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Author: KevinHM

chapter5/networking_layer_revisited.md

@@ -1 +1,59 @@
 # 网络层回访
+
+还有一个机会来进一步接受我们函数反应型编程的理念，那就是我们的网络层 `FRPPhotoImporter`,我们先来看看下载图片的方法：
+
+```
++ (void)downloadThumbnailForPhotoModel:(FRPPhotoModel *)photoModel {
+	[self download:photoModel.thumbnailURL withCompletion:^(NSData *data) {
+		photoModel.thumbnailData = data;
+	}];
+}
+
++ (void)downloadFullsizedImageForPhotoModel:(FRPPhotoModel *)photoModel {
+	[self download:photoModel.fullsizedURL withCompletion:^(NSData *data){
+		photoModel.fullsizedData = data;
+	}];
+}
+
++ (void)download:(NSString *)urlString withCompletion:(void (^)(NSData *data))completion {
+	NSAssert(urlString, @"URL must not be nil");
+	
+	NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:urlString]];
+	[NSURLConnection sendAsynchronousRequest:request 
+								queue:[NSOperationQueue mainQueue] 
+								completionHandler:
+									 ^(NSURLResponse *response, NSData *data, NSError *connectionError) {
+											if(completion) {
+												completion(data);
+											}  
+									 }];
+}
+
+```
+Completion blocks?这是另外一个使用Signals的机会。更深入一点来说，我们可以使用`NSURLConnection`的ReactiveCocoa的扩展。下面我们来重写上面的方法：
+
+```
++ (void)downloadThumbnailForPhotoModel:(FRPPhotoModel *)photoModel {
+	RAC(photoModel, thumbnailData) = [self download:photoModel.thumbnailURL];
+}
+
++ (void)downloadFullsizedImageForPhotoModel:(FRPPhotoModel *)photoModel {
+	RAC(photoModel,fullsizedData) = [self download:photoModel.fullsizedURL];
+}
+
++ (RACSignal *)download:(NSString *)urlString {
+	NSAssert(urlString , @"URL must not be nil");
+	
+	NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString: urlString]];
+	
+	return [[[NSURLConnection rac_sendAsynchronousRequest:request] 
+				map:^id (RACTuple *value) {
+					return [value second];
+				}] deliverOn:[RACScheduler mainThreadScheduler]];
+}
+
+```
+这里有两个大的不同：
+
+  1. asdf
+  2. asdf