38fcbe21d4
This converts the style property classes (CirclePaintProperties and so on) to the same tuple-based approach as gl::Attribute and gl::Uniform. The approach is outlined in https://github.com/mapbox/cpp/blob/master/C%2B%2B%20Structural%20Metaprogramming.md. The main advantage of this approach is it allows writing algorithms that work on sets of style properties, without resorting to code generation or manually repetitive code. This lets us iterate on approaches to data-driven properties more easily. Another advantage is that the cascading, unevaluated, and evaluated states of a set of properties exist as independent structures, instead of individual properties holding their own state. This is a more functional approach that makes data flow clearer and reduces state.
54 lines
1.5 KiB
C++
54 lines
1.5 KiB
C++
#pragma once
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#include <mbgl/util/variant.hpp>
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#include <mbgl/style/function.hpp>
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namespace mbgl {
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namespace style {
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class Undefined {};
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inline bool operator==(const Undefined&, const Undefined&) { return true; }
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inline bool operator!=(const Undefined&, const Undefined&) { return false; }
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template <class T>
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class PropertyValue {
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private:
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using Value = variant<Undefined, T, Function<T>>;
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Value value;
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template <class S> friend bool operator==(const PropertyValue<S>&, const PropertyValue<S>&);
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public:
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PropertyValue() : value() {}
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PropertyValue( T constant) : value(constant) {}
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PropertyValue(Function<T> function) : value(function) {}
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bool isUndefined() const { return value.which() == 0; }
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bool isConstant() const { return value.which() == 1; }
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bool isFunction() const { return value.which() == 2; }
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const T & asConstant() const { return value.template get< T >(); }
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const Function<T>& asFunction() const { return value.template get<Function<T>>(); }
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explicit operator bool() const { return !isUndefined(); };
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template <typename Evaluator>
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auto evaluate(const Evaluator& evaluator) const {
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return Value::visit(value, evaluator);
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}
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};
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template <class T>
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bool operator==(const PropertyValue<T>& lhs, const PropertyValue<T>& rhs) {
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return lhs.value == rhs.value;
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}
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template <class T>
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bool operator!=(const PropertyValue<T>& lhs, const PropertyValue<T>& rhs) {
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return !(lhs == rhs);
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}
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} // namespace style
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} // namespace mbgl
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