观察数据类属性变化的可变实时数据 [英] Observe mutable live data of a data class's property changes

问题描述

在 Android Kotlin 基础知识，代码提到使用后备属性将 MutableLiveData 封装在 ViewModel 中，以便只有 ViewModel 本身可以更改所述可变值.与实践不同，我使用的是数据类.我想观察数据类的属性变化并在UI中显示.

In Android Kotlin Fundamentals, the code mentions of using a backing property to encapsulate the MutableLiveData in ViewModel so that only the ViewModel itself can change said mutable value. Different from the practice, I'm using a data class instead. I would like to observe the changes to the property of the data class and display it in the UI.

这是我目前所拥有的.

class CourseViewModel : ViewModel() {
    private var _lastAccessedCourse: MutableLiveData<Course>
    val lastAccessedCourse: LiveData<Course>
        get() = _lastAccessedCourse
    // Some other code...

    fun updateProgress() {
        if (_lastAccessedCourse.value != null)
            _lastAccessedCourse.value = _lastAccessedCourse.value!!.let {
                it.progress += 5
                it
            }
    }

然后，我将在我的 UI 中观察 lastAccessedCourse，如下所示.单击按钮会更新变量 progress，而我正在观察这种变化.

Then, I will observe the lastAccessedCourse in my UI like below. A button click updates the variable progress and that change is what I am observing.

binding.fragHomeLaReadButton.setOnClickListener {
    courseViewModel.updateProgress()
}

courseViewModel.lastAccessedCourse.observe(viewLifecycleOwner, Observer {
    binding.fragHomeLaProgress.progress = it.progress
})

我的数据类很简单.

data class Course(
    var name: String,
    var category: String,
    var progress: Int
)

这有效.不过，我有一个顾虑.当我尝试 updateProgress() 而不设置 _lastAccessedCourse 变量(而只是将 5 添加到进度)时，似乎更改不是观察到的.我怀疑这是因为它没有观察到数据类的属性变化.这是正确的吗?

This works. However, I have a concern. When I tried to updateProgress() without setting _lastAccessedCourse variable (and instead just adding 5 to progress), it seems like the change isn't observed. I suspect that this is because it doesn't observe the data class's property changes. Is this correct?

可以看出，每次调用 updateProgress() 时，我都会向 _lastAccessedCourse 变量返回一个新的 Course.这有性能缺陷吗?有没有更好的做法来实现我想要的?

As can be seen, I'm returning a new Course to _lastAccessedCourse variable every time I call updateProgress(). Does this have a performance drawback? Is there a better practice to achieve what I want?

推荐答案

MutableLiveData 仅在 value 更改时通知其观察者.未观察到对课程的更改(在您的情况下为 Course).

A MutableLiveData only notifies its observers if the value is changed. Changes to the class (Course in your case) are not observed.

为了最佳实践，请考虑为 Course 使用一个不可变的数据类，并每次为 LiveData 分配一个新的数据类:

For best practice, consider using an immutable data class for Course and assigning a new one to the LiveData each time:

data class Course(
    val name: String,
    val category: String,
    val progress: Int
)

val course = /* TODO: get current course */
val newCourse = course.copy(progress = course.progress + 5)
_lastAccessedCourse.value = newCourse

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