Why are MutableLiveData variables declared with equals sign (=)?

Question

In Kotlin, when declaring the type of variable, a colon is used. Even when declaring LiveData, a colon is used. So why is an equals sign used for MutableLiveData? I haven't been able to figure this out. I spent about 2 hours a few days ago trying to understand why my MutableLiveData variable wasn't working only to realize I needed an equals instead of a colon.

Example:

private val _liveData = MutableLiveData<Int>()  
val liveData: LiveData<Int>

Thanks in advance!

score 4 · Answer 1 · answered Jun 24 '20 at 13:26

val liveData: LiveData<Int>

is not meaningful by itself. It also needs to be initialized. I suspect you are looking at a rather common idiom in Android programming, but missed that the next line is still part of definition of liveData:

val liveData: LiveData<Int>
  get() = _liveData

Here the reason you need : LiveData<Int> is only because otherwise Kotlin would infer the same type MutableLiveData<Int> for liveData as for _liveData, and the entire point of this idiom is to stop other classes from calling postValue/setValue on it.

score 3 · Accepted Answer · answered Jun 23 '20 at 23:09

Kotlin auto detects the type, so you do not need to specify it. These are equivalent

val foo: Int = 123
val foo = 123

However, if you have a variable that is initialized later, you must provide the type, as otherwise the compiler can't determine what type it is. For instance,

class MyClass {
    val foo: Int  // must specify type

    init {
        foo = /* compute foo */
    }
}

It has nothing to do with LiveData or MutableLiveData.

CommonsWare · Answer 3 · 2020-06-24T13:45:13.177

2

So why is an equals sign used for MutableLiveData?

Because you are creating an instance of MutableLiveData, via its constructor. You are not providing a type — the Kotlin compiler is inferring it from the type of the object you are assigning to the property.

Basically, your statement is shorthand for:

private val _liveData: MutableLiveData<Int> = MutableLiveData<Int>()

You can learn more about type inference in Google's Kotlin docs.

edited Jun 24 '20 at 13:45

answered Jun 23 '20 at 23:08

CommonsWare

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So this means that the compiler infers a type of LiveData? I understand that if I write a class, I can declare a variable of that class using the equals sign. Is LiveData a type and MutableLiveData a class? – James Futures Jun 23 '20 at 23:14
@JamesFutures: "So this means that the compiler infers a type of LiveData?" -- no, it infers a type of `MutableLiveData`. You are creating an instance of `MutableLiveData`. "Is LiveData a type and MutableLiveData a class? " -- [`LiveData` is a class](https://developer.android.com/reference/kotlin/androidx/lifecycle/LiveData). `MutableLiveData` is [a subclass of `LiveData`](https://developer.android.com/reference/kotlin/androidx/lifecycle/MutableLiveData). – CommonsWare Jun 23 '20 at 23:18
1

`: MutableLiveData`, rather. – Alexey Romanov Jun 24 '20 at 13:21

Why are MutableLiveData variables declared with equals sign (=)?

3 Answers3