Object-c block

block 在开发中的用途多种多样，一下是我遇到的block的常见用法

• block 保存代码

#import 

@interface CellItem : NSObject

@property (nonatomic, strong) NSString *title;

// 保存每个cell做的事情
@property (nonatomic, strong) void(^block)();

+ (instancetype)itemWithTitle:(NSString *)title;

@end

/************************/
#import "CellItem.h"

@implementation CellItem
+ (instancetype)itemWithTitle:(NSString *)title
{
    CellItem *item = [[self alloc] init];
    
    item.title = title;
    
    return item;
}
@end

/************************/
#import "TableViewController.h"
#import "CellItem.h"
@interface TableViewController ()
@property (nonatomic, strong) NSArray *items;
@end

@implementation TableViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 创建模型
    CellItem *item1 = [CellItem itemWithTitle:@"打电话"];
    item1.block = ^{
        NSLog(@"打电话");
    };
    CellItem *item2 = [CellItem itemWithTitle:@"发短信"];
    item2.block = ^{
        NSLog(@"发短信");
    };
    CellItem *item3 = [CellItem itemWithTitle:@"发邮件"];
    item3.block = ^{
        NSLog(@"发邮件");
    };
    _items = @[item1,item2,item3];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return _items.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *ID = @"cell";
    
    // 1.从缓存池取
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ID];
    
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:ID];
    }
    
    CellItem *item = self.items[indexPath.row];
    cell.textLabel.text = item.title;
    
    return cell;
}

// 点击cell就会调用
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    // 把要做的事情(代码)保存到模型

    CellItem *item = self.items[indexPath.row];
    
    if (item.block) {
        item.block();
    }
}
@end

• block传值

#import 
@interface ModalViewController : UIViewController
@property(nonatomic,strong)void(^block)(NSString *);
@end

/**********************/

#import "ModalViewController.h"

@interface ModalViewController ()

@end

@implementation ModalViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    if (_block) {
        _block(@"123");
    }
    [self dismissViewControllerAnimated:YES completion:nil];
}
@end

/**********************/
#import "ViewController.h"
#import "ModalViewController.h"
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    ModalViewController *modal = [[ModalViewController alloc]init];
    modal.view.backgroundColor = [UIColor blueColor];
    modal.block = ^(NSString *value){
        NSLog(@"value = %@",value);
    };
    
    [self presentViewController:modal animated:YES completion:nil];
}

@end

• block内存管理

  • MRC:管理block

    • 只要Block引用外部局部变量,block放在栈里面.

  • block只能使用copy,不能使用retain,使用retain,block还是在栈里面

  • ARC:管理block

    • 只要block引用外部局部变量,block放在堆里面

  • block使用strong.最好不要使用copy

• 只要block没有引用外部局部变量,block放在全局区

• block造成的循环引用

block中到底什么时候用weakSelf和strongSelf

• 在看AFNetworking框架时我发现一种特别的block循环引用问题

#import "ModalViewController.h"
@interface ModalViewController ()
@property (nonatomic, strong) void(^block)();
@end

@implementation ModalViewController

- (void)dealloc
{
    NSLog(@"ModalViewController销毁");
}
- (void)viewDidLoad {
    [super viewDidLoad];
    __weak typeof(self) weakSelf = self;
    _block = ^{
        __strong typeof(weakSelf) strongSelf = weakSelf;
        /**
         这种block两次引用第一次是把self修饰改成__weak,第二次的修饰改成了__strong又变成了强引用，而dispatch_after的block是由系统帮我们管理生命周期，所以两秒之后页面才会销毁
         */
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"%@",strongSelf);
        });
    };
    _block();
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self dismissViewControllerAnimated:YES completion:nil];
}
@end

• block变量传递

  • 如果block是局部变量，block是值传递

  • 如果是静态比那两，全局变量__block修饰的变量,block都是指针传递

• block作为参数传递

#import 
@interface Manager : NSObject
@property(nonatomic,assign)NSInteger result ;
-(void)cacultor:(NSInteger(^)(NSInteger value))cacultorBlock;
@end
/*****************/
#import "Manager.h"

@implementation Manager
-(void)cacultor:(NSInteger (^)(NSInteger))cacultorBlock
{
    if (cacultorBlock) {
        _result = cacultorBlock(_result);
    }
}
@end
/*****************/
#import "ViewController.h"
#import "Manager.h"
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // 把block当做参数,并不是马上就调用Block,什么时候调用,由方法内部决定
// 什么时候需要把block当做参数去使用:做的事情由外界决定,但是什么时候做由内部决定.
    Manager *mgr = [[Manager alloc]init];
    [mgr cacultor:^NSInteger(NSInteger value) {
        value +=2;
        value *=2;
        NSLog(@"value = %ld",(long)value);
        return value;
    }];
}
@end

• 在看masonry框架时发现一种block更加神奇的用法。

#import 
@interface Manager : NSObject
@property(nonatomic,assign)NSInteger result ;
-(Manager *(^)(NSInteger value))add;
@end
/***************/
#import "Manager.h"
@implementation Manager
-(Manager *(^)(NSInteger))add
{
    return ^(NSInteger value){
        _result += value;
        return self;
    };
}
@end
/***************/
#import "ViewController.h"
#import "Manager.h"
@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    Manager *mgr = [[Manager alloc]init];
    mgr.add(5).add(5).add(3);
    NSLog(@"result = %ld",(long)mgr.result);
    self.test();
}
-(void(^)())test
{
    return ^(){
        NSLog(@"block");
    };
    //    void(^block)() = ^{
    //     写法等同于上面
    //    };
}

@end