[swift-evolution] Improve Swift APIs for NSExpression and NSIncrementalStore

[Andrew Tetlaw]

Apologies if this has been discussed, I couldn't find any mention in the
list or on https://github.com/apple/swift-3-api-guidelines-review.

The NSIncrementalStore method:

    func referenceObjectForObjectID(_ objectID: NSManagedObjectID) ->
AnyObject

Can thrown an exception, but it's not marked as throws. From the
documentation:

"This method raises an NSInvalidArgumentException if the object ID was not
created by the receiving store."

NSExpression has a constantValue property:

    var constantValue: AnyObject { get }

that can raise an exception at runtime, but it's not marked as throws. From
the documentation:

'Accessing this property raises an exception if it is not applicable to the
expression."

The NSExpression constantValue property can also return nil at runtime, but
it's return type is AnyObject; not an optional.

If you construct an NSComparisonPredicate with a format string like:

    "property == nil"

and you examine the expression returned by predicate.rightExpression. You
can't call expression.constantValue from Swift because it'll crash with
EXC_BAD_ACCESS. You also can't protect against a nil value because the
return type is not optional.


To protect Swift against both of these situations, you have to use
Objective-C wrapper functions. For example to protect against exceptions:

NSException *_Nullable IADCatchObjCException(void (^_Nullable block)())
{
    NSException *exception = nil;
    @try {
        if (block) {
            block();
        }
    } @catch (NSException *e) {
        exception = e;
    }
    return exception;
}

To protect against a surprise nil at runtime:

inline _Nullable id IADMaybeNilValue(_Nullable id value)
{
    return value ?: nil;
}

I was hoping this could be improved for the Swift API?

--
Andrew Tetlaw
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