Represent $type as member selector expression in predicates

[1ec5]

MGLVectorStyleLayer.predicate should recognize expressions invoking the isKindOfClass: selector on self and convert them to comparisons involving $type. The MGLVectorStyleLayer.predicate documentation currently defines the special $type attribute as the following:

The type of geometry represented by the feature. A feature’s type is guaranteed to be one of the following strings:

• Point for point features, corresponding to the MGLPointAnnotation class
• LineString for polyline features, corresponding to the MGLPolyline class
• Polygon for polygon features, corresponding to the MGLPolygon class

These four special strings are discoverable only via documentation, and $type can’t occur inside a predicate format string because $ denotes a predicate variable. Adding a string enumeration for these strings could help with discoverability, but I think a more self-consistent approach would be to use isKindOfClass: in conjunction with the classes we’ve already mapped to the vector feature types, since this is how you filter types when filtering NSArrays. What we currently represent as:

[NSPredicate predicateWithFormat:@"%K in { 'LineString', 'Polygon' }", @"$type"]

NSPredicate(format: "%K in { 'LineString', 'Polygon' }", "$type")

turns into:

[NSPredicate predicateWithFormat:@"(self isKindOfClass: %@) OR (self isKindOfClass: %@)", [MGLPolyline class], [MGLPolygon class]]

NSPredicate(format: "(self isKindOfClass: %@) OR (self isKindOfClass: %@)", MGLPolyline.self, MGLPolygon.self)

Under the hood, self is parsed as +[NSExpression expressionForEvaluatedObject] and has an expression type of NSEvaluatedObjectExpressionType. The isKindOfClass: expression is initialized with -[NSComparisonPredicate initWithLeftExpression:rightExpression:customSelector:], and customSelector contains isKindOfClass:.

/ref #6970
/cc @frederoni @incanus @boundsj

[1ec5]

Per #7552, $type is currently always represented as an integer: 1 for points, 2 for polylines, and 3 for polygons.

[1ec5]

As part of #8074, the geometry-type operator would be translated into an isKindOfClass: method call.

[stale]

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