无法在Firebase上设置null的属性“variable” [英] Cannot set property 'variable' of null on Firebase

问题描述

我有这段代码来检查连接到互联网的Firebase。但问题表明我的 this.total_downloaded = 1 无法设置，即使我在 construct（）之前声明为 public total_downloaded = 0 。

I have this code to check either the Firebase connected to the internet or not. But the problem shows that my this.total_downloaded=1 cannot be set even I was declared before construct() as public total_downloaded=0.

为什么会这样？我不应该能够轻松使用我的功能或设置变量吗？

Why does it happen like that? Shouldn't I be able to use my function or set the variable easily?

有人能帮帮我吗？谢谢。这是我的代码：

Can someone help me? Thanks. Here's my code:

public this.total_downloaded = 0;
//...
var connectedRef = firebase.database().ref('/.info/connected');
connectedRef.once("value", function(snap) {
    if (snap.val() === true) {
        firebase.database()
            .ref("last_update")
            .once('value', (snap_0) => {
                if (data.rows.item(0).value != snap_0.val().value) {
                    this.update_hok_baghu(db);

                    var query_insert_last_update = "UPDATE last_update SET value =" + snap_0.val().value + "";
                    db.executeSql(query_insert_last_update, []).then(() => {
                    }, (error) => {
                        console.log("ERROR on update to last_update: " + JSON.stringify(error));
                    });

                } else {
                    this.total_downloaded = 1;
                }
            });
    } else {
        this.total_downloaded = 1;
    }
});

推荐答案

您正在使用常规函数作为回调功能（SNAP）。
所以 else 条件中的 this 指的是回调函数，而不是你的类。

You are using the regular function as callback function(snap). So the this in your else condition refers to the callback function and not your class.

使用箭头功能：

connectedRef.once("value", (snap)=> {
    if (snap.val() === true) {
        firebase.database()
            .ref("last_update")
            .once('value', (snap_0) => {
                if (data.rows.item(0).value != snap_0.val().value) {
                    this.update_hok_baghu(db);

                    var query_insert_last_update = "UPDATE last_update SET value =" + snap_0.val().value + "";
                    db.executeSql(query_insert_last_update, []).then(() => {
                    }, (error) => {
                        console.log("ERROR on update to last_update: " + JSON.stringify(error));
                    });

                } else {
                    this.total_downloaded = 1;
                }
            });
    } else {
        this.total_downloaded = 1;//this will refer to class now.
    }
});

这篇关于无法在Firebase上设置null的属性“variable”的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！