ios学习笔记block回调的应用（一个简单的例子）

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


 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


 @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

3、Person.h 

 #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

5、Main.m 

 #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

原文地址：http://ff20081528.iteye.com/blog/1670433