mirror of
https://github.com/facebook/react-native.git
synced 2025-11-01 09:14:26 +00:00
Joshua Gross
cafbf67d0d
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