ASIHttpRequest中Block代码块的使用和注意事项

使用ASIHttpRequest的setCompletionBlock、setFailedBlock时碰到一些诡异的内存泄漏和莫名其妙的行为（如：无法release对象）。

1. 声明ASIHttpRequest时一定要使用__block关键字

__block关键字告诉block不要retain request，这对于防止循环retain非常重要！！因为request总是会retain block

2. 谨慎处理block与对象的关系

当setCompletionBlock/setFailedBlock内部使用对象的instance var时，self会被retain（If you access an instance variable by reference, self is retained;）。所以在request结束前向对象发送release消息不会导致对象的释放（dealloc），亦即：该对象依然可进行所有操作，这将导致诸多你意想不到的结果。

3. 解决第2点的问题

3.1 仔细拿捏block与对象的关系 + 按值的方式访问instance var（If you access an instance variable by value, the variable is retained.）
3.2 不使用block，而使用ASIHttpRequestDelegate

4. 参考资料

4.1 http://allseeing-i.com/ASIHTTPRequest/How-to-use - Using blocks
4.2 Blocks Programming Topics - Object and Block Variables

接口部分:

//
//  LYHASIRequestBlock.h
//  ASIBlockTest
//
//  Created by Charles Leo on 14-7-23.
//  Copyright (c) 2014年 Charles Leo. All rights reserved.
//
 
#import <Foundation/Foundation.h>
#import "ASIHTTPRequest.h"
#import "ASIFormDataRequest.h"
typedef void (^RequestBlock) (void);
@interface LYHASIRequestBlock : NSObject <ASIHTTPRequestDelegate>
{
    ASIHTTPRequest * getRequest;
    ASIFormDataRequest * postRequest;
    NSURL * url;
    RequestBlock finishBlock;
    RequestBlock failBlock;
    RequestBlock startBlock;
    NSString * requestType;
}
 
//接收到的数据
@property (strong,nonatomic) NSData * receiveData;
//请求完成的block
- (void)didFinishBlock:(RequestBlock)block;
//请求失败的block
- (void)didFailedBlock:(RequestBlock)block;
//请求开始的block
- (void)didStartBlock:(RequestBlock)block;
//取消请求
- (void)cancelRequst;
//get请求方法
- (void)getRequest:(NSString *)getUrl;
//post请求方法
- (void)postRequest:(NSString *)postUrl andKeys:(NSArray *)keyArray andValues:(NSArray *)valueArray;
@end

实现部分:

//
//  LYHASIRequestBlock.m
//  ASIBlockTest
//
//  Created by Charles Leo on 14-7-23.
//  Copyright (c) 2014年 Charles Leo. All rights reserved.
//
 
#import "LYHASIRequestBlock.h"
 
@implementation LYHASIRequestBlock
 
- (void)getRequest:(NSString *)getUrl
{
    requestType = @"GET";
    getRequest  = [[ASIHTTPRequest alloc]initWithURL:[NSURL URLWithString:getUrl]];
    getRequest.delegate = self;
    getRequest.timeOutSeconds = 15;
    [getRequest startAsynchronous];
}
#pragma mark -GET请求的代理方法
//开始请求
- (void)requestStarted:(ASIHTTPRequest *)request
{
    startBlock();
}
//请求完成
- (void)requestFinished:(ASIHTTPRequest *)request
{
    self.receiveData = request.responseData;
    finishBlock();
}
//请求失败
- (void)requestFailed:(ASIHTTPRequest *)request
{
    failBlock();
}
//post请求
-(void)postRequest:(NSString *)postUrl andKeys:(NSArray *)keyArray andValues:(NSArray *)valueArray
{
    postRequest = [[ASIFormDataRequest alloc]initWithURL:[NSURL URLWithString:postUrl]];
    postRequest.timeOutSeconds = 15;
    postRequest.delegate = self;
    for (int i = 0; i<keyArray.count; i++) {
        [postRequest setPostValue:[valueArray objectAtIndex:i] forKey:[keyArray objectAtIndex:i]];
    }
    [postRequest setDidFinishSelector:@selector(didFinishPostRequest:)];
    [postRequest setDidStartSelector:@selector(didStartPostRequest:)];
    [postRequest setDidFailSelector:@selector(didFailPostRequest:)];
    [postRequest startAsynchronous];
}
 
#pragma mark - POST请求的代理方法
//请求开始
- (void)didStartPostRequest:(ASIFormDataRequest *)request
{
    startBlock();
}
//请求完成
- (void)didFinishPostRequest:(ASIFormDataRequest *)request
{
    finishBlock();
}
//请求失败
- (void)didFailPostRequest:(ASIFormDataRequest *)request
{
    failBlock();
}
 
//设置Blocks
//设置开始块
- (void)didStartBlock:(RequestBlock)block
{
    [startBlock release];
    startBlock = [block copy];
}
//设置完成块
-(void)didFinishBlock:(RequestBlock)block
{
    [finishBlock release];
    finishBlock = [block copy];
}
//设置失败块
-(void)didFailedBlock:(RequestBlock)block
{
    [failBlock release];
    failBlock = [block copy];
}
//取消请求
- (void)cancelRequst{
    if ([requestType isEqualToString:@"GET"])
    {
        [getRequest cancel];
    }
    else if([requestType isEqualToString:@"POST"])
    {
        [postRequest cancel];
    }
}
 
-(void)dealloc
{
    [getRequest release];
    [postRequest release];
    [startBlock release];
    [finishBlock release];
    [failBlock release];
    [super dealloc];
}
@end

参考：http://www.cocoachina.com/bbs/read.php?tid=95100
http://code4app.com/snippets/one/%E7%94%A8Block%E5%B0%81%E8%A3%85ASIHttpRequest/5428f5b8933bf0bf118b51d8