (strong) NSString *strongStringProperty;
@property (weak) NSArray *weakArrayProperty;
@property (strong) NSArray *strongArrayProperty;
@end
@implementation DemoViewController
- (void)viewDidLoad {
[super viewDidLoad];
strongArray = [[NSArray alloc] initWithObjects:@"one",@"two", nil];
weakArray = strongArray;
NSLog(@"Round:1 strongArray is %@.", strongArray);
NSLog(@"Round:1 weakArray is %@.", weakArray);
strongArray = nil;
NSLog(@"Round:2 strongArray is %@.", strongArray);
NSLog(@"Round:2 weakArray is %@.", weakArray);
self.strongArrayProperty = [[NSArray alloc] initWithObjects:@"one",@"two", nil];
self.weakArrayProperty = self.strongArrayProperty;
NSLog(@"Round:1 strongArrayProperty is %@.", self.strongArrayProperty);
NSLog(@"Round:1 weakArrayProperty is %@.", self.weakArrayProperty);
self.strongArrayProperty = nil;
NSLog(@"Round:2 strongArrayProperty is %@.", self.strongArrayProperty);
NSLog(@"Round:2 weakArrayProperty is %@.", self.weakArrayProperty);
strongString = [[NSString alloc]initWithFormat:@"instanceVariable"];
weakString = strongString;
NSLog(@"Round:1 strongString is %@.", strongString);
NSLog(@"Round:1 weakString is %@.", weakString);
strongString = nil;
NSLog(@"Round:2 strongString is %@.", strongString);
NSLog(@"Round:2 weakString is %@.", weakString);
self.strongStringProperty = [[NSString alloc]initWithFormat:@"Property"];
self.weakStringProperty = self.strongStringProperty;
NSLog(@"Round:1 strongStringProperty is %@.", self.strongStringProperty);
NSLog(@"Round:1 weakStringProperty is %@.", self.weakStringProperty);
self.strongStringProperty = nil;
NSLog(@"Round:2 strongStringProperty is %@.", self.strongStringProperty);
NSLog(@"Round:2 weakStringProperty is %@.", self.weakStringProperty);
}
@end
以下是生成的日志:
Round:1 strongArray is (
one,
two
).
Round:1 weakArray is (
one,
two
).
Round:2 strongArray is (null).
Round:2 weakArray is (null).
Round:1 strongArrayProperty is (
one,
two
).
Round:1 weakArrayProperty is (
one,
two
).
Round:2 strongArrayProperty is (null).
Round:2 weakArrayProperty is (
one,
two
). —???
Round:1 strongString is instanceVariable.
Round:1 weakString is instanceVariable.
Round:2 strongString is (null).
Round:2 weakString is (null).
Round:1 strongStringProperty is Property.
Round:1 weakStringProperty is Property.
Round:2 strongStringProperty is (null).
Round:2 weakStringProperty is Property. ——??
当弱引用对象被设置为nil后,两个弱实例变量都会打印(null),这是符合预期的。但我想知道为什么弱属性weakStringProperty和weakArrayProperty仍然打印它们之前的值,并且表现得像它们强指向strongStringProperty和strongArrayProperty一样?
谢谢 :)