iOS开发中的 ARC

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:
            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:
            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:
            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:
            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:
            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:
            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:
            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:
            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

NSError and __autoreleasing

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.ir

and without ARC

clang -fno-objc-arc test.m -S -emit-llvm -o mrc.ir

and 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.

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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.