iOS中block的用法

__block 变量

一般来说，在block内只能读取在同一个作用域的变数而且没有办法修改在block外定义的任何变数，此时若我们想要这些变数能够在block中被修改，就必须在前面挂上__block的修饰词，以上面第一个例子中的 multiplier 来说，这个变数在 block 中是唯读的，所以 multiplier = 7 指定完后，在 block 中的 multiplier 就只能是 7 不能修改，若我们在 block 中修改 multiplier ，在编辑时就会产生错误，因此若想要在 block 中修改 multiplier ，就必须在 multiplier 前面加上 __block 的修饰词，请参考下面的范例：

   1: __block int multiplier = 7 ;

   2: int (^myBlock)( int ) = ^( int num)

   3:                         {

   4:                             if (num > 5 )

   5:                             {

   6:                                   multiplier = 7 ;

   7:                             }

   8:                             else

   9:                             {

  10:                                   multiplier = 10 ;

  11:                             }

  12:                             return num * multiplier;

  13:                         };

 

PS：简单的使用block进行网络请求回调，非常的方便，避免了delegate的繁琐

HttpRequest.h

//
//  HttpRequest.h
//  UseBlockCallBack
//
//  Created by Michael on 2/13/14.
//  Copyright (c) 2014 EIMS. All rights reserved.
//

#import <Foundation/Foundation.h>

typedef void (^FinishBlock)(NSString *dataString);

@interface HttpRequest : NSObject<NSURLConnectionDataDelegate>

@property (strong, nonatomic) NSMutableData *resultData;
@property (strong, nonatomic) FinishBlock finishBlock;

+ (void)postRequestWithURL:(NSString *)urlStr
                 paramters:(NSString *)paramters
              finshedBlock:(FinishBlock)block;

@end

HttpRequest.m

//
//  HttpRequest.m
//  UseBlockCallBack
//
//  Created by Michael on 2/13/14.
//  Copyright (c) 2014 EIMS. All rights reserved.
//

#import "HttpRequest.h"

@implementation HttpRequest

+ (void)postRequestWithURL:(NSString *)urlStr
                 paramters:(NSString *)paramters
              finshedBlock:(FinishBlock)block
{
    HttpRequest *httpRequest = [[HttpRequest alloc]init];
    httpRequest.finishBlock = block;
    
    NSURL *url = [NSURL URLWithString:urlStr];
    NSMutableURLRequest *requset = [[NSMutableURLRequest alloc]initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:30];
    [requset setHTTPBody:[paramters dataUsingEncoding:NSUTF8StringEncoding]];
    
    NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:requset delegate:httpRequest];
    NSLog(connection ? @"连接创建成功" : @"连接创建失败");
}
/**
 *  接收到服务器回应的时回调
 */
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    if (!self.resultData) {
        self.resultData = [[NSMutableData alloc]init];
    } else {
        [self.resultData setLength:0];
    }
    
    if ([response respondsToSelector:@selector(allHeaderFields)]) {
        NSDictionary *dic = [httpResponse allHeaderFields];
        NSLog(@"[network]allHeaderFields:%@",[dic description]);
    }
}
/**
 *  接收到服务器传输数据的时候调用，此方法根据数据大小执行若干次
 */
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [self.resultData appendData:data];
}
/**
 *  数据传完之后调用此方法
 */
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSString *resultStr = [[NSString alloc]initWithData:self.resultData
                                               encoding:NSUTF8StringEncoding];
    if (self.finishBlock) {
        self.finishBlock(resultStr);
    }
}
/**
 *  网络请求过程中，出现任何错误（断网，连接超时等）会进入此方法
 */
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"network error : %@", [error localizedDescription]);
    
    if (self.finishBlock) {
        self.finishBlock(nil);
    }
}


@end

在ViewController中调用：

[HttpRequest postRequestWithURL:@"http://www.baidu.com"
                          paramters:@""
                       finshedBlock:^(NSString *dataString) {
                           NSLog(@"finish callback block, result: %@", dataString);
}];