block循环引用

block可以造成循环引用，导致对象不能释放，造成内存泄露。

所以在使用的时候，要用__block 来对变量或属性进行修饰。
而且block的释放跟一般的变量不一样

布局为一个根视图控制器，一个firstViewcontroller。这里仅显示了firstViewcontroller的代码

#import "FirstViewController.h"

typedef void(^BLOCK)(NSString *);

@interface FirstViewController ()

{
    UILabel *label;
}

@property(nonatomic,copy)BLOCK block;

@end

@implementation FirstViewController


- (void)dealloc
{


#pragma mark-----------------block的释放跟变量不一样
    Block_release(_block);



    [super dealloc];
}

- (void)viewDidLoad {
    [super viewDidLoad];


    label = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];


    self.view.backgroundColor = [UIColor cyanColor];

    NSLog(@"++++++%lu",[_block retainCount]);

    NSLog(@"-------%lu",self.retainCount);


    __block FirstViewController *firstVC = self;

    //在block中如果用到了self或者当前类的实例变量，一定要记得用__block修饰，否则会造成循环引用。
    self.block = ^void(NSString *text){
        firstVC.title = text;
    };

    NSLog(@"++++++%lu",[_block retainCount]);

    NSLog(@"-------%lu",self.retainCount);


    _block(@"First");


    // Do any additional setup after loading the view.
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}