Fix generated props CPP default values

Summary:
This one is fun.

# What?

Previously, the codegen'd constructor for a prop value in CPP was defined like so: `value(convertRawProp(rawProps, "value", sourceProps.value, value))`.

The fourth argument there is the default value of the result of `convertRawProps`. What that is saying is: the default value of `value` is `value` - the default value is itself.

The assumption was that because value is defined as `T value{someDefaultValue}` in the struct, in other words, because it has a default value in the `.h` file, that it will /start out/ being equal to `someDefaultValue`, and then be overridden later optionally, or be set to itself, which should be `someDefaultValue`.

However, that is not how initialization of class members in C++ constructors work. If a class member is in the member initializer list, [then the default value is not set](https://en.cppreference.com/w/cpp/language/initializer_list). That means that if the `defaultValue` as passed is /ever/ used, we incur undefined behavior.

# When is the defaultValue used?

The defaultValue is only used when no prop or a null value is sent from JS to C++: https://our.intern.facebook.com/intern/diffusion/FBS/browse/master/xplat/js/react-native-github/ReactCommon/fabric/core/propsConversions.h?commit=becfded106d706e6028e705d7883483051061e9f&lines=60

In most cases, the `sourceProps.value` is actually used as a fallback (the previous props value). The first time props are ever constructed, `sourceProps` should have valid values since it goes through a different path where only the defaultValues from props.h are used.

# Why wasn't this crashing before?

Most codegen'd properties are ints, floats, doubles, and bools. They might get wacky values, but they won't crash. I wouldn't be surprised if this diff solves some subtle visual or layout bugs, but I have no evidence of this yet.

# How do non-codegen'd props.cpp initialize default values?

Same as this diff does: defaultValue should be explicit everywhere: https://our.intern.facebook.com/intern/diffusion/FBS/browse/master/xplat/js/react-native-github/ReactCommon/fabric/components/scrollview/ScrollViewProps.cpp?commit=2813789c292dfdf1220b88f203af6b33ba9e42de&lines=51

# So... what have we learned?

C++ is tricky!

Reviewed By: yungsters, shergin

Differential Revision: D16955421

fbshipit-source-id: 75bb3f22822299e17df1c36abecdb6ce49012406

packages/react-native-codegen/src/generators/components/CppHelpers.js

@@ -96,8 +96,88 @@ function generateStructName(
   return `${componentName}${additional}Struct`;
 }
 
+function getEnumName(componentName: string, propName: string): string {
+  const uppercasedPropName = toSafeCppString(propName);
+  return `${componentName}${uppercasedPropName}`;
+}
+
+function getEnumMaskName(enumName: string): string {
+  return `${enumName}Mask`;
+}
+
+function convertDefaultTypeToString(
+  componentName: string,
+  prop: PropTypeShape,
+): string {
+  const typeAnnotation = prop.typeAnnotation;
+  switch (typeAnnotation.type) {
+    case 'BooleanTypeAnnotation':
+      return String(typeAnnotation.default);
+    case 'StringTypeAnnotation':
+      if (typeAnnotation.default == null) {
+        return '';
+      }
+      return `"${typeAnnotation.default}"`;
+    case 'Int32TypeAnnotation':
+      return String(typeAnnotation.default);
+    case 'DoubleTypeAnnotation':
+      const defaultDoubleVal = typeAnnotation.default;
+      return parseInt(defaultDoubleVal, 10) === defaultDoubleVal
+        ? typeAnnotation.default.toFixed(1)
+        : String(typeAnnotation.default);
+    case 'FloatTypeAnnotation':
+      const defaultFloatVal = typeAnnotation.default;
+      return parseInt(defaultFloatVal, 10) === defaultFloatVal
+        ? typeAnnotation.default.toFixed(1)
+        : String(typeAnnotation.default);
+    case 'NativePrimitiveTypeAnnotation':
+      switch (typeAnnotation.name) {
+        case 'ColorPrimitive':
+          return '';
+        case 'ImageSourcePrimitive':
+          return '';
+        case 'PointPrimitive':
+          return '';
+        default:
+          (typeAnnotation.name: empty);
+          throw new Error('Received unknown NativePrimitiveTypeAnnotation');
+      }
+    case 'ArrayTypeAnnotation': {
+      switch (typeAnnotation.elementType.type) {
+        case 'StringEnumTypeAnnotation':
+          if (typeAnnotation.elementType.default == null) {
+            throw new Error(
+              'A default is required for array StringEnumTypeAnnotation',
+            );
+          }
+          const enumName = getEnumName(componentName, prop.name);
+          const enumMaskName = getEnumMaskName(enumName);
+          const defaultValue = `${enumName}::${toSafeCppString(
+            typeAnnotation.elementType.default || '',
+          )}`;
+          return `static_cast<${enumMaskName}>(${defaultValue})`;
+        default:
+          return '';
+      }
+    }
+    case 'ObjectTypeAnnotation': {
+      return '';
+    }
+    case 'StringEnumTypeAnnotation':
+      return `${getEnumName(componentName, prop.name)}::${toSafeCppString(
+        typeAnnotation.default,
+      )}`;
+    default:
+      (typeAnnotation: empty);
+      throw new Error('Received invalid typeAnnotation');
+  }
+}
+
 module.exports = {
+  convertDefaultTypeToString,
   getCppTypeForAnnotation,
+  getEnumName,
+  getEnumMaskName,
   getImports,
   toSafeCppString,
   generateStructName,

packages/react-native-codegen/src/generators/components/GeneratePropsCpp.js

@@ -11,7 +11,7 @@
 'use strict';
 
 import type {ComponentShape, SchemaType} from '../../CodegenSchema';
-const {getImports} = require('./CppHelpers');
+const {convertDefaultTypeToString, getImports} = require('./CppHelpers');
 
 // File path -> contents
 type FilesOutput = Map<string, string>;
@@ -45,12 +45,13 @@ const componentTemplate = `
       {}
 `.trim();
 
-function generatePropsString(component: ComponentShape) {
+function generatePropsString(componentName: string, component: ComponentShape) {
   return component.props
     .map(prop => {
+      const defaultValue = convertDefaultTypeToString(componentName, prop);
       return `${prop.name}(convertRawProp(rawProps, "${
         prop.name
-      }", sourceProps.${prop.name}, ${prop.name}))`;
+      }", sourceProps.${prop.name}, {${defaultValue}}))`;
     })
     .join(',\n' + '    ');
 }
@@ -104,7 +105,7 @@ module.exports = {
             const component = components[componentName];
             const newName = `${componentName}Props`;
 
-            const propsString = generatePropsString(component);
+            const propsString = generatePropsString(componentName, component);
             const extendString = getClassExtendString(component);
 
             const imports = getImports(component.props);

packages/react-native-codegen/src/generators/components/GeneratePropsH.js

@@ -11,7 +11,10 @@
 'use strict';
 
 const {
+  convertDefaultTypeToString,
   getCppTypeForAnnotation,
+  getEnumMaskName,
+  getEnumName,
   toSafeCppString,
   generateStructName,
   getImports,
@@ -239,80 +242,6 @@ function getNativeTypeFromAnnotation(
   }
 }
 
-function convertDefaultTypeToString(componentName: string, prop): string {
-  const typeAnnotation = prop.typeAnnotation;
-  switch (typeAnnotation.type) {
-    case 'BooleanTypeAnnotation':
-      return String(typeAnnotation.default);
-    case 'StringTypeAnnotation':
-      if (typeAnnotation.default == null) {
-        return '';
-      }
-      return `"${typeAnnotation.default}"`;
-    case 'Int32TypeAnnotation':
-      return String(typeAnnotation.default);
-    case 'DoubleTypeAnnotation':
-      const defaultDoubleVal = typeAnnotation.default;
-      return parseInt(defaultDoubleVal, 10) === defaultDoubleVal
-        ? typeAnnotation.default.toFixed(1)
-        : String(typeAnnotation.default);
-    case 'FloatTypeAnnotation':
-      const defaultFloatVal = typeAnnotation.default;
-      return parseInt(defaultFloatVal, 10) === defaultFloatVal
-        ? typeAnnotation.default.toFixed(1)
-        : String(typeAnnotation.default);
-    case 'NativePrimitiveTypeAnnotation':
-      switch (typeAnnotation.name) {
-        case 'ColorPrimitive':
-          return '';
-        case 'ImageSourcePrimitive':
-          return '';
-        case 'PointPrimitive':
-          return '';
-        default:
-          (typeAnnotation.name: empty);
-          throw new Error('Received unknown NativePrimitiveTypeAnnotation');
-      }
-    case 'ArrayTypeAnnotation': {
-      switch (typeAnnotation.elementType.type) {
-        case 'StringEnumTypeAnnotation':
-          if (typeAnnotation.elementType.default == null) {
-            throw new Error(
-              'A default is required for array StringEnumTypeAnnotation',
-            );
-          }
-          const enumName = getEnumName(componentName, prop.name);
-          const enumMaskName = getEnumMaskName(enumName);
-          const defaultValue = `${enumName}::${toSafeCppString(
-            typeAnnotation.elementType.default || '',
-          )}`;
-          return `static_cast<${enumMaskName}>(${defaultValue})`;
-        default:
-          return '';
-      }
-    }
-    case 'ObjectTypeAnnotation': {
-      return '';
-    }
-    case 'StringEnumTypeAnnotation':
-      return `${getEnumName(componentName, prop.name)}::${toSafeCppString(
-        typeAnnotation.default,
-      )}`;
-    default:
-      (typeAnnotation: empty);
-      throw new Error('Received invalid typeAnnotation');
-  }
-}
-
-function getEnumName(componentName, propName): string {
-  const uppercasedPropName = toSafeCppString(propName);
-  return `${componentName}${uppercasedPropName}`;
-}
-
-function getEnumMaskName(enumName: string): string {
-  return `${enumName}Mask`;
-}
-
 function convertValueToEnumOption(value: string): string {
   return toSafeCppString(value);
 }