榆树当前日期 [英] Elm Current Date

问题描述

在榆树中，我有一个包含 currentDate 的模型，它是一个表示日期的字符串。我用一个信号更新：

In elm I have a model that includes a currentDate, it's a string representing the date. I update it with a signal:

Signal.map (SetCurrentDate << timeToDateString) (Time.every Time.second)

SetCurrentDate 是更新模型的动作（ SetCurrentDate date - > {model | currentDate< - date} ）和 timeToDateString 将时间转换为字符串如yyyy-mm-dd。

SetCurrentDateis an action that updates the model (SetCurrentDate date -> { model | currentDate <- date}), and timeToDateString converts from time to a string like "yyyy-mm-dd".

但是，有两个问题：

• model.currentDate 只能在一秒后正确设置。所以在应用程序的开始有一秒钟，当前的日期没有正确设置。


• model.currentDate 每秒设置一次，即使它每天都在变化。

• model.currentDate is only correctly set after one second. So there's a second at the start of the app where the currentDate is not properly set.
• model.currentDate is set every second, even though it only changes every day.

有没有办法解决这个问题？所以currentDate是设置在应用程序的开始（没有一秒的延迟），而且它每天只更新一次？

Is there a way to fix this? So the currentDate is set at the start of the app (without the one second delay), and that it's only updated one a day?

额外的发现：

如果我将信号更改为 Signal.dropRepeats< | Signal.map（SetCurrentDate<< timeToDateString）（Time.every Time.second）（所以我放弃重复），信号只在当天发生变化时被触发，也不会在开始应用程序。

If I change the signal to Signal.dropRepeats <| Signal.map (SetCurrentDate << timeToDateString) (Time.every Time.second) (so I drop the repeats), the signal is only fired when the day changes, and not also once on the start of the app.

编辑：如果在Elm应用程序中有更好的方式了解currentDate，我很乐意听到。我的谷歌没有任何东西。

If there is a better way of knowing the currentDate in an Elm application, I would love to hear it. My Googling gave nothing.

谢谢！

推荐答案

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您是否使用 StartApp 或者您有自己的 foldp ？如果您正在使用 StartApp ，最好查找开始函数的代码，并内联它，以便您可以访问 foldp 。

Using Signals

Are you using StartApp or do you have your own foldp? If you are using StartApp it's best to look up the code for the start function and inline it so you have access to the foldp.

Signal.foldp 对信号的初始值不做任何事情。因此，如果您使用 Time.every Second 信号，则只能在一秒钟后获取更新，只有在使用日期转换后的一天内才能获得 Signal.dropRepeats 。您可以使用 Signal.Extra.foldp' 从第三方 signal-extra library *来修复。它需要一个功能来从输入的初始值创建 foldp 的初始状态。

Signal.foldp doesn't do anything with the initial value of a signal. So if you use the Time.every second signal, you only get your update after a second, and only after a day if you use the date conversion and Signal.dropRepeats. You can use Signal.Extra.foldp' from the 3rd party signal-extra library* to fix that. It takes a function to create the initial state of the foldp from the initial value of the input.

*全面披露：我是图书馆的作者

*Full disclosure: I'm the author of the library

在任务教程库调用 getCurrentTime 。我认为这可以满足您的需求，而不需要每秒更新一次的信号。除了 Task.sleep ，你可能会得到一些只能每天检查一次或两次的东西。

There is a very useful task in the task-tutorial library called getCurrentTime. I think that can serve your needs without needing a signal that updates every second. Along with Task.sleep, you can probably get something that only checks the time once or twice a day.

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