在火疮上是什么意思?

当查询的数据发生更改时,将在 QuerySnapshot . 但在那张快照里是关于发生了什么变化的信息。如果只想对新添加的文档执行某些操作,请检查该文档是否使用此 sample from the documentation :

db.collection("cities").where("state", "==", "CA")
    .onSnapshot(function(snapshot) {
        snapshot.docChanges().forEach(function(change) {
            if (change.type === "added") {
                console.log("New city: ", change.doc.data());
                // This is equivalent to child_added
            }
            if (change.type === "modified") {
                console.log("Modified city: ", change.doc.data());
                // This is equivalent to child_changed
            }
            if (change.type === "removed") {
                console.log("Removed city: ", change.doc.data());
                // This is equivalent to child_removed
            }
        });
    });

请注意,此侦听器的未修改数据是从本地缓存读取的,因此不必从服务器重新读取。