MAPlistTypeChecking

An easier way to deal with type-checking the objects in property lists, JSON, and the like.

To use:

1. Wrap the top-level object of your property list in an MAErrorReportingDictionary or MAErrorReportingArray.
2. Extract sub-objects as usual.
3. Type-check them by using ma_castRequiredObject: or ma_castOptionalObject:. These will return nil if the types do not match.
4. Type-checking is generally optional. Code that expects plain property list types will get them. (One exception to this currently: keys which do not exist will not produce nil, but an instance of MAErrorReportingObject wrapping nil.)
5. When done, call -errors on the top-level object. A list of errors produced by the type-checking cast methods will be returned. An empty array means no errors.
6. Each error contains a code indicating whether it was due to a missing value or a value that was present but not of the correct type. It also contains the full key path used to obtain the value, stored in the userInfo under MAErrorReportingContainersKeyPathUserInfoKey.

Property list objects are wrapped in proxies that forward to the original object while adding the error-tracking facilities that make the rest possible. Arrays and dictionaries are wrapped in special objects which return proxies for their contents, while other classes (strings, numbers, data, dates) have more generic wrappers. Objects of other types are supported, but they get wrapped in instances of MAErrorReportingObject which do not directly proxy their contents. Instead, the wrapped object must be accessed by calling -wrappedObject, or by using one of the ma_cast methods, which will return the interior object if it is of the correct type.

Because of the proxy wrappers for property list types, the containers can generally be passed into code that is unaware of them, as they will still get valid, usable NSString, NSNumber, NSDictionary, etc. instances from the containers. One exception to this is that nil will be wrapped, so if the code does an explicit check for nil, it will not work properly.

Additional class cluster types can be given proxy wrappers fairly easily. To wrap non-container types such as NSString:

1. Create a new class subclassing the class cluster.
2. In the body of the @interface section, put MA_ERROR_REPORTING_INTERFACE to add the necessary interface portions.
3. In the @implementation section, put MA_ERROR_REPORTING_IVARS in brackets to declare the necessary instance variables.
4. In the body of the @implementation section, put MA_ERROR_REPORTING_METHODS to add the necessary methods.
5. Just below that, put MA_ERROR_REPORTING_AUTO_CLUSTER_FORWARD to automatically forward all methods from the main class cluster class to the wrapped object.

To wrap container types such as NSArray, the steps are similar, but require some manual intervention:

1. Create a new class subclassing the class cluster.
2. In the body of the @interface section, put MA_ERROR_REPORTING_INTERFACE to add the necessary interface portions.
3. In the @implementation section, put MA_ERROR_REPORTING_IVARS in brackets to declare the necessary instance variables.
4. In the body of the @implementation section, put MA_ERROR_REPORTING_METHODS to add the necessary methods.
5. Override the class cluster's primitive methods. For any methods that return an object in the container, wrap those objects using -[MAErrorReportingObject wrapObject:parent:key:] before returning them to the caller.

Pull requests are appreciated!