浅谈Mantle

- (id)modelFromJSONDictionary:(NSDictionary *)JSONDictionary error:(NSError **)error {
// 开始 处理由一堆相关的class来map同一类字典，少见，可忽略，为了结构清晰，删除了这部分代码
// 结束 处理由一堆相关的class来map同一类字典，少见，可忽略
// 开始 常规的解析流程
	NSMutableDictionary *dictionaryValue = [[NSMutableDictionary alloc] initWithCapacity:JSONDictionary.count];
	for (NSString *propertyKey in [self.modelClass propertyKeys]) {
    // 开始 单个property的解析
		id JSONKeyPaths = self.JSONKeyPathsByPropertyKey[propertyKey];
		if (JSONKeyPaths == nil) continue;
		id value;
    // 开始 花式keypath处理
    // 同样，这部分代码也省略了
    // 结束花式keypath处理
		if (value == nil) continue;
		@try {
      // 开始 常规的property解析流程
			NSValueTransformer *transformer = self.valueTransformersByPropertyKey[propertyKey];
			if (transformer != nil) {
				// Map NSNull -> nil for the transformer, and then back for the
				// dictionary we're going to insert into.
				if ([value isEqual:NSNull.null]) value = nil;
				if ([transformer respondsToSelector:@selector(transformedValue:success:error:)]) {
					id<MTLTransformerErrorHandling> errorHandlingTransformer = (id)transformer;
					BOOL success = YES;
					value = [errorHandlingTransformer transformedValue:value success:&success error:error];
					if (!success) return nil;
				} else {
					value = [transformer transformedValue:value];
				}
				if (value == nil) value = NSNull.null;
			}
			dictionaryValue[propertyKey] = value;
      // 结束 常规的property解析流程
		} @catch (NSException *ex) {
			NSLog(@"*** Caught exception %@ parsing JSON key path \"%@\" from: %@", ex, JSONKeyPaths, JSONDictionary);
		}
    // 结束 单个property的解析
	}
	id model = [self.modelClass modelWithDictionary:dictionaryValue error:error];
	return [model validate:error] ? model : nil;
// 结束 常规解析流程
}

+ (NSDictionary *)valueTransformersForModelClass:(Class)modelClass {
	NSParameterAssert(modelClass != nil);
	NSParameterAssert([modelClass conformsToProtocol:@protocol(MTLJSONSerializing)]);
	NSMutableDictionary *result = [NSMutableDictionary dictionary];
	for (NSString *key in [modelClass propertyKeys]) {
    // keyTransformer 的生成
		SEL selector = MTLSelectorWithKeyPattern(key, "JSONTransformer");
		if ([modelClass respondsToSelector:selector]) {
			IMP imp = [modelClass methodForSelector:selector];
			NSValueTransformer * (*function)(id, SEL) = (__typeof__(function))imp;
			NSValueTransformer *transformer = function(modelClass, selector);
			if (transformer != nil) result[key] = transformer;
			continue;
		}
		if ([modelClass respondsToSelector:@selector(JSONTransformerForKey:)]) {
			NSValueTransformer *transformer = [modelClass JSONTransformerForKey:key];
			if (transformer != nil) {
				result[key] = transformer;
				continue;
			}
		}
		objc_property_t property = class_getProperty(modelClass, key.UTF8String);
		if (property == NULL) continue;
		mtl_propertyAttributes *attributes = mtl_copyPropertyAttributes(property);
		@onExit {
			free(attributes);
		};
		NSValueTransformer *transformer = nil;
    // type transformer 的生成
		if (*(attributes->type) == *(@encode(id))) {
			Class propertyClass = attributes->objectClass;
			if (propertyClass != nil) {
				transformer = [self transformerForModelPropertiesOfClass:propertyClass];
			}
			// For user-defined MTLModel, try parse it with dictionaryTransformer.
			if (nil == transformer && [propertyClass conformsToProtocol:@protocol(MTLJSONSerializing)]) {
				transformer = [self dictionaryTransformerWithModelClass:propertyClass];
			}
			if (transformer == nil) transformer = [NSValueTransformer mtl_validatingTransformerForClass:propertyClass ?: NSObject.class];
		} else {
			transformer = [self transformerForModelPropertiesOfObjCType:attributes->type] ?: [NSValueTransformer mtl_validatingTransformerForClass:NSValue.class];
		}
		if (transformer != nil) result[key] = transformer;
	}
	return result;
}