Block的使用和循環(huán)引用的隱患

Block本身會產(chǎn)生Retain么

Block其實就是一段代碼段,假設(shè)我在ClassA中使用ClassB的Block,block實際存在于ClassA的內(nèi)存段p:0xXXXXX.也就是說在運行的時候如果ClassB的Block被使用,他會等待Block的代碼段出現(xiàn),如下.

typedef void(^CompletionBlock)(NSString *message);
//ClassB.h
- (void)classBFunction:(CompletionBlock)completion;
//ClassB.m
- (void)classBFunction:(CompletionBlock)completion {
completion(@"wait Block");
}

此時,我在ClassA中聲明ClassB,并且調(diào)用該函數(shù)

//ClassA.m
ClassB *b = [[ClassB alloc]init];
[b classBFunction:^(NSString *message) {
    NSLog(@"%@",message);
    }];

此時NSLog(@"%@",message)其實是ClassA中的代碼片段,處于地址p.也就是說如果ClassB想在classBFunction:內(nèi)部completion執(zhí)行傳遞來的NSLog,那么ClassA的內(nèi)存千萬不能被釋放.也就是說ClassB必須用Block對ClassA產(chǎn)生一個Retain,一個強引用.

Block分幾種

Block一共分三種

  1. NSGlobalBlock：沒有使用除了Block參數(shù)以外的變量；
  2. NSStackBlock：使用了Block參數(shù)以外的變量,位于棧內(nèi)存,函數(shù)返回后Block就被釋放；
  3. NSMallocBlock：位于堆內(nèi)存,使用引用計數(shù)。

示例

BlkSum blk1 = ^ long (int a, int b) {
  return a + b;
};
NSLog(@"blk1 = %@", blk1);// blk1 = <__NSGlobalBlock__: 0x47d0>

int base = 100;
BlkSum blk2 = ^ long (int a, int b) {
  return base + a + b;
};
NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8>

BlkSum blk3 = [[blk2 copy] autorelease];
NSLog(@"blk3 = %@", blk3); // blk3 = <__NSMallocBlock__: 0x902fda0>

Block如何產(chǎn)生的循環(huán)引用

在較早版本的Xcode,由于Block會對.m本身的ClassA產(chǎn)生一個Retain,而ClassA如果想調(diào)用ClassB的Block,自然也會對ClassB產(chǎn)生一個Retain.所以Block作為傳送指針的通道,就讓AB互相強引用了,例如Bang的文獻(xiàn)里說的.

@interface ClassAViewController ()
@property (nonatomic, strong) ClassB *classB;
@end

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    _classB = [[ClassB alloc]init];
    [_classB blockFunction:^(UIView *view) {
        [self completionBlock];
    }];
}

- (void)completionBlock {
    NSLog(@"ClassAViewController Comlpetion Block");
}

- (void)dealloc {
    NSLog(@"ClassAViewController did dealloc");
}

由于testFunction的block要調(diào)用ClassA(self)里的函數(shù),為了防止這一段函數(shù)被釋放,必須對ClassA產(chǎn)生一個Retain.在以前的版本為了避免產(chǎn)生循環(huán)引用,會使用一個weak指針指向self,然后通過weak指針調(diào)用self的函數(shù).

_b = [[ClassB alloc]init];
id weakself = self;
[_b testFunction:^(UIView *view) {
    [weakself completionBlock];
}];

<font color=red>經(jīng)過Xcode7.2實際測試,這樣書寫已經(jīng)不會產(chǎn)生循環(huán)引用,dealloc會被正常調(diào)用</font>

還有一種可能的情況,有3個類形成通過block循環(huán)引用,但是很少見.

+-----------+           +-----------+           +-----------+
|   self    |           |    obj    |           |   Block   |
|  ClassA   | --------> |   ClassC  | --------> |  ClassB   |
| retain 1  |           |  retain 1 |           | retain 1  |
|           |           |           |           |           |
+-----------+           +-----------+           +-----------+
     ^                                                |
     |                    strong                      |
     +------------------------------------------------+

我去專門復(fù)現(xiàn)也無法復(fù)現(xiàn)出來,因為ClassC中調(diào)用ClassB的Block,訪問的是ClassC的內(nèi)存段,ClassB基本很少有手段對ClassA產(chǎn)生Retain.因為總需要一個指針把ClassA傳遞給ClassB,如果使用弱指針傳遞,就相當(dāng)舊版本時的weakself了

+-----------+           +-----------+           +-----------+
|   self    |           |    obj    |           |   Block   |
|  ClassA   | --------> |   ClassC  | --------> |  ClassB   |
| retain 1  |           |  retain 1 |           | retain 1  |
|           |           |           |           |           |
+-----------+           +-----------+           +-----------+
                            ^                           |
                            |            weak           |
                            +---------------------------+

那是不是ARC下Block不會產(chǎn)生循環(huán)引用了

不是,與其擔(dān)心Block造成循環(huán)引用,更應(yīng)該關(guān)心透穿時使用什么指針

+-----------+           +-----------+           +-----------+
|   self    |   strong  |    obj    |   strong  |   Block   |
|  ClassA   | --------> |   ClassC  | --------> |  ClassB   |
| retain 1  |  ?pointA  |  retain 1 |   pointA  | retain 1  |
|           | <---------|           | --------> |           |
+-----------+           +-----------+           +-----------+
                            ^                           |
                            |            weak           |
                            +---------------------------+

循環(huán)引用例子

  1. 主ViewController用來Present出ClassAVC
  2. ClassAVC中有兩個按鈕,一個是dismiss自己,一個是present出ClassCVC
  3. ClassC中聲明pointA,用于把ClassA傳遞給ClassC
  4. 在ClassC中初始化ClassB,執(zhí)行閉包,傳入pointA指針.

在上述過程中,pointA應(yīng)該使用weak,如果使用了strong,ClassA對ClassC本身就是強引用,如果pointA也是強引用,就形成了循環(huán)引用,這種前后兩個界面互相傳遞的情況比較常見,一不小心就會形成循環(huán)引用,這也是為什么推薦使用代碼潔癖weak的prorerty的原因.

+-----------+                   +-----------+   
|   self    |   strong          |    obj    | 
|  ClassA   | --------------->  |   ClassC  |
| retain 1  |   strong-pointA   |  retain 1 |
|           | <---------------- |           | 
+-----------+                   +-----------+

ClassA

//ClassA.h
@interface ClassAViewController : UIViewController
- (void)completionBlock;
@end
//ClassA.m
#import "ClassCViewController.h"

@interface ClassAViewController ()
@property (nonatomic, strong) UIButton *btn;
@property (nonatomic, strong) ClassCViewController *classCVC;
@property (nonatomic, strong) UIButton *dismissBtn;
@end

@implementation ClassAViewController

- (void)touchUpInsideButton:(UIButton *)button {
    _classCVC = [[ClassCViewController alloc]init];
    _classCVC.classAVC = self;
    [self presentViewController:_classCVC animated:YES completion:nil];
}

- (void)touchUpInsideDismissBtn:(UIButton *)button {
    [self dismissViewControllerAnimated:YES completion:nil];
}

- (void)completionBlock {
    NSLog(@"ClassAViewController Comlpetion Block");
}   

- (void)dealloc {
    NSLog(@"ClassAViewController did dealloc%p",self);
}
@end

ClassC

//ClassC.h
@interface ClassCViewController : UIViewController
//此處是pointA,使用了Strong,形成循環(huán)引用
@property (nonatomic, strong)  ClassAViewController *classAVC;
@end
//ClassC.m
#import "ClassB.h"
#import "ClassAViewController.h"

@interface ClassCViewController ()
@property (nonatomic, strong) UIButton *btn;
@property (nonatomic, strong) ClassB *classB;
@end

@implementation ClassAViewController

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    _classB = [[ClassB alloc]init];
    [_classB blockFunction:^(UIView *view) {
        [_classAVC completionBlock];
    }];
}

- (void)touchUpInsideButton:(UIButton *)button {
    [self dismissViewControllerAnimated:YES completion:nil];
}

- (void)dealloc {
    NSLog(@"ClassCViewController did dealloc%p",self);
}
@end

ClassB

//ClassB.h
@interface ClassB : NSObject
- (void)blockFunction:(void(^)(UIView *view))completion;
@end
//ClassB.m
- (void)blockFunction:(void(^)(UIView *view))completion {
    UIView *view = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 50, 50)];
    [view setBackgroundColor:[UIColor redColor]];
    completion(view);
}

變量進(jìn)入Block是傳值還是傳址

使用了局部變量的NSStackBlock,對局部變量采用的是傳值形式的調(diào)用,進(jìn)入Block后更改不會影響原變量.如果想進(jìn)行傳址,需要加上__block標(biāo)注

__block int base = 100;
BlkSum blk2 = ^ long (int a, int b) {
    base += 10;
    return base + a + b;
};
NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8>

參考文獻(xiàn)

[1] http://git.devzeng.com/blog/ios-arc-block-retain-cycle.html
[2] http://tanqisen.github.io/blog/2013/04/19/gcd-block-cycle-retain/
[3] http://git.devzeng.com/blog/ios-arc-block-retain-cycle.html

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