iOS platform KVC-based JSON and data object binding

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It's not difficult to manipulate JSON data on the iOS platform, but we have a simpler solution that uses KVC, the full name is Key-value Coding.

Suppose the developer (you) developed an application whose data comes from external to the Web service, retrieving some JSON data from the Web service, with the following data:

{"count": 3, "sum": 9.0, "average": 3.0}

To get data from the server, you need to call Nsjsonserializationalization's Jsonobjectwithdata method and retrieve the data from the serialized dictionary, such as:

NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];NSLog(@"%d", [[dictionary objectForKey:@"count"] intValue]); // prints 3RdNSLog(@"%.1f", [[dictionary objectForKey:@"sum"] doubleValue]); // prints 9.0NSLog(@"%.1f", [[dictionary objectForKey:@"average"] doubleValue]); // prints 3.0

However, the above values are more fragmented, and it may be easier to interact directly with strongly typed data objects when doing application development. For example, you might want to create a statistics statistic class that represents the type of data returned through a Web service, as follows:

@interface Statistics : NSObject@property (nonatomic) int count;@property (nonatomic) double sum;@property (nonatomic) double average;@end

You can then extract the values from the dictionary to populate the above objects:

Statistics *statistics = [[Statistics alloc] init];statistics.count = [[dictionary objectForKey:@"count"] intValue];statistics.sum = [[dictionary objectForKey:@"sum"] doubleValue];statistics.average = [[dictionary objectForKey:@"average"] doubleValue];

To make things easier and avoid code duplication, you can put this code in the initialization of the statistics class:

- (instancetype)initWithDictionary:(NSDictionary *)dictionary {    self = [super init];    if (self) {        self.count = [[dictionary objectForKey:@"count"] intValue];        self.sum = [[dictionary objectForKey:@"sum"] doubleValue];        self.average = [[dictionary objectForKey:@"average"] doubleValue];    }    return self;}

The code-binding JSON responds to the statistics instance as follows:

Statistics *statistics = [[Statistics alloc] initWithDictionary:dictionary];

In any case, you can use the properties of this strongly typed data object to access the data returned from the server:

NSLog(@"%d", statistics.count); // prints 3NSLog(@"%.1f", statistics.sum); // prints 9.0NSLog(@"%.1f", statistics.average); // prints 3.0

The code above works fine, and mapping JSON data to strongly typed data objects is a very good approach.

However, there is a simpler solution: KVC. NSObject Setvaluesforkeyswithdictionary: Method can be used to automatically map all values on a given dictionary to the properties of an object. Using this method, Initwithdictionary: The method is simplified as follows:

- (instancetype)initWithDictionary:(NSDictionary *)dictionary {    self = [super init];    if (self) {        [self setValuesForKeysWithDictionary:dictionary];    }    return self;}

There is no need to manually map a dictionary into an item-to-attribute value, declaring the property with the appropriate name and type is sufficient, and the following code works well in Swift:

class Statistics: NSObject {    var count: Int = 0    var sum: Double = 0    var average: Double = 0    init(dictionary: [String:AnyObject]) {        super.init()        setValuesForKeysWithDictionary(dictionary);    }}

In addition, if you need to customize the assignment of property names or property values, you can simply override the Setvalue:forkey: method. For example, suppose the server refers to an average attribute with a different name:

{"count": 3, "sum": 9.0, "mean": 3.0}

You can override the Setvalue:forkey: method to ensure that the values are correctly mapped to the attribute:

- (void)setValue:(id)value forKey:(NSString *)key {    if ([key isEqual:@"mean"]) {        key = @"average";    }    [super setValue:value forKey:key];}

Finally, you can use KVC to ignore the values you don't want. For example, suppose the server's response also contains a property named "median":

{"count": 3, "sum": 9.0, "average": 3.0, "median": 3.0}

Because the statistics class does not have a "Median" attribute defined, the Setvaluesforkeyswithdictionary: Method throws a Nsunknownkeyexception exception. To avoid throwing this exception, you can simply rewrite the Setvalue:forundefinedkey:: Method.

- (void)setValue:(id)value forUndefinedKey:(NSString *)key {    // No-op}

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iOS platform KVC-based JSON and data object binding