1 files changed, 77 insertions, 0 deletions
diff --git a/src/mbgl/style/transitioning_property.hpp b/src/mbgl/style/transitioning_property.hpp
new file mode 100644
index 0000000000..5456322b33
--- /dev/null
+++ b/src/mbgl/style/transitioning_property.hpp
@@ -0,0 +1,77 @@
+#pragma once
+
+#include <mbgl/style/property_value.hpp>
+#include <mbgl/style/data_driven_property_value.hpp>
+#include <mbgl/style/transition_options.hpp>
+#include <mbgl/util/interpolate.hpp>
+
+#include <utility>
+
+namespace mbgl {
+namespace style {
+
+template <class Value>
+class TransitioningProperty {
+public:
+    TransitioningProperty() = default;
+
+    TransitioningProperty(Value value_,
+                          TransitioningProperty<Value> prior_,
+                          TransitionOptions transition,
+                          TimePoint now)
+        : begin(now + transition.delay.value_or(Duration::zero())),
+          end(begin + transition.duration.value_or(Duration::zero())),
+          value(std::move(value_)) {
+        if (transition.isDefined()) {
+            prior = { std::move(prior_) };
+        }
+    }
+
+    template <class Evaluator>
+    auto evaluate(const Evaluator& evaluator, TimePoint now) {
+        auto finalValue = value.evaluate(evaluator);
+        if (!prior) {
+            // No prior value.
+            return finalValue;
+        } else if (now >= end) {
+            // 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);
+        } else {
+            // Interpolate between recursively-calculated prior value and final.
+            float t = std::chrono::duration<float>(now - begin) / (end - begin);
+            return util::interpolate(prior->get().evaluate(evaluator, now), finalValue,
+                                     util::DEFAULT_TRANSITION_EASE.solve(t, 0.001));
+        }
+    }
+
+    bool hasTransition() const {
+        return bool(prior);
+    }
+
+    bool isUndefined() const {
+        return value.isUndefined();
+    }
+
+    const Value& getValue() const {
+        return value;
+    }
+
+private:
+    optional<mapbox::util::recursive_wrapper<TransitioningProperty<Value>>> prior;
+    TimePoint begin;
+    TimePoint end;
+    Value value;
+};
+
+} // namespace style
+} // namespace mbgl