ViewModel และ LiveData

ปัญหาของ Activity

ViewModel คืออะไร

การสร้าง ViewModel

// app/build.gradle:
// implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.7.0'
// implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.7.0'

import androidx.lifecycle.ViewModel

class CounterViewModel : ViewModel() {

    var count = 0
        private set

    fun increment() {
        count++
    }

    fun reset() {
        count = 0
    }
}

การรับ ViewModel ใน Activity

import androidx.activity.viewModels

class CounterActivity : AppCompatActivity() {

    private lateinit var binding: ActivityCounterBinding
    // Android manages the ViewModel lifecycle — never instantiate it yourself
    private val viewModel: CounterViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityCounterBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // This shows the count is preserved after rotation!
        binding.tvCount.text = viewModel.count.toString()

        binding.btnPlus.setOnClickListener {
            viewModel.increment()
            binding.tvCount.text = viewModel.count.toString()
        }
    }
}

LiveData คืออะไร

LiveData ใน ViewModel

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class CounterViewModel : ViewModel() {

    private val _count = MutableLiveData(0)
    val count: LiveData<Int> = _count  // read-only for the UI

    fun increment() {
        _count.value = (_count.value ?: 0) + 1
    }

    fun reset() {
        _count.value = 0
    }
}

การสังเกต LiveData ใน Activity

class CounterActivity : AppCompatActivity() {

    private val viewModel: CounterViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // ...

        // Automatically updates TextView whenever count changes
        viewModel.count.observe(this) { count ->
            binding.tvCount.text = count.toString()
        }

        binding.btnPlus.setOnClickListener { viewModel.increment() }
        binding.btnReset.setOnClickListener { viewModel.reset() }
    }
}

LiveData เทียบกับ State Flow

ViewModel ที่มีตัวสังเกตหลายตัว

ตรวจสอบความเข้าใจ

ทบทวน: ViewModel และ LiveData