block循環引用

block可以造成循環引用，導致對象不能釋放，造成內存泄露。

所以在使用的時候，要用__block 來對變量或屬性進行修飾。
而且block的釋放跟一般的變量不一樣

佈局爲一個根視圖控制器，一個firstViewcontroller。這裏僅顯示了firstViewcontroller的代碼

#import "FirstViewController.h"

typedef void(^BLOCK)(NSString *);

@interface FirstViewController ()

{
    UILabel *label;
}

@property(nonatomic,copy)BLOCK block;

@end

@implementation FirstViewController


- (void)dealloc
{


#pragma mark-----------------block的釋放跟變量不一樣
    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或者當前類的實例變量，一定要記得用__block修飾，否則會造成循環引用。
    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.
}