IOS——简化Web服务的连接

给NSURLConnection包一层更简单的接口，就可以减少每次的下载工作，省去大量的重复。
XYConnection继承自NSURLConnection，这个类管理临时的数据结构，并管理通常由控制器对象负责的进度，这样发起调用的代码只需在下载完成，数据准备好之后进行响应就行了。也创建了可选的挂钩机制，可以见识下载进度。
包含的其他信息有：

1，目标URL
2，起始NSURLRequest
3，预计的下载大小
4，目前一完成的下载量

代码，XYConnection.h：

#import <Foundation/Foundation.h>
@class XYConnection;
typedef void (^XYConnectionProgressBlock)(XYConnection *connection);
typedef void (^XYConnectionCompletionBlock)(XYConnection *connection, NSError *error);

@interface XYConnection : NSURLConnection<NSURLConnectionDataDelegate>

@property (nonatomic, copy, readonly) NSURL *url;
@property (nonatomic, copy, readonly) NSURLRequest *urlRequest;
@property (nonatomic, assign, readonly) NSInteger contentLength;
@property (nonatomic, retain, readonly) NSMutableData *downloadData;
@property (nonatomic, assign, readonly) float percentComplete;
@property (nonatomic, assign) NSUInteger progressThreshold;

+ (id)connectionWithURL:(NSURL *)requestURL progressBlock:(XYConnectionProgressBlock)progress completionBlock:(XYConnectionCompletionBlock)completion;
+ (id)connectionWithRequest:(NSURLRequest *)request progressBlock:(XYConnectionProgressBlock)progress completionBlock:(XYConnectionCompletionBlock)completion;

- (id)initWithURL:(NSURL *)requestURL progressBlock:(XYConnectionProgressBlock)progress completionBlock:(XYConnectionCompletionBlock)completion;

- (id)initWithRequest:(NSURLRequest *)request progressBlock:(XYConnectionProgressBlock)progress completionBlock:(XYConnectionCompletionBlock)completion;

- (void)start;
- (void)stop;

@end

我们可直接传入目标URL和可选的用于处理下载进度、完成、失败的Block，创建连接。-start和-stop方法用于显式的开始或取消连接。Block部分，一个报告进度的增加，另一个报告完成或失败。

contentLength属性表示连接的相应头中的Content-Length值。这个值是在接收到标准的NSURLContent委托方法-connection:didReceiveResponse:的调用时设置的。

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    if ([response isKindOfClass:[NSHTTPURLResponse class]]){
        NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
        if ([httpResponse statusCode] == 200){
            NSDictionary *header = [httpResponse allHeaderFields];
            NSString *contentLen = [header valueForKey:@"Content-Lenght"];
            NSInteger length = self.contentLength = [contentLen integerValue];
            self.downloadData = [NSMutableData dataWithCapacity:length];
        }
    }
}

使用这个值和已下载的数据量计算percentComplete属性

- (float)percentComplete{
    if (self.contentLength <= 0) return 0;
    return (([self.downloadData length] * 1.0f) / self.contentLength) * 100;
}

每当percentComplete达到制定的progressThreshold的倍数时，报告进的的Block就会被调用。

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    [self.downloadData appendData:data];
    float pctComplete = floorf([self percentComplete]);
    if ((pctComplete - self.previousMilestone) >= self.progressThreshold){
        self.previousMilestone = pctComplete;
        if  (self.progressBlock) self.progressBlock(self);
    }
}

注意，对于大数据量的下载，需要准备一个NSInputStream,拿到数据时就把它写到固态硬盘，以防止内存不足。

还有关于内存管理，本博文的完整代码在内存管理上做的不太好，能改进的地方请指正。