viewmodel

官方地址：https://developer.android.google.cn/topic/libraries/architecture/viewmodel

B站视频：https://www.bilibili.com/video/BV1fY4y1i7BG?p=7&vd_source=c9e619eb6c2ba53337eccc49eb025732

执行add方法，+1+1+1+1，i为4，屏幕旋转i会变回0

class MainActivity : AppCompatActivity() {
    private lateinit var btnAdd:Button
    private lateinit var tvCount: TextView
    private var i = 0
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        btnAdd = findViewById(R.id.btnAdd)
        tvCount = findViewById(R.id.tvCount)
        btnAdd.setOnClickListener(View.OnClickListener {
            add()
        })
    }
    fun add(){
        i++
        tvCount.text = i.toString()
    }
}

使用viewmodel，横竖屏数据是不会变为0的

class MyViewModel :ViewModel() {

    private var testNum = MutableLiveData<Int>()

    private var i = 0

    fun getTestNum():MutableLiveData<Int>{
        return testNum
    }

    fun add(){
        i ++
        testNum.value = i
    }

}

class MainActivity : AppCompatActivity() {
    private lateinit var btnAdd:Button
    private lateinit var tvCount: TextView
    val model: MyViewModel by viewModels()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        btnAdd = findViewById(R.id.btnAdd)
        tvCount = findViewById(R.id.tvCount)
        val model: MyViewModel by viewModels()
        model.getTestNum().observe(this, Observer {
            tvCount.text = it.toString()
        })
        btnAdd.setOnClickListener(View.OnClickListener {
            add()
        })
    }
    fun add(){
        model.add()
        val testNum = model.getTestNum().value
        tvCount.text = testNum.toString()
    }
}

在 Fragment 之间共享数据

class MainActivity : AppCompatActivity() {

    private val viewModel by lazy {
        ViewModelProvider(this).get(LeftViewModel::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main2)
        val btnAdd = findViewById<Button>(R.id.btnAdd)
        btnAdd.setOnClickListener(View.OnClickListener {
            viewModel.add()
        })
    }
}

class LeftFragment : Fragment() {

    private lateinit var tvTest:TextView

    private val viewModel by lazy {
        ViewModelProvider(requireActivity()).get(LeftViewModel::class.java)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.left_fragment, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        tvTest = view.findViewById(R.id.tvTest)
        viewModel.getLiveData().observe(requireActivity()){
            tvTest.text = it.toString()
        }
    }

}

class LeftViewModel : ViewModel() {

    private var testLiveData = MutableLiveData<Int>()

    private var i = 0

    fun getLiveData(): MutableLiveData<Int> {
        return testLiveData
    }

    fun add(){
        i ++
        testLiveData.value = i
    }

}

class RightFragment : Fragment() {

    private lateinit var tvTest: TextView

    private val viewModel by lazy {
        ViewModelProvider(requireActivity()).get(LeftViewModel::class.java)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_right, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        tvTest = view.findViewById(R.id.tvTest)
        viewModel.getLiveData().observe(requireActivity()){
            tvTest.text = it.toString()
        }
    }

}