react-native/ReactCommon/react/renderer/core/RawPropsParser.cpp

/*
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

#include "RawPropsParser.h"

#include <folly/Likely.h>
#include <react/renderer/core/RawProps.h>

#ifndef NDEBUG
#include <glog/logging.h>
#endif

namespace facebook {
namespace react {

// During parser initialization, Props structs are used to parse
// "fake"/empty objects, and `at` is called repeatedly which tells us
// which props are accessed during parsing, and in which order.
RawValue const *RawPropsParser::at(
    RawProps const &rawProps,
    RawPropsKey const &key) const noexcept {
  if (UNLIKELY(!ready_)) {
    // Check against the same key being inserted more than once.
    // This happens commonly with nested Props structs, where the higher-level
    // struct may access all fields, and then the nested Props struct may
    // access fields a second (or third, etc) time.
    // Without this, multiple entries will be created for the same key, but
    // only the first access of the key will return a sensible value.
    // The complexity of this is (n + (n - 1) + (n - 2) + ... + (n - (n - 1) +
    // 1))) or n*n - (1/2)(n*(n+1)). If there are 100 props, this will result in
    // 4950 lookups and equality checks on initialization of the parser, which
    // happens exactly once per component.
    for (int i = 0; i < size_; i++) {
      if (keys_[i] == key) {
        return nullptr;
      }
    }
    // This is not thread-safe part; this happens only during initialization of
    // a `ComponentDescriptor` where it is actually safe.
    keys_.push_back(key);
    nameToIndex_.insert(key, size_);
    size_++;
    return nullptr;
  }

  // Normally, keys are looked up in-order. For performance we can simply
  // increment this key counter, and if the key is equal to the key at the next
  // index, there's no need to do any lookups. However, it's possible for keys
  // to be accessed out-of-order or multiple times, in which case we start
  // searching again from index 0.
  // To prevent infinite loops (which can occur if
  // you look up a key that doesn't exist) we keep track of whether or not we've
  // already looped around, and log and return nullptr if so. However, we ONLY
  // do this in debug mode, where you're more likely to look up a nonexistent
  // key as part of debugging. You can (and must) ensure infinite loops are not
  // possible in production by: (1) constructing all props objects without
  // conditionals, or (2) if there are conditionals, ensure that in the parsing
  // setup case, the Props constructor will access _all_ possible props. To
  // ensure this performance optimization is utilized, always access props in
  // the same order every time. This is trivial if you have a simple Props
  // constructor, but difficult or impossible if you have a shared sub-prop
  // Struct that is used by multiple parent Props.
#ifndef NDEBUG
  bool resetLoop = false;
#endif
  do {
    rawProps.keyIndexCursor_++;

    if (UNLIKELY(rawProps.keyIndexCursor_ >= size_)) {
#ifndef NDEBUG
      if (resetLoop) {
        LOG(ERROR) << "Looked up RawProps key that does not exist: "
                   << (std::string)key;
        return nullptr;
      }
      resetLoop = true;
#endif
      rawProps.keyIndexCursor_ = 0;
    }
  } while (UNLIKELY(key != keys_[rawProps.keyIndexCursor_]));

  auto valueIndex = rawProps.keyIndexToValueIndex_[rawProps.keyIndexCursor_];
  return valueIndex == kRawPropsValueIndexEmpty ? nullptr
                                                : &rawProps.values_[valueIndex];
}

void RawPropsParser::postPrepare() noexcept {
  ready_ = true;
  nameToIndex_.reindex();
}

void RawPropsParser::preparse(RawProps const &rawProps) const noexcept {
  rawProps.keyIndexToValueIndex_.resize(size_, kRawPropsValueIndexEmpty);

  // Resetting the cursor, the next increment will give `0`.
  rawProps.keyIndexCursor_ = size_ - 1;

  switch (rawProps.mode_) {
    case RawProps::Mode::Empty:
      return;

    case RawProps::Mode::JSI: {
      auto &runtime = *rawProps.runtime_;
      auto object = rawProps.value_.asObject(runtime);

      auto names = object.getPropertyNames(runtime);
      auto count = names.size(runtime);
      auto valueIndex = RawPropsValueIndex{0};

      for (auto i = 0; i < count; i++) {
        auto nameValue = names.getValueAtIndex(runtime, i).getString(runtime);
        auto value = object.getProperty(runtime, nameValue);

        auto name = nameValue.utf8(runtime);

        auto keyIndex = nameToIndex_.at(name.data(), name.size());
        if (keyIndex == kRawPropsValueIndexEmpty) {
          continue;
        }

        rawProps.keyIndexToValueIndex_[keyIndex] = valueIndex;
        rawProps.values_.push_back(
            RawValue(jsi::dynamicFromValue(runtime, value)));
        valueIndex++;
      }

      break;
    }

    case RawProps::Mode::Dynamic: {
      auto const &dynamic = rawProps.dynamic_;
      auto valueIndex = RawPropsValueIndex{0};

      for (auto const &pair : dynamic.items()) {
        auto name = pair.first.getString();

        auto keyIndex = nameToIndex_.at(name.data(), name.size());
        if (keyIndex == kRawPropsValueIndexEmpty) {
          continue;
        }

        rawProps.keyIndexToValueIndex_[keyIndex] = valueIndex;
        rawProps.values_.push_back(RawValue{pair.second});
        valueIndex++;
      }
      break;
    }
  }
}

} // namespace react
} // namespace facebook