问livedata和mutableLiveData有什么区别？

EN

class SampleViewModel(): ViewModel() {
    private val _sampleString = MutableLiveData<String>()
    val sampleString: LiveData<String> get() = _sampleString

    fun setSampleString(temp: String) {
        _sampleString.value  = temp
    }
}

class MainActivity: AppCompatActivtiy {
   private val viewModel: SampleViewModel by viewModels()

   //in onCreate
    viewModel.sampleString.observe(this) {
        //here you will have the value sampleString, that you can use as you want.
    }   
    
    //that is how you can update the value of MutableLiveData 
    viewModel.setSampleString("random string")

}