在用delegate回调数据时，避免避免持续调用 [_delegate respondsToSelector:@selector(someThingFunction)

GCNetworkFetcher.h

定义protocol

#import <Foundation/Foundation.h>
@class GCNetworkFetcher;

@protocol GCNetworkFetcherDelegate <NSObject>

@optional
- (void)networkFetcher:(GCNetworkFetcher *)fetcher didReceiveData:(NSData *)data;
- (void)networkFetcher:(GCNetworkFetcher *)fetcher didFaileWithError:(NSError *)error;
- (void)networkFetcher:(GCNetworkFetcher *)fetcher didUpdateProgressTo:(float)progress;

@end

@interface GCNetworkFetcher : NSObject

@property (nonatomic, weak) id<GCNetworkFetcherDelegate>delegate;

@end

GCNetworkFetcher.m

#import "GCNetworkFetcher.h"

@interface GCNetworkFetcher () {
    //用位段的数据类型缓存方法的响应能力，避免每次回调数据的时候都去检测一下 避免持续调用 if( [_delegate respondsToSelector:@selector(networkFetcher:didUpdateProgressTo:)]) {}
    //用结构体来缓存委托对象是否能响应特定的选择器
    struct {
        unsigned int didReceivedData     : 1;
        unsigned int didFaileWithError   : 1;
        unsigned int didUpdateProgressTo : 1;
    }_delegateFlags;
}

@end

@implementation GCNetworkFetcher

// 实现缓存功能所有的代码可以写在delegate属性所对应的设置方法里
- (void)setDelegate:(id<GCNetworkFetcherDelegate>)delegate {
    _delegate = delegate;
    _delegateFlags.didReceivedData      = [delegate respondsToSelector:@selector(networkFetcher:didReceiveData:)];
    _delegateFlags.didFaileWithError    = [delegate respondsToSelector:@selector(networkFetcher:didFaileWithError:)];
    _delegateFlags.didUpdateProgressTo  = [delegate respondsToSelector:@selector(networkFetcher:didUpdateProgressTo:)];
}

- (void)getData:(NSData *)data {
    if (_delegateFlags.didReceivedData) {
        [_delegate networkFetcher:self didReceiveData:data];
    }
}

- (void)setError:(NSError *)error {
    if (_delegateFlags.didFaileWithError) {
        [_delegate networkFetcher:self didFaileWithError:error];
    }
}

- (void)progress:(float)progress {
    if (_delegateFlags.didUpdateProgressTo) {
        [_delegate networkFetcher:self didUpdateProgressTo:progress];
    }
}