For data-driven paint setters, transition immediately to target value (#8306)

Closes #8237

include/mbgl/style/property_value.hpp

@@ -29,6 +29,7 @@ public:
     bool isUndefined()      const { return value.which() == 0; }
     bool isConstant()       const { return value.which() == 1; }
     bool isCameraFunction() const { return value.which() == 2; }
+    bool isDataDriven()     const { return false; }
 
     const                T & asConstant()       const { return value.template get<               T >(); }
     const CameraFunction<T>& asCameraFunction() const { return value.template get<CameraFunction<T>>(); }

src/mbgl/style/paint_property.hpp

@@ -51,6 +51,12 @@ public:
             // Transition from prior value is now complete.
             prior = {};
             return finalValue;
+        } else if (value.isDataDriven()) {
+            // Transitions to data-driven properties are not supported.
+            // We snap immediately to the data-driven value so that, when we perform layout,
+            // we see the data-driven function and can use it to populate vertex buffers.
+            prior = {};
+            return finalValue;
         } else if (now < begin) {
             // Transition hasn't started yet.
             return prior->get().evaluate(evaluator, now);

test/style/paint_property.test.cpp

@@ -22,6 +22,22 @@ float evaluate(UnevaluatedPaintProperty<PropertyValue<float>>& property, Duratio
     return property.evaluate(evaluator, parameters.now);
 }
 
+PossiblyEvaluatedPropertyValue<float> evaluate(UnevaluatedPaintProperty<DataDrivenPropertyValue<float>>& property, Duration delta = Duration::zero()) {
+    PropertyEvaluationParameters parameters {
+        0,
+        TimePoint::min() + delta,
+        ZoomHistory(),
+        Duration::zero()
+    };
+    
+    DataDrivenPropertyEvaluator<float> evaluator {
+        parameters,
+        0.0f
+    };
+    
+    return property.evaluate(evaluator, parameters.now);
+}
+
 TEST(UnevaluatedPaintProperty, EvaluateDefaultValue) {
     UnevaluatedPaintProperty<PropertyValue<float>> property;
     ASSERT_EQ(0.0f, evaluate(property));
@@ -86,3 +102,32 @@ TEST(UnevaluatedPaintProperty, EvaluateTransitionedConstantWithDelay) {
     ASSERT_FLOAT_EQ(0.823099f, evaluate(t1, 1500ms));
     ASSERT_FLOAT_EQ(1.0f, evaluate(t1, 2500ms));
 }
+
+TEST(UnevaluatedPaintProperty, EvaluateDataDrivenValue) {
+    TransitionOptions transition;
+    transition.delay = { 1000ms };
+    transition.duration = { 1000ms };
+    
+    UnevaluatedPaintProperty<DataDrivenPropertyValue<float>> t0 {
+        DataDrivenPropertyValue<float>(0.0f),
+        UnevaluatedPaintProperty<DataDrivenPropertyValue<float>>(),
+        TransitionOptions(),
+        TimePoint::min()
+    };
+    
+    SourceFunction<float> sourceFunction = {
+        "property_name",
+        IdentityStops<float>()
+    };
+    
+    UnevaluatedPaintProperty<DataDrivenPropertyValue<float>> t1 {
+        DataDrivenPropertyValue<float>(sourceFunction),
+        t0,
+        transition,
+        TimePoint::min()
+    };
+    
+    ASSERT_TRUE(evaluate(t0, 0ms).isConstant());
+    ASSERT_FALSE(evaluate(t1, 0ms).isConstant()) <<
+        "A paint property transition to a data-driven evaluates immediately to the final value (see https://github.com/mapbox/mapbox-gl-native/issues/8237).";
+}