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

user_13715932 0
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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!

observers mutablelivedata android-livedata kotlin android
3 Answers
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.

alexey_r_92040

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.

user_27412730

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.

user_1151450
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