Block本身会产生Retain么
Block其实就是一段代码段,假设我在ClassA中使用ClassB的Block,block实际存在于ClassA的内存段p:0xXXXXX.也就是说在运行的时候如果ClassB的Block被使用,他会等待Block的代码段出现,如下.
typedef void(^CompletionBlock)(NSString *message);
//ClassB.h
- (void)classBFunction:(CompletionBlock)completion;
//ClassB.m
- (void)classBFunction:(CompletionBlock)completion {
completion(@"wait Block");
}
此时,我在ClassA中声明ClassB,并且调用该函数
//ClassA.m
ClassB *b = [[ClassB alloc]init];
[b classBFunction:^(NSString *message) {
NSLog(@"%@",message);
}];
此时NSLog(@"%@",message)其实是ClassA中的代码片段,处于地址p.也就是说如果ClassB想在classBFunction:内部completion执行传递来的NSLog,那么ClassA的内存千万不能被释放.也就是说ClassB必须用Block对ClassA产生一个Retain,一个强引用.
Block分几种
Block一共分三种
- NSGlobalBlock：没有使用除了Block参数以外的变量；
- NSStackBlock：使用了Block参数以外的变量,位于栈内存,函数返回后Block就被释放；
- NSMallocBlock：位于堆内存,使用引用计数。
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如何产生的循环引用
在较早版本的Xcode,由于Block会对.m本身的ClassA产生一个Retain,而ClassA如果想调用ClassB的Block,自然也会对ClassB产生一个Retain.所以Block作为传送指针的通道,就让AB互相强引用了,例如Bang的文献里说的.
@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要调用ClassA(self)里的函数,为了防止这一段函数被释放,必须对ClassA产生一个Retain.在以前的版本为了避免产生循环引用,会使用一个weak指针指向self,然后通过weak指针调用self的函数.
_b = [[ClassB alloc]init];
id weakself = self;
[_b testFunction:^(UIView *view) {
[weakself completionBlock];
}];
经过Xcode7.2实际测试,这样书写已经不会产生循环引用,dealloc会被正常调用
还有一种可能的情况,有3个类形成通过block循环引用,但是很少见.
+-----------+ +-----------+ +-----------+
| self | | obj | | Block |
| ClassA | --------> | ClassC | --------> | ClassB |
| retain 1 | | retain 1 | | retain 1 |
| | | | | |
+-----------+ +-----------+ +-----------+
^ |
| strong |
+------------------------------------------------+
我去专门复现也无法复现出来,因为ClassC中调用ClassB的Block,访问的是ClassC的内存段,ClassB基本很少有手段对ClassA产生Retain.因为总需要一个指针把ClassA传递给ClassB,如果使用弱指针传递,就相当旧版本时的weakself了
+-----------+ +-----------+ +-----------+
| self | | obj | | Block |
| ClassA | --------> | ClassC | --------> | ClassB |
| retain 1 | | retain 1 | | retain 1 |
| | | | | |
+-----------+ +-----------+ +-----------+
^ |
| weak |
+---------------------------+
那是不是ARC下Block不会产生循环引用了
不是,与其担心Block造成循环引用,更应该关心透穿时使用什么指针
+-----------+ +-----------+ +-----------+
| self | strong | obj | strong | Block |
| ClassA | --------> | ClassC | --------> | ClassB |
| retain 1 | ?pointA | retain 1 | pointA | retain 1 |
| | <---------| | --------> | |
+-----------+ +-----------+ +-----------+
^ |
| weak |
+---------------------------+
循环引用例子
- 主ViewController用来Present出ClassAVC
- ClassAVC中有两个按钮,一个是dismiss自己,一个是present出ClassCVC
- ClassC中声明pointA,用于把ClassA传递给ClassC
- 在ClassC中初始化ClassB,执行闭包,传入pointA指针.
在上述过程中,pointA应该使用weak,如果使用了strong,ClassA对ClassC本身就是强引用,如果pointA也是强引用,就形成了循环引用,这种前后两个界面互相传递的情况比较常见,一不小心就会形成循环引用,这也是为什么推荐使用代码洁癖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
//此处是pointA,使用了Strong,形成循环引用
@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);
}
变量进入Block是传值还是传址
使用了局部变量的NSStackBlock,对局部变量采用的是传值形式的调用,进入Block后更改不会影响原变量.如果想进行传址,需要加上__block标注
__block int base = 100;
BlkSum blk2 = ^ long (int a, int b) {
base += 10;
return base + a + b;
};
NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8>
参考文献
[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