Refactor NSExpression unit tests

[birkskyum]

Remove all XCTExpectFailure from macos/ios/darwin test suites introduced in #411 by refactoring the tests to the new format. Related to #331

[birkskyum]

Working in progress in here: https://github.com/maplibre/maplibre-gl-native/tree/refactor-nsexpression-unit-tests

[birkskyum]

@roblabs , do you know how to write

[NSExpression expressionWithFormat:@"MGL_FUNCTION('image', 'Icon Image')"];

With the new syntax?

[roblabs]

@birkskyum — the expression MGL_FUNCTION is curious and still needs attention.

While I was researching the source for #331 & PR #411, I found documentation for the expression in "Predicates and Expressions.md":

https://maplibre.org/maplibre-gl-native/ios/api/predicates-and-expressions.html#code-mgl_function-code

---

But the Objective-C source mentions that it is not implemented.

https://github.com/maplibre/maplibre-gl-native/blob/main/platform/ios/platform/darwin/src/NSExpression+MGLAdditions.mm#L256,L259

- (id)MGL_FUNCTION:(id)firstArgument, ... {
    [NSException raise:NSInvalidArgumentException
                format:@"Mapbox GL function expressions lack underlying Objective-C implementations."];
    return nil;
}

---

In general, I found that writing the expressions in Swift first was easier to sort out than in Objective-C.

[louwers]

The MapLibre GL JS update is blocked by this because the test files are generated. #816

[birkskyum]

I found that some of these tests only can be enabled again by porting them to swift. The swift syntax is more flexible and we have cases that aren't supported by obj-c anymore.

[birkskyum]

Because of that, I don't plan to look further into refactoring the obj-c code as it seemed futile.

[louwers]

OK thanks for the update.

[1ec5]

But the Objective-C source mentions that it is not implemented.

This just tells you that it’s an abstract class.

MGL_FUNCTION is a generic aftermarket expression that developers can use for any style-spec function that isn’t specifically implemented yet as an aftermarket expression.

/ref mapbox/mapbox-gl-native#11472

[1ec5]

The swift syntax is more flexible and we have cases that aren't supported by obj-c anymore.

There isn’t a separate Swift syntax. You may be referring to the “MGL” convenience initializers on NSExpression, which are also available in Objective-C. These initializers are more type-safe but not as flexible as anything that uses an expression function.

The tests were originally intended to catch the situation where the longstanding trick to installing an aftermarket function ceased to be supported on iOS or macOS. If that has finally happened, then some code can switch to the initializers as in the examples in #411, but more dynamic code needs a different solution.

As noted in #331 (comment), the initializers may not work in some cases, because they’re converted to the aftermarket functions under the hood anyways.

[louwers]

The unit tests are generated so should not be edited manually.

Instead the template that generates them should be fixed: platform/ios/platform/darwin/test/MGLStyleLayerTests.mm.ejs platform/ios/platform/darwin/test/MGLBackgroundStyleLayerTests.mm