Android-Jetpack笔记--ViewModel

         这一节了解一下ViewModel，ViewModel 类旨在以注重生命周期的方式存储和管理界面相关的数据。ViewModel 类让数据可在发生屏幕旋转等配置更改后继续留存。架构组件为界面控制器提供了 ViewModel 辅助程序类，该类负责为界面准备数据。在配置更改期间会自动保留 ViewModel对象，以便它们存储的数据立即可供下一个 activity 或 fragment 实例使用。ViewModel管理的数据为什么不会消失呢，是因为ViewModel的生命周期，如图:

例子:

 ViewModel的另一个特点就是同一个Activity的Fragment之间可以使用ViewModel实现共享数据。来看一个小栗子:

第一步:添加依赖

  implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.1.0'
  implementation 'androidx.lifecycle:lifecycle-extensions:2.1.0'

如果不想findviewById,需添加

app下gradle：

plugins {
    id 'kotlin-android-extensions'
}

项目下gradle:

classpath "org.jetbrains.kotlin:kotlin-android-extensions:1.3.61"

主要代码:

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

class SharedViewModel: ViewModel() {
    val inputNumber = MutableLiveData()
}

class InputFragment : Fragment() {

    private var sharedViewModel: SharedViewModel? =null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
       activity?.let {
           sharedViewModel = ViewModelProviders.of(it).get(SharedViewModel::class.java)
       }

        et_input?.addTextChangedListener(object : TextWatcher {
            override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
            }

            override fun afterTextChanged(p0: Editable?) {
            }

            override fun onTextChanged(txt: CharSequence?, p1: Int, p2: Int, p3: Int) {
               txt?.let {
                   var input = 0
                   if(txt.toString().isNotEmpty()) {
                       input = txt.toString().toInt()
                   }
                   sharedViewModel?.inputNumber?.postValue(input)
               }
            }

        })
    }
    companion object {
    }
}

class OutputFragment : Fragment() {

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

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_output, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
     activity?.let {
         val sharedViewModel = ViewModelProviders.of(it).get(SharedViewModel::class.java)
         observeInput(sharedViewModel)
     }
    }

    @SuppressLint("SetTextI18n")
    private fun observeInput(sharedViewModel: SharedViewModel) {
        sharedViewModel.inputNumber.observe(viewLifecycleOwner, Observer {
            it?.let {
                tv_output?.text = "2 x $it = ${2*it}"
            }
        })
    }

    companion object {
    }
}

class ViewModelActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.viewmodel_main)

        supportFragmentManager.beginTransaction().add(R.id.layout_top,InputFragment()).commit()
        supportFragmentManager.beginTransaction().add(R.id.layout_bottom,OutputFragment()).commit()

        val message = resources.getString(R.string.show_input)

        val sharedViewModel = ViewModelProviders.of(this).get(SharedViewModel::class.java)

        sharedViewModel.inputNumber.observe(this, Observer {
            it?.let {
              tv_show_input.text = "$message  $it"
            }
        })

    }
}

布局文件：
viewmodel_main.xml

fragment_input.xml

fragment_output.xml

strings.xml

    Your input is 
    Enter a number
    Result

参考:
官方文档:ViewModel

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