[移动开发]Android-Jetpack笔记--ViewModel

?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<Int>()
}

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

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <FrameLayout
        android:id="@+id/layout_top"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

    <TextView
        android:id="@+id/tv_show_input"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="You input is"
        android:textSize="18sp" />

    <FrameLayout
        android:id="@+id/layout_bottom"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="3" />

</LinearLayout>

fragment_input.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#79ce98"
    android:orientation="vertical"
    android:padding="10dp">

    <EditText
        android:id="@+id/et_input"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/input_hint"
        android:inputType="numberSigned"
        android:maxLength="4"
        android:maxLines="1" />

</LinearLayout>

fragment_output.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#81c1d4"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="10dp">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/result"
        android:textSize="18dp" />
    <TextView
        android:id="@+id/tv_output"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="16sp" />

</LinearLayout>

strings.xml

    <string name="show_input">Your input is </string>
    <string name="input_hint">Enter a number</string>
    <string name="result">Result</string>

参考:
官方文档:ViewModel

谷歌实验室