ios学习笔记之block在ios开发中的应用

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

Java代码



#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);

}

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

Java代码



#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

2、Dog.m

Java代码



#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

Java代码



#import <Foundation/Foundation.h>

#import "Dog.h"

@interface Person : NSObject

{

Dog *_dog;

}

@property (retain) Dog *dog;

@end

4、Person.m

Java代码



#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

Java代码



#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;

}

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