Top-level aftermarket expression functions fail to parse

[1ec5]

As of iOS 11.4 and macOS 10.15.4, if an “aftermarket” expression function like the ones added in mapbox/mapbox-gl-native#11472 appears at the top level of an NSExpression format string, it may raise an “Unable to parse the format string” exception unless certain syntaxes are used to make it an explicit function call. For example:

• mgl_round(3.14159) 💥
• mgl_round:(3.14159) ✔️
• (mgl_round(3.14159)) ✔️
• mgl_round(3.14159) + 0 ✔️
• abs(mgl_round(3.14159)) ✔️

This issue affects any of the functions installed by +[MGLAftermarketExpressionInstaller installFunctions]:

mapbox-gl-native-ios/platform/darwin/src/NSExpression+MGLAdditions.mm

Lines 65 to 88 in 23b5b46

	// Install method-like functions, taking the number of arguments implied by
	// the selector name.
	INSTALL_METHOD(mgl_join:);
	INSTALL_METHOD(mgl_round:);
	INSTALL_METHOD(mgl_interpolate:withCurveType:parameters:stops:);
	INSTALL_METHOD(mgl_step:from:stops:);
	INSTALL_METHOD(mgl_coalesce:);
	INSTALL_METHOD(mgl_does:have:);
	INSTALL_METHOD(mgl_acos:);
	INSTALL_METHOD(mgl_cos:);
	INSTALL_METHOD(mgl_asin:);
	INSTALL_METHOD(mgl_sin:);
	INSTALL_METHOD(mgl_atan:);
	INSTALL_METHOD(mgl_tan:);
	INSTALL_METHOD(mgl_log2:);
	INSTALL_METHOD(mgl_attributed:);
	// Install functions that resemble control structures, taking arbitrary
	// numbers of arguments. Vararg aftermarket functions need to be declared
	// with an explicit and implicit first argument.
	INSTALL_CONTROL_STRUCTURE(MGL_LET);
	INSTALL_CONTROL_STRUCTURE(MGL_MATCH);
	INSTALL_CONTROL_STRUCTURE(MGL_IF);
	INSTALL_CONTROL_STRUCTURE(MGL_FUNCTION);

I suspect this regression is caused by a change in Apple’s iOS/macOS SDK, not the Mapbox Maps SDK. It’s possible that one of these private methods is responsible for raising the error. However, there probably isn’t much we can do about this issue, because we’re trying to avoid any potential App Store submission issues that would arise from calling any private method. We’re only extending the _NSPredicateUtilities class, which we’re only obtaining indirectly using official documented APIs.

In the absence of a fix, we should document this edge case in the “Predicates and Expressions” guide.

/cc @mapbox/maps-ios

[BRZErez]

Is this still an issue?
I've spent a full day yesterday trying to make MGL_MATCH, MGL_IF, MGL_FUNCTION, TERNARY work and the app crashes all the time and couldn't understand why until I found this.
So why didn't you fix the docs to say that non of this is working anymore and need to use mgl_interpolate the very long way instead?
10x

[1ec5]

All the aftermarket expression functions do continue to work, but you need to surround them in parentheses if they’re at the top level of the expresion or predicate format string. See the ✔️ marks above.

TERNARY() works regardless. This is a syntax built into the Foundation framework, not a Mapbox-specific function.