以下是一个函数，它可以检查子文档是否更新并返回父文档中所有值的总和，并将其作为 Cloud Function 部署到 Firebase 云：

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp();

exports.calculateSum = functions.firestore.document('parentCollection/{parentId}/childCollection/{childId}').onWrite((change, context) => {
    const parentId = context.params.parentId;
    const childId = context.params.childId;

    const parentRef = admin.firestore().collection('parentCollection').doc(parentId);
    const childRef = parentRef.collection('childCollection').doc(childId);

    return childRef.get().then((childDoc) => {
        if (childDoc.exists) {
            console.log('Child document with ID', childId, 'has been updated');

            return parentRef.collection('childCollection')
                .get()
                .then((snapshot) => {
                    let sum = 0;

                    snapshot.forEach((doc) => {
                        const data = doc.data();

                        if (data.value !== undefined) {
                            sum += data.value;
                        }
                    });

                    console.log('Calculated sum:', sum);

                    return parentRef.update({
                        sumOfValues: sum
                    });
                })
        } else {
            console.log('Child document with ID', childId, 'does not exist');
            return null;
        }
    }).catch((error) => {
        console.error('Error getting child document:', error);
        return null;
    });
});

此函数监视了“parentCollection/{parentId}/childCollection/{childId}”路径下的所有写入事件。如果子文档被更新，则该函数会获取 parentCollection 集合中的所有文档，并返回所有文档中值的总和。然后，它会将总和保存在父文档中的 sumOfValues 字段中。