Dereferencing a __weak pointer is not allowed due to possible null value caused by race condition, a

报错误Dereferencing a __weak pointer is not allowed due to possible null value caused by race condition, assign it to strong variable first

//

//  ViewController.m

//  __block__weak

//

//  Created by apple on 15/12/25.

//  Copyright © 2015年 apple. All rights reserved.

//

#import "ViewController.h"

#import "ViewController2.h"

typedef void(^ViewBlock)();

@interface ViewController (){

    

    NSString * str;

}

@property (nonatomic,copy) ViewBlock block;

@end

@implementation ViewController

- (void)dealloc {

    

    NSLog(@"v1 dealloc!");

}

- (void)viewDidLoad {

    [super viewDidLoad];

    str = @"123";

    __weak typeof(self) blockSelf = self;

    self.block = ^(){

        //报错误Dereferencing a __weak pointer is not allowed due to possible null value caused by race condition, assign it to strong variable first

        //需要加下面一个来强引用blockSelf

        ViewController *strongSelf = blockSelf;

        NSLog(@"%@",strongSelf->str);

        

    };

}

@end