项目需求：从网络获取Json后，将Json自动转存到CoreData中。

继上一篇日志，那么这篇的主要内容是：将Json存到CoreData中。

说话实话，无非就是KVC赋值，思路清晰明了，但是我在想一个问题，有没有办法做到通用呢？那么问题来了～挖机技术哪家强！

好了不扯淡了，虽然KVC暂时满足我项目需求，那个通用办法我还在寻找中，能力有限，不过我会努力。

顺便分享一篇 讲述 这个网络请求数据存放到CoreData的介绍

Process remote service data into Core Data

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Well, you have our data persisting to disk in a Property List format. But what you really want to do is to process it into Core Data. This is where you will be doing some heavy lifting and getting into the nitty gritty details. This is the part where the real magic happens! 

To start off you will first need a way to retrieve the files from disk. Add -JSONDictionaryForClassWithName: in SDSyncEngine.m:

- (NSDictionary *)JSONDictionaryForClassWithName:(NSString *)className {
    NSURL *fileURL = [NSURL URLWithString:className relativeToURL:[self JSONDataRecordsDirectory]];
    return [NSDictionary dictionaryWithContentsOfURL:fileURL];
}

One caveat to the NSDictionary that -JSONDictionaryForClassWithName: returns is that the information you are interested in is will be in an NSArray with the key “results”. So to make things easier for processing purposes, add another method to access the data in the NSArray and spice it up a little to allow for sorting of the records by a specified key. 

Add this beneath -JSONDictionaryForClassWithName: in SDSyncEngine.m:

- (NSArray *)JSONDataRecordsForClass:(NSString *)className sortedByKey:(NSString *)key {
    NSDictionary *JSONDictionary = [self JSONDictionaryForClassWithName:className];
    NSArray *records = [JSONDictionary objectForKey:@"results"];
    return [records sortedArrayUsingDescriptors:[NSArray arrayWithObject:
                                                 [NSSortDescriptor sortDescriptorWithKey:key ascending:YES]]];
}

This method calls the previous method you implemented, and returns an NSArray of all the records in the response, sorted by the specified key. 

You won’t really need the JSON responses that were saved to disk much past this point, so add another method to delete them when you’re finished with them. Add the following method above -JSONDictionaryForClassWithName:

- (void)deleteJSONDataRecordsForClassWithName:(NSString *)className {
    NSURL *url = [NSURL URLWithString:className relativeToURL:[self JSONDataRecordsDirectory]];
    NSError *error = nil;
    BOOL deleted = [[NSFileManager defaultManager] removeItemAtURL:url error:&error];
    if (!deleted) {
        NSLog(@"Unable to delete JSON Records at %@, reason: %@", url, error);
    }
}

In order to translate records from JSON to NSManagedObjects, you will need a few methods. First, you will need to translate the JSON values to Objective-C properties; the method you use will vary based on the remote service you are working with. In this case, you are using Parse which has a few “special” data types. The data you’ll be concerned with here are Files and Dates. Files are returned as URLs to the file’s location, and Dates are returned in the following format:

{
  "__type": "Date",
  "iso": "2011-08-21T18:02:52.249Z"
}

Since the date is in the format of a string, you will want some methods to convert from a Parse formatted date string to an NSDate and back to an NSString. NSDateFormatter can help with this, but they are very expensive to allocate — so first add a new NSDateFormatter property that you can re-use.

Add the dateFormatter property in your private category:

@interface SDSyncEngine ()

@property (nonatomic, strong) NSMutableArray *registeredClassesToSync;
@property (nonatomic, strong) NSDateFormatter *dateFormatter;

@end

And add these three methods above the #pragma mark -File Management.

- (void)initializeDateFormatter {
    if (!self.dateFormatter) {
        self.dateFormatter = [[NSDateFormatter alloc] init];
        [self.dateFormatter setDateFormat:@"yyyy-MM-dd‘T‘HH:mm:ss‘Z‘"];
        [self.dateFormatter setTimeZone:[NSTimeZone timeZoneWithName:@"GMT"]];
    }
}

- (NSDate *)dateUsingStringFromAPI:(NSString *)dateString {
    [self initializeDateFormatter];
    // NSDateFormatter does not like ISO 8601 so strip the milliseconds and timezone
    dateString = [dateString substringWithRange:NSMakeRange(0, [dateString length]-5)];
    
    return [self.dateFormatter dateFromString:dateString];
}

- (NSString *)dateStringForAPIUsingDate:(NSDate *)date {
    [self initializeDateFormatter];
    NSString *dateString = [self.dateFormatter stringFromDate:date];
    // remove Z
    dateString = [dateString substringWithRange:NSMakeRange(0, [dateString length]-1)];
    // add milliseconds and put Z back on
    dateString = [dateString stringByAppendingFormat:@".000Z"];
    
    return dateString;
}

The first method -initializeDateFormatter will initialize your dateFormatter property. The second method -dateUsingStringFromAPI: receives an NSString and returns an NSDate object. The third method -dateStringForAPIUsingDate: receives an NSDate and returns an NSString. 

Take a little closer look, there, detective — the second and third methods do something a little strange. Parse uses timestamps in the ISO 8601 format which do not translate to NSDate objects very well, so you need to do some stripping and appending of the milliseconds and Z flag (used to denote the timezone). (Oh standards…there are so many wonderful ones to choose from!) :] 

Next add this method below mostRecentUpdatedAtDateForEntityWithName:

- (void)setValue:(id)value forKey:(NSString *)key forManagedObject:(NSManagedObject *)managedObject {
    if ([key isEqualToString:@"createdAt"] || [key isEqualToString:@"updatedAt"]) {
        NSDate *date = [self dateUsingStringFromAPI:value];
        [managedObject setValue:date forKey:key];
    } else if ([value isKindOfClass:[NSDictionary class]]) {
        if ([value objectForKey:@"__type"]) {
            NSString *dataType = [value objectForKey:@"__type"];
            if ([dataType isEqualToString:@"Date"]) {
                NSString *dateString = [value objectForKey:@"iso"];
                NSDate *date = [self dateUsingStringFromAPI:dateString];
                [managedObject setValue:date forKey:key];
            } else if ([dataType isEqualToString:@"File"]) {
                NSString *urlString = [value objectForKey:@"url"];
                NSURL *url = [NSURL URLWithString:urlString];
                NSURLRequest *request = [NSURLRequest requestWithURL:url];
                NSURLResponse *response = nil;
                NSError *error = nil;
                NSData *dataResponse = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
                [managedObject setValue:dataResponse forKey:key];
            } else {
                NSLog(@"Unknown Data Type Received");
                [managedObject setValue:nil forKey:key];
            }
        }
    } else {
        [managedObject setValue:value forKey:key];
    }
}

This method accepts a value, key, and managedObject. If the key is equal to createdDate or updatedAt, you will be converting them to NSDates. If the key is an NSDictionary you will check the __type key to determine the data type Parse returned. If it is a Date, you will convert the value from an NSString to an NSDate. If it is a File, you will do a little more work since you are interested in getting the image itself! 

To get the image, send off a request to download the image file. It is important to note that downloading the image data can take a considerable amount of time, so this may only work efficiently with smaller data sets. Another solution would be to fetch the image data when the record is accessed (lazy loading), but it would only be available if the user has an Internet connection at the time of lazy loading. 

If the data type is anything other than a File or Date there is no way to know what to do with it so set the value to nil. In any other case you will simply pass the value and key through untouched and set them on the managedObject. 

后话：

耐心看到这里，其实我就是想证明一点，果然还是KVC！

iOS 网络请求Json自动转存到CoreData(二)