block(程序块)

             之前程序中一直用block这里把它进行归类一下。block是ios4.0之后出来的一门新技术，应用也非常广泛，只要我们撑握其使用的翘门，使 用其来也是非常简单的，而且它能将传统程序简单化

       程序中" ^ "代表程序块也就是block,在我们接下来的程序中都会有^

            block的声明一般都是      返回类型(^block名称)(参数,...,...)   如 void(^firstBlock)(float ,float);

       block的定义 可以是如下  firstBlock=^(float r,float t){ NSLog(@"value is %f",r*t };

      我们也可以将声明和定义放在一块实现如下：

       void(^firstBlock)(float,float)=^(float r,float t){NSLog(@"value is %f",r*t};

     当我们需要使用它的时候执行如下  firstBlock(5.0,6.0);

     在这里需要注意一下，我们使用它的时候应该在能使用其范围内才能使用它，就跟变量一样，只有在其作用域 内才能使用它，写到这里觉得block跟变量差不多,说到跟变量一样这里提一下

     就是block引用外部的非对象型变量的时候一般是不能更改它的，除非在变量前加上__block,对于对象型的变量在block的内部引用的时候一般引用计数都会加1，为了打破这种 retain circle，可以在对象前加__block，这样block块就不会维护这个对象了

      block不仅能单独使用，还可以作为方法的参数如下

     -(void)changeValue:(void(^)(float,float))block,还过这里的block的写法和上面不一样，请注意

    使用也很简单 [self changeValue:firstBlock];

    

    再就是block还可以代替delegate进行回调，如有A和B两个UIViewController,给Ｂ定义一个属性block,从Ａ界面到Ｂ中时设置Ｂ的block,当我们需要在Ｂ中的任何时候执行回调的时候，直接执行block就行了，说到这里就不得不说一下作为属性block是不能进行retain的只能进行copy.还有block还可以用于多线程，这个以后再说

    

#import <UIKit/UIKit.h>@interface BIDViewController : UIViewController- (IBAction)clicked:(UIButton *)sender;- (IBAction)secondClicked:(id)sender;- (IBAction)thirdClicked:(id)sender;@property (retain, nonatomic) IBOutlet UILabel *lb;@end

#import "BIDViewController.h"#import "FirstViewController.h"@interface BIDViewController (){    float (^oneFrom)(float);}@end@implementation BIDViewController- (void)viewDidLoad{    [super viewDidLoad];// Do any additional setup after loading the view, typically from a nib.    oneFrom=^(float aFloat){        float result=aFloat-1.0;        return result;    };}- (void)didReceiveMemoryWarning{    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}- (IBAction)clicked:(UIButton *)sender {    NSLog(@"%f",oneFrom(10.0));            void(^thirdfrom)(float)=^(float r){        NSLog(@"%f",r*5.0);    };    [self changeValue:thirdfrom];}- (IBAction)secondClicked:(id)sender {    __block CGFloat height=10.0;    void(^twoFrom)(float)=^(float t){        height+=t;    };    NSLog(@"height is %f",height);    twoFrom(5.0);     NSLog(@"height  is %f",height);}- (IBAction)thirdClicked:(id)sender {    FirstViewController* first=[[FirstViewController alloc] init];    __block UILabel* blockLb=_lb;    //回调    first.testBlock=^(float r){        blockLb.text=[NSString stringWithFormat:@"%f",r];    };    [self presentModalViewController:first animated:YES];}-(void)changeValue:(void(^)(float))from{    [NSThread sleepForTimeInterval:3.0];    from(10.0);}-(void)dealloc{    [_lb release];    [super dealloc];}- (void)viewDidUnload {    [self setLb:nil];    [super viewDidUnload];}@end

#import <UIKit/UIKit.h>@interface FirstViewController : UIViewController@property(nonatomic,copy)void(^testBlock)(float);- (IBAction)btnClicked:(id)sender;@end

#import "FirstViewController.h"@interface FirstViewController ()@end@implementation FirstViewController@synthesize testBlock;- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];    if (self) {        // Custom initialization    }    return self;}- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view from its nib.}- (void)didReceiveMemoryWarning{    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}- (void)dealloc {    [super dealloc];}- (void)viewDidUnload {    [super viewDidUnload];}- (IBAction)btnClicked:(id)sender {    if (testBlock) {        testBlock(4.0);    }    [self dismissModalViewControllerAnimated:YES];}@end

有什么问题请多多指教