第三輪遍歷（oldFiber沒遍歷完且newChildren沒遍歷完）：

• 把剩下的oldFiber放入existingChildren中；
• 聲明lastPlacedIndex變量，表示不需要移動(dòng)的老節(jié)點(diǎn)的索引；
• 如果在map中能找到相同key相同type的節(jié)點(diǎn)復(fù)用老fiber，并從map中刪除;
• 如果在map中找不到相同key相同type的節(jié)點(diǎn)創(chuàng)建新fiber;
• 如果復(fù)用老fiber，老fiber的索引大于lastPlacedIndex則更新lastPlacedIndex為老fiber的索引；
• 如果復(fù)用老fiber，老fiber的索引小于lastPlacedIndex則移動(dòng)fiber, lastPlacedIndex不變；
• 把map中剩下的fiber全部標(biāo)記為刪除

代碼詳情

1. main.jsx

function FunctionComponent() {
  const [number, setNumber] = React.useState(0);
  return number === 0 ? (
    <ul key="container" onClick={() => setNumber(number + 1)}>
      <li key="A">A</li>
      <li key="B" id="b">
        B
      </li>
      <li key="C">C</li>
      <li key="D">D</li>
      <li key="E">E</li>
      <li key="F" id="F">
        F
      </li>
    </ul>
  ) : (
    <ul key="container" onClick={() => setNumber(number + 1)}>
      <li key="A">A2</li>
      <li key="C">C2</li>
      <li key="E">E2</li>
      <li key="B" id="b2">
        B2
      </li>
      <li key="G">G</li>
      <li key="D">D2</li>
    </ul>
  );
}
let element = <FunctionComponent />;

const root = createRoot(document.getElementById("root"));
console.log("root", root);
root.render(element);

2. ReactChildFiber

function placeChild(newFiber, lastPlacedIndex, newIdx) {
    newFiber.index = newIdx;
    if (!shouldTrackSideEffects) {
      return lastPlacedIndex;
    }
    const current = newFiber.alternate;
    if (current !== null) {
      const oldIndex = current.index;
      if (oldIndex < lastPlacedIndex) {
        newFiber.flags |= Placement;
        return lastPlacedIndex;
      } else {
        return oldIndex;
      }
    } else {
      newFiber.flags |= Placement;
      return lastPlacedIndex;
    }
  }

 function mapRemainingChildren(returnFiber, currentFirstChild) {
    const existingChildren = new Map();
    let existingChild = currentFirstChild;
    while (existingChild != null) {
      if (existingChild.key != null) {
        existingChildren.set(existingChild.key, existingChild);
      } else {
        existingChildren.set(existingChild.index, existingChild);
      }
      existingChild = existingChild.sibling;
    }
    return existingChildren;
  }

function updateTextNode(returnFiber, current, textContent) {
    if (current !== null || current.tag !== HostText) {
      const created = createFiberFromText(textContent);
      created.return = returnFiber;
      return created;
    } else {
      const existing = useFiber(current, textContent);
      existing.return = returnFiber;
      return existing;
    }
  }

function updateFromMap(existingChildren, returnFiber, newIdx, newChild) {
    if (
      (typeof newChild === "string" && newChild !== "") ||
      typeof newChild === "number"
    ) {
      const matchedFiber = existingChildren.get(newIdx) || null;
      return updateTextNode(returnFiber, matchedFiber, "" + newChild);
    }
    if (typeof newChild === "object" && newChild !== null) {
      switch (newChild.$$typeof) {
        case REACT_ELEMENT_TYPE: {
          const matchedFiber =
            existingChildren.get(
              newChild.key === null ? newIdx : newChild.key
            ) || null;
          return updateElement(returnFiber, matchedFiber, newChild);
        }
      }
    }
  }

 function reconcileChildrenArray(returnFiber, currentFirstChild, newChildren) {
    let resultingFirstChild = null;
    let previousNewFiber = null;
    let newIdx = 0;
    let oldFiber = currentFirstChild;
    let nextOldFiber = null;
    let lastPlacedIndex = 0;
    ...
    const existingChildren = mapRemainingChildren(returnFiber, oldFiber);
    for (; newIdx < newChildren.length; newIdx++) {
      const newFiber = updateFromMap(
        existingChildren,
        returnFiber,
        newIdx,
        newChildren[newIdx]
      );
      if (newFiber !== null) {
        if (shouldTrackSideEffects) {
          if (newFiber.alternate !== null) {
            existingChildren.delete(
              newFiber.key === null ? newIdx : newFiber.key
            );
          }
        }
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          resultingFirstChild = newFiber;
        } else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
    }
    if (shouldTrackSideEffects) {
      existingChildren.forEach((child) => deleteChild(returnFiber, child));
    }
    return resultingFirstChild;
  }

domdiff

DOM操作