一、什么是Blocks      Block是一个C级别的语法以及运行时的一个特性，和标准C中的函数（函数指针）类似，但是其运行需要编译器和运行时支持，从ios4.0开始就很好的支持Block。

二、在ios开发中，什么情况下使用Block      Block除了能够定义参数列表、返回类型外，还能够获取被定义时的词法范围内的状态（比如局部变量），并且在一定条件下（比如使用__block变量）能够修改这些状态。此外，这些可修改的状态在相同词法范围内的多个block之间是共享的，即便出了该词法范围（比如栈展开，出了作用域），仍可以继续共享或者修改这些状态。通常来说，block都是一些简短代码片段的封装，适用作工作单元，通常用来做并发任务、遍历、以及回调。

三、block如何申明（对比于c语言中的函数申明） 四、c函数指正和blocks调用      int (*CFunc) (int a) 函数调用      int result = CFunc(10);      int (^BFunc)  (int  a)  函数调用      int result = BFunc(10);

五、__block  关键字      一个Block的内部时可以引用自身作用域外的变量的，包括static变量，extern变量或自由变量（定义一个变量的时候，如果不加存储修饰符，默认情况下就是自由变量auto，auto变量保存在stack中的。除了auto之外还存在register，static等存储修饰符），对于自由变量，在Block中只读的。在引入block的同时，还引入了一种特殊的__block关键字变量存储修饰符。

六、block的几个小例子

#import <Cocoa/Cocoa.h>


int main(int argc, char *argv[])
{
    @autoreleasepool {
        NSLog(@"Hello world");
        void (^myblocks) (void) = NULL;
        myblocks = ^(void) {
            NSLog(@"in blocks");
        };
        NSLog(@"before myblocks");
        myblocks();
        NSLog(@"after myblocks");


        int (^myblocks2) (int a, int b) = ^(int a, int b) {
            int c = a + b;
            return c;
        };
        NSLog(@"before blocks2");
        int ret = myblocks2(10, 20);
        NSLog(@"after blocks2 ret %d", ret);

        //此处如果不加__block会报错
        __blockint sum = 0;
        int (^myblocks3) (int a, int b) = ^(int a, int b) {
            sum = a + b;
            return3;
        };
        myblocks3(20, 30);
        NSLog(@"sum is %d", sum);
    }
    returnNSApplicationMain(argc, (constchar **)argv);
}

#import 
    
     int main(int argc, char *argv[]){    @autoreleasepool {        NSLog(@"Hello world");        void (^myblocks) (void) = NULL;        myblocks = ^(void) {            NSLog(@"in blocks");        };        NSLog(@"before myblocks");        myblocks();        NSLog(@"after myblocks");                        int (^myblocks2) (int a, int b) = ^(int a, int b) {            int c = a + b;            return c;        };        NSLog(@"before blocks2");        int ret = myblocks2(10, 20);        NSLog(@"after blocks2 ret %d", ret);                //此处如果不加__block会报错        __blockint sum = 0;        int (^myblocks3) (int a, int b) = ^(int a, int b) {            sum = a + b;            return3;        };        myblocks3(20, 30);        NSLog(@"sum is %d", sum);    }    returnNSApplicationMain(argc, (constchar **)argv);}
    

打印结果如下 2012-09-03 10:23:20.878 blockTest[407:403] Hello world 2012-09-03 10:23:20.880 blockTest[407:403] before myblocks 2012-09-03 10:23:20.881 blockTest[407:403] in blocks 2012-09-03 10:23:20.881 blockTest[407:403] after myblocks 2012-09-03 10:23:20.882 blockTest[407:403] before blocks2 2012-09-03 10:23:20.882 blockTest[407:403] after blocks2 ret 30 2012-09-03 10:23:20.882 blockTest[407:403] sum is 50

七、block写的回调例子 1、Dog.h

#import <Foundation/Foundation.h>


@interface Dog : NSObject {
    int _ID;
    NSTimer *timer;
    int barkCount;

    //定义一个blocks变量
    void (^BarkCallback) (Dog *thisDog, int count);
}
@property (assign) int ID;


//向外暴露一个接口
-(void) setBark:( void (^) (Dog *thisDog, int count) ) eachBark;


@end

#import 
    
     @interface Dog : NSObject {    int _ID;    NSTimer *timer;    int barkCount;        //定义一个blocks变量    void (^BarkCallback) (Dog *thisDog, int count);}@property (assign) int ID;//向外暴露一个接口-(void) setBark:( void (^) (Dog *thisDog, int count) ) eachBark;@end
    

2、Dog.m

#import "Dog.h"


@implementation Dog
@synthesize ID = _ID;


-(id) init
{
    self = [superinit];
    if (self) {
        //每隔1s调用一次updateTimer方法
        timer = [NSTimerscheduledTimerWithTimeInterval:1.0ftarget:selfselector:@selector(updateTimer:) userInfo:nilrepeats:YES];

    }
    returnself;
}


-(void) updateTimer:(id) arg
{
    barkCount ++;
    NSLog(@"dog %d bark count %d", _ID, barkCount);
    //向Person对象进行汇报
    if (BarkCallback) {
        //调用从Person传过来的Blocks
        BarkCallback(self, barkCount);
    }
}




-(void) setBark:(void (^)(Dog *, int))eachBark
{
    [BarkCallbackrelease];
    BarkCallback = [eachBark copy];
}


-(void) dealloc
{
    [BarkCallbackrelease];
    [superdealloc];
}
@end

#import "Dog.h"@implementation Dog@synthesize ID = _ID;-(id) init{    self = [superinit];    if (self) {        //每隔1s调用一次updateTimer方法        timer = [NSTimerscheduledTimerWithTimeInterval:1.0ftarget:selfselector:@selector(updateTimer:) userInfo:nilrepeats:YES];            }    returnself;}-(void) updateTimer:(id) arg{    barkCount ++;    NSLog(@"dog %d bark count %d", _ID, barkCount);    //向Person对象进行汇报    if (BarkCallback) {        //调用从Person传过来的Blocks        BarkCallback(self, barkCount);    }}-(void) setBark:(void (^)(Dog *, int))eachBark{    [BarkCallbackrelease];    BarkCallback = [eachBark copy];}-(void) dealloc{    [BarkCallbackrelease];    [superdealloc];}@end

3、Person.h

#import <Foundation/Foundation.h>
#import "Dog.h"


@interface Person : NSObject
{
    Dog *_dog;
}


@property (retain) Dog *dog;


@end

#import 
    
     #import "Dog.h"@interface Person : NSObject{    Dog *_dog;}@property (retain) Dog *dog;@end
    

4、Person.m

#import "Person.h"


@implementation Person
@synthesize dog = _dog;


-(void) setDog:(Dog *)dog
{
    if (_dog != dog) {
        [_dogrelease];
        _dog = [dog retain];

        [_dogsetBark:^(Dog *thisDog, int count) {
            NSLog(@"person dog %d count %d", [thisDog ID], count);
        }];
    }
}


-(Dog *) dog
{
    return_dog;
}


-(void) dealloc
{
    self.dog = nil;
    [superdealloc];
}


@end

#import "Person.h"@implementation Person@synthesize dog = _dog;-(void) setDog:(Dog *)dog{    if (_dog != dog) {        [_dogrelease];        _dog = [dog retain];                [_dogsetBark:^(Dog *thisDog, int count) {            NSLog(@"person dog %d count %d", [thisDog ID], count);        }];    }}-(Dog *) dog{    return_dog;}-(void) dealloc{    self.dog = nil;    [superdealloc];}@end

5、Main.m

#import <Foundation/Foundation.h>
#import "Person.h"
#import "Dog.h"


int main(int argc, constchar * argv[])
{


    @autoreleasepool {

        // insert code here...
        NSLog(@"Hello, World!");
        Person *person = [[Personalloc] init];
        Dog *dog = [[Dogalloc] init];
        [dog setID:10];
        [person setDog:dog];
        [dog release];
        while (1) {
            [[NSRunLoopcurrentRunLoop] run];
        }
        [person release];

    }
    return 0;
}

#import 
    
     #import "Person.h"#import "Dog.h"int main(int argc, constchar * argv[]){    @autoreleasepool {                // insert code here...        NSLog(@"Hello, World!");        Person *person = [[Personalloc] init];        Dog *dog = [[Dogalloc] init];        [dog setID:10];        [person setDog:dog];        [dog release];        while (1) {            [[NSRunLoopcurrentRunLoop] run];        }        [person release];            }    return 0;}
    

6、打印结果 2012-09-03 11:21:08.551 blockDelegate[549:403] Hello, World! 2012-09-03 11:21:09.554 blockDelegate[549:403] dog 10 bark count 1 2012-09-03 11:21:09.555 blockDelegate[549:403] person dog 10 count 1 2012-09-03 11:21:10.554 blockDelegate[549:403] dog 10 bark count 2 2012-09-03 11:21:10.555 blockDelegate[549:403] person dog 10 count 2 2012-09-03 11:21:11.553 blockDelegate[549:403] dog 10 bark count 3 2012-09-03 11:21:11.554 blockDelegate[549:403] person dog 10 count 3 2012-09-03 11:21:12.554 blockDelegate[549:403] dog 10 bark count 4 2012-09-03 11:21:12.555 blockDelegate[549:403] person dog 10 count 4 2012-09-03 11:21:13.553 blockDelegate[549:403] dog 10 bark count 5 2012-09-03 11:21:13.553 blockDelegate[549:403] person dog 10 count 5 2012-09-03 11:21:14.553 blockDelegate[549:403] dog 10 bark count 6 2012-09-03 11:21:14.554 blockDelegate[549:403] person dog 10 count 6