Replace compile-time polymorphism with runtime polymorphism in the conversion system

2 files changed, 120 insertions, 107 deletions

diff --git a/platform/darwin/src/MGLConversion.h b/platform/darwin/src/MGLConversion.h
index d6363b28eb..0d18d4e716 100644
--- a/platform/darwin/src/MGLConversion.h
+++ b/platform/darwin/src/MGLConversion.h
@@ -1,9 +1,4 @@
-#import <Foundation/Foundation.h>
-
-#include <mbgl/util/logging.hpp>
 #include <mbgl/style/conversion.hpp>
-#include <mbgl/util/feature.hpp>
-#include <mbgl/util/optional.hpp>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -11,128 +6,147 @@ namespace mbgl {
 namespace style {
 namespace conversion {
 
-/**
- A minimal wrapper class conforming to the requirements for `objectMember(v, name)` (see mbgl/style/conversion.hpp)
- This is necessary because using `NSObject*` as the value type in `optional<NSObject*>` causes problems for the ARC,
- due to things like `optional(const value_type& __v)`
- */
-class OptionalNSObjectValue {
+// A wrapper class for `id`, so as not to confuse ARC.
+class Holder {
 public:
-    OptionalNSObjectValue(NSObject * _Nullable _value) : value(_value) {}
-    
-    explicit operator bool() const {
-        return value;
+    Holder(const id v) : value(v) {}
+    const id value;
+};
+
+template <>
+class ConversionTraits<Holder> {
+public:
+    static bool isUndefined(const Holder& holder) {
+        const id value = holder.value;
+        return !value || value == [NSNull null];
     }
-    
-    NSObject * _Nullable operator*() {
-        NSCAssert(this, @"Expected non-null value.");
-        return value;
+
+    static bool isArray(const Holder& holder) {
+        const id value = holder.value;
+        return [value isKindOfClass:[NSArray class]];
     }
-private:
-    NSObject * _Nullable value;
-};
 
-inline bool isUndefined(const id value) {
-    return !value || value == [NSNull null];
-}
+    static bool isObject(const Holder& holder) {
+        const id value = holder.value;
+        return [value isKindOfClass:[NSDictionary class]];
+    }
 
-inline bool isArray(const id value) {
-    return [value isKindOfClass:[NSArray class]];
-}
+    static std::size_t arrayLength(const Holder& holder) {
+        const id value = holder.value;
+        NSCAssert([value isKindOfClass:[NSArray class]], @"Value must be an NSArray for getLength().");
+        NSArray *array = value;
+        auto length = [array count];
+        NSCAssert(length <= std::numeric_limits<size_t>::max(), @"Array length out of bounds.");
+        return length;
+    }
 
-inline bool isObject(const id value) {
-    return [value isKindOfClass:[NSDictionary class]];
-}
+    static Holder arrayMember(const Holder& holder, std::size_t i) {
+        const id value = holder.value;
+        NSCAssert([value isKindOfClass:[NSArray class]], @"Value must be an NSArray for get(int).");
+        NSCAssert(i < NSUIntegerMax, @"Index must be less than NSUIntegerMax");
+        return {[value objectAtIndex: i]};
+    }
 
-inline std::size_t arrayLength(const id value) {
-    NSCAssert([value isKindOfClass:[NSArray class]], @"Value must be an NSArray for getLength().");
-    NSArray *array = value;
-    auto length = [array count];
-    NSCAssert(length <= std::numeric_limits<size_t>::max(), @"Array length out of bounds.");
-    return length;
-}
+    static optional<Holder> objectMember(const Holder& holder, const char *key) {
+        const id value = holder.value;
+        NSCAssert([value isKindOfClass:[NSDictionary class]], @"Value must be an NSDictionary for get(string).");
+        NSObject *member = [value objectForKey: @(key)];
+        if (member && member != [NSNull null]) {
+            return {member};
+        } else {
+            return {};
+        }
+    }
 
-inline NSObject *arrayMember(const id value, std::size_t i) {
-    NSCAssert([value isKindOfClass:[NSArray class]], @"Value must be an NSArray for get(int).");
-    NSCAssert(i < NSUIntegerMax, @"Index must be less than NSUIntegerMax");
-    return [value objectAtIndex: i];
-}
+// Compiler is wrong about `Fn` parameter missing a nullability specifier.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wnullability-completeness"
+    template <class Fn>
+    static optional<Error> eachMember(const Holder&, Fn&&) {
+#pragma clang diagnostic pop
+        // Not implemented (unneeded for MGLStyleFunction conversion).
+        NSCAssert(NO, @"eachMember not implemented");
+        return {};
+    }
 
-inline OptionalNSObjectValue objectMember(const id value, const char *key) {
-    NSCAssert([value isKindOfClass:[NSDictionary class]], @"Value must be an NSDictionary for get(string).");
-    NSObject *member = [value objectForKey: @(key)];
-    if (member && member != [NSNull null]) {
-        return { member };
-    } else {
-        return { nullptr };
+    static optional<bool> toBool(const Holder& holder) {
+        const id value = holder.value;
+        if (_isBool(value)) {
+            return ((NSNumber *)value).boolValue;
+        } else {
+            return {};
+        }
     }
-}
 
-// Not implemented (unneeded for MGLStyleFunction conversion):
-// optional<Error> eachMember(const NSObject*, Fn&&)
+    static optional<float> toNumber(const Holder& holder) {
+        const id value = holder.value;
+        if (_isNumber(value)) {
+            return ((NSNumber *)value).floatValue;
+        } else {
+            return {};
+        }
+    }
 
-inline bool _isBool(const id value) {
-    if (![value isKindOfClass:[NSNumber class]]) return false;
-    // char: 32-bit boolean
-    // BOOL: 64-bit boolean
-    NSNumber *number = value;
-    return ((strcmp([number objCType], @encode(char)) == 0) ||
-            (strcmp([number objCType], @encode(BOOL)) == 0));
-}
-    
-inline bool _isNumber(const id value) {
-    return [value isKindOfClass:[NSNumber class]] && !_isBool(value);
-}
-    
-inline bool _isString(const id value) {
-    return [value isKindOfClass:[NSString class]];
-}
+    static optional<double> toDouble(const Holder& holder) {
+        const id value = holder.value;
+        if (_isNumber(value)) {
+            return ((NSNumber *)value).doubleValue;
+        } else {
+            return {};
+        }
+    }
 
-inline optional<bool> toBool(const id value) {
-    if (_isBool(value)) {
-        return ((NSNumber *)value).boolValue;
-    } else {
-        return {};
+    static optional<std::string> toString(const Holder& holder) {
+        const id value = holder.value;
+        if (_isString(value)) {
+            return std::string(static_cast<const char *>([value UTF8String]));
+        } else {
+            return {};
+        }
     }
-}
 
-inline optional<float> toNumber(const id value) {
-    if (_isNumber(value)) {
-        return ((NSNumber *)value).floatValue;
-    } else {
-        return {};
+    static optional<mbgl::Value> toValue(const Holder& holder) {
+        const id value = holder.value;
+        if (isUndefined(value)) {
+            return {};
+        } else if (_isBool(value)) {
+            return { *toBool(holder) };
+        } else if ( _isString(value)) {
+            return { *toString(holder) };
+        } else if (_isNumber(value)) {
+            // Need to cast to a double here as the float is otherwise considered a bool...
+           return { static_cast<double>(*toNumber(holder)) };
+        } else {
+            return {};
+        }
     }
-}
 
-inline optional<double> toDouble(const id value) {
-    if (_isNumber(value)) {
-        return ((NSNumber *)value).doubleValue;
-    } else {
+    static optional<GeoJSON> toGeoJSON(const Holder& holder, Error& error) {
+        error = { "toGeoJSON not implemented" };
         return {};
     }
-}
 
-inline optional<std::string> toString(const id value) {
-    if (_isString(value)) {
-        return std::string(static_cast<const char *>([value UTF8String]));
-    } else {
-        return {};
+private:
+    static bool _isBool(const id value) {
+        if (![value isKindOfClass:[NSNumber class]]) return false;
+        // char: 32-bit boolean
+        // BOOL: 64-bit boolean
+        NSNumber *number = value;
+        return ((strcmp([number objCType], @encode(char)) == 0) ||
+                (strcmp([number objCType], @encode(BOOL)) == 0));
     }
-}
 
-inline optional<mbgl::Value> toValue(const id value) {
-    if (isUndefined(value)) {
-        return {};
-    } else if (_isBool(value)) {
-        return { *toBool(value) };
-    } else if ( _isString(value)) {
-        return { *toString(value) };
-    } else if (_isNumber(value)) {
-        // Need to cast to a double here as the float is otherwise considered a bool...
-       return { static_cast<double>(*toNumber(value)) };
-    } else {
-        return {};
+    static bool _isNumber(const id value) {
+        return [value isKindOfClass:[NSNumber class]] && !_isBool(value);
     }
+
+    static bool _isString(const id value) {
+        return [value isKindOfClass:[NSString class]];
+    }
+};
+
+inline Convertible makeConvertible(const id value) {
+    return Convertible(Holder(value));
 }
 
 } // namespace conversion
@@ -140,4 +154,3 @@ inline optional<mbgl::Value> toValue(const id value) {
 } // namespace mbgl
 
 NS_ASSUME_NONNULL_END
-
diff --git a/platform/darwin/src/MGLStyleValue_Private.h b/platform/darwin/src/MGLStyleValue_Private.h
index 2155c657bd..5914e0a2aa 100644
--- a/platform/darwin/src/MGLStyleValue_Private.h
+++ b/platform/darwin/src/MGLStyleValue_Private.h
@@ -124,9 +124,9 @@ public:
         if ([value isKindOfClass:[MGLConstantStyleValue class]]) {
             return toMBGLConstantValue((MGLConstantStyleValue<ObjCType> *)value);
         } else if ([value isKindOfClass:[MGLStyleFunction class]]) {
-            auto rawValue = toRawStyleSpecValue((MGLStyleFunction<ObjCType> *) value);
             mbgl::style::conversion::Error error;
-            auto result = mbgl::style::conversion::convert<mbgl::style::DataDrivenPropertyValue<MBGLType>>(rawValue, error);
+            auto result = mbgl::style::conversion::convert<mbgl::style::DataDrivenPropertyValue<MBGLType>>(
+                mbgl::style::conversion::makeConvertible(toRawStyleSpecValue((MGLStyleFunction<ObjCType> *) value)), error);
             NSCAssert(result, @(error.message.c_str()));
             return *result;
         } else {