1. weak属性(弱引用)
被weak修饰的对象叫弱引用,不算对象持有者,一个方法执行完后会导致这个对象自动释放掉,并将对象的指针设置成nil,我使用GCD延时1000ms来验证,1000ms之后,其对象是否还在.
#import "RootViewController.h" @interface RootViewController () @property (nonatomic, weak) NSString *str; @end @implementation RootViewController /** 延时多少毫秒 @param microseconds 毫秒 @param queue 线程池 @param block 执行代码处 @return none */ - (void)delayTime:(int64_t)microSeconds inQueue:(dispatch_queue_t)queue block:(void (^)(dispatch_queue_t queue))block { dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, microSeconds * USEC_PER_SEC); dispatch_after(popTime, queue, ^(void){ block(queue); }); } - (void)viewDidLoad { [super viewDidLoad]; // 赋值 _str = [NSString stringWithFormat:@"weak"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", _str); }]; } @end
打印信息:
2014-03-31 14:48:29.360 ARC[3387:60b] (null)
被__weak修饰的对象也是弱引用,如下所示,其打印信息也为nil
// 赋值 __weak NSString *str = [NSString stringWithFormat:@"weak"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", str); }];
2. strong属性(强引用)
被strong修饰的对象叫强引用,是对象持有者,一个方法执行完后这个对象不会被释放,我使用GCD延时1000ms来验证,1000ms之后,其对象是否还在.
#import "RootViewController.h" @interface RootViewController () @property (nonatomic, strong) NSString *str; @end @implementation RootViewController /** 延时多少毫秒 @param microseconds 毫秒 @param queue 线程池 @param block 执行代码处 @return none */ - (void)delayTime:(int64_t)microSeconds inQueue:(dispatch_queue_t)queue block:(void (^)(dispatch_queue_t queue))block { dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, microSeconds * USEC_PER_SEC); dispatch_after(popTime, queue, ^(void){ block(queue); }); } - (void)viewDidLoad { [super viewDidLoad]; // 赋值 _str = [NSString stringWithFormat:@"strong"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", _str); }]; } @end
打印信息:
2014-03-31 14:59:57.445 ARC[3599:60b] strong
默认方式创建的对象以及__strong方式修饰的对象都是强引用,其打印信息是"strong"
-默认方式-
// 赋值 NSString *str = [NSString stringWithFormat:@"strong"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", str); }];
-__strong修饰方式-
// 赋值 __strong NSString *str = [NSString stringWithFormat:@"strong"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", str); }];
strong修饰的对象没有释放,则weak还是可以用的
// 赋值 NSString *str = [NSString stringWithFormat:@"strong"]; __weak NSString *tmp = str; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", tmp); NSLog(@"%@", str); }];
打印信息:
2014-03-31 15:27:48.894 ARC[4144:60b] strong
2014-03-31 15:27:48.897 ARC[4144:60b] strong
以下例子按理说stringTest中retStr属于强引用,但其值赋给tmp时,却打印为nil,为什么呢?
ARC下，当一个函数返回一个NSObject指针时，编译器会帮我们实现autorelease调用,也就是retStr与返回值不是一个东西了.
- (void)viewDidLoad { [super viewDidLoad]; // 赋值 NSString *str = [NSString stringWithFormat:@"strong"]; __weak NSString *tmp = [self stringTest]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", tmp); NSLog(@"%@", str); }]; } - (NSString *)stringTest { __strong NSString *retStr = [NSString stringWithFormat:@"strongVer2"]; // 延时1000毫秒 [self delayTime:1000 inQueue:dispatch_get_main_queue() block:^(dispatch_queue_t queue) { NSLog(@"%@", retStr); }]; return retStr; }
打印信息:
2014-03-31 15:30:19.185 ARC[4172:60b] strongVer2
2014-03-31 15:30:19.188 ARC[4172:60b] (null)
2014-03-31 15:30:19.188 ARC[4172:60b] strong
3. __unsafe_unretained
该关键字与__weak一样，也是弱引用，与__weak的区别只是是否执行nil赋值。需要注意变量所指的对象已经被释放了，但地址还存在，如果还是访问该对象，将引起「BAD_ACCESS」错误。
4. __autoreleasing
本人并没有明白__autoreleasing有什么作用,看例子也没明白,提供链接供读者参考
http://stackoverflow.com/questions/20949886/need-more-explanation-on-usage-of-autoreleasing
'm desperately trying to understand the usage of __autoreleasing keyword in Objective-C. I have thoroughly read answers to the following questions:
In which situations do we need to write the __autoreleasing ownership qualifier under ARC?
Use of __autoreleasing in code snippet example
问:非arc项目中使用了arc编译的静态库,为什么不会报警告?
http://stackoverflow.com/questions/18609935/strong-qualifier-used-in-non-arc-project
The project is non-ARC enabled, however we are (mistakingly) using ARC compliant code libraries - specifically one to create singleton objects like so defined in GCDSingleton.h:
我的项目是非arc的,然而,我无意间使用了用arc编译的库,尤其是其中的一个单例对象,在GCDSingleton.h文件中定义的:
#define DEFINE_SHARED_INSTANCE
+(id)sharedInstance
{staticdispatch_once_t pred =0;
__strong static id _sharedObject = nil;
dispatch_once(&pred,^{
_sharedObject =^{return[[self alloc] init];}();});return _sharedObject;}This seems to work even though the shared object is defined with an __strong qualifier. I'm wondering why this doesn't cause an error or at least a warning (latest Xcode 4.6 and ios 6 sdk). Also, since the project is not ARC enabled, what exactly is that __strong qualifier doing, if anything?
即使这个对象被定义成了__strong,但还是可以使用.我在想,为什么不会引起一个警告.__strong在非arc项目中到底做了什么,任何一点解释都行.
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In MRC code, __strong is simply ignored.
在非arc代码中,__strong被忽略掉了.
I tried to compile a simple example
#import <Foundation/Foundation.h>
int main(int argc,charconst*argv[]){
__strong NSString* foo =[[NSString alloc] initWithFormat:@"Hello, %s", argv[1]];
NSLog(@"%@", foo);
}with ARC
clang -fobjc-arc test.m -S -emit-llvm -o arc.irand without ARC
clang -fno-objc-arc test.m -S -emit-llvm -o mrc.irand to diff the llvm IR output.
Here's the result of diff mrc.ir arc.ir
54a55,56>%17= bitcast %0**%foo to i8**> call void@objc_storeStrong(i8**%17, i8* null) nounwind
63a66,67> declare void@objc_storeStrong(i8**, i8*)>So basically the only difference between ARC and MRC is the addition of a objc_storeStrong call.
By the way the same code without the __strong qualifier will produce the same exact results, since __strong is the default qualifier for variables in ARC.