在 Firebase 中制作 POJO 时，您可以使用 ServerValue.TIMESTAMP 吗? [英] When making a POJO in Firebase, can you use ServerValue.TIMESTAMP?

问题描述

当您制作要从 Firebase 序列化和反序列化到 Firebase 的普通旧 Java 对象时，有没有办法使用 ServerValue.TIMESTAMP 值?

When you're making a Plain Old Java Object that's meant to be serialized from and deserialized to Firebase, is there a way to use the ServerValue.TIMESTAMP value?

例如，假设我想要一个对象，其中一个属性是上次编辑时的属性，并且您想使用 ServerValue.TIMESTAMP 值.

For example, let's say I want to have an object where one of the properties is the last time it was edited and you'd like to use the ServerValue.TIMESTAMP value.

在 POJO 类中，您可能有:

In the POJO class, you might have:

private String timeLastChanged;

或

private Map<String, String> timeLastChanged;

在带有 String 的第一个示例中，我遇到了设置 timeLastChange = ServerValue.TIMESTAMP; 的问题，因为 ServerValue.TIMESTAMP是一张地图.

In the first example with the String, I run into the issue of setting timeLastChange = ServerValue.TIMESTAMP;, because ServerValue.TIMESTAMP is a Map.

在带有 Map<String, String> 的第二个示例中，我收到无法去抖动"错误，因为它无法将存储在数据库中的 long 正确反序列化为 Map<字符串，字符串>.有没有办法解决这个问题?

In the second example with the Map<String, String> I get a "failed to debounce" error because it can't properly deserialize the long stored in the database into a Map<String, String>. Is there any work around for this?

推荐答案

12/27/2016 更新

为 @Exclude 正如很多人提到的.

Switched out @JsonIgnore for @Exclude as many have mentioned.

我终于想出了一个灵活的解决方案来处理日期和 ServerValue.TIMESTAMP.这适用于 Ivan V、Ossama 和 puf.

I finally came up with a flexible solution for working with Dates and ServerValue.TIMESTAMP. This is working off of examples from Ivan V, Ossama, and puf.

我想不出一种方法来处理 long 和 HashMap 之间的转换，但是如果您将属性嵌套在更通用的HashMap 它可以作为单个长值(日期"、1443765561874")或作为 ServerValue.TIMESTAMP 哈希映射(日期"，{.sv"，服务器时间"}).然后当你把它拉出来的时候，它总是一个带有 ("date", "some long number") 的 HashMap.然后，您可以使用 @JsonIgnore @Exclude 注释在您的 POJO 类中创建一个辅助方法(这意味着 Firebase 将忽略它，而不是将其视为与数据库进行序列化的方法)以轻松获取返回的 HashMap 中要在您的应用中使用的 long 值.

I couldn't figure out a way to deal with the conversion between long and HashMap<String, String>, but if you nest the property in a more generic HashMap<String, Object> it can go into the database as either a single long value ("date", "1443765561874") or as the ServerValue.TIMESTAMP hash map ("date", {".sv", "servertime"}). Then when you pull it out, it will always be a HashMap with ("date", "some long number"). You can then create a helper method in your POJO class using the @JsonIgnore @Exclude annotation (meaning Firebase will ignore it and not treat it as a method for serializing to/from the database) to easily get the long value from the returned HashMap to use in your app.

POJO 类的完整示例如下:

Full example of a POJO class is below:

import com.google.firebase.database.Exclude;
import com.firebase.client.ServerValue;

import java.util.HashMap;
import java.util.Map;

public class ExampleObject {
    private String name;
    private String owner;
    private HashMap<String, Object> dateCreated;
    private HashMap<String, Object> dateLastChanged;

    /**
     * Required public constructor
     */
    public ExampleObject() {
    }

    public ExampleObject(String name, String owner, HashMap<String,Object> dateCreated) {
        this.name = name;
        this.owner = owner;
        this.dateCreated = dateCreated;

        //Date last changed will always be set to ServerValue.TIMESTAMP
        HashMap<String, Object> dateLastChangedObj = new HashMap<String, Object>();
        dateLastChangedObj.put("date", ServerValue.TIMESTAMP);
        this.dateLastChanged = dateLastChangedObj;
    }

    public String getName() {
        return name;
    }

    public String getOwner() {
        return owner;
    }

    public HashMap<String, Object> getDateLastChanged() {
        return dateLastChanged;
    }

    public HashMap<String, Object> getDateCreated() {
      //If there is a dateCreated object already, then return that
        if (dateCreated != null) {
            return dateCreated;
        }
        //Otherwise make a new object set to ServerValue.TIMESTAMP
        HashMap<String, Object> dateCreatedObj = new HashMap<String, Object>();
        dateCreatedObj.put("date", ServerValue.TIMESTAMP);
        return dateCreatedObj;
    }

// Use the method described in https://stackoverflow.com/questions/25500138/android-chat-crashes-on-datasnapshot-getvalue-for-timestamp/25512747#25512747
// to get the long values from the date object.
    @Exclude
    public long getDateLastChangedLong() {

        return (long)dateLastChanged.get("date");
    }

    @Exclude
    public long getDateCreatedLong() {
        return (long)dateCreated.get("date");
    }

}

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