Android Jetpack之ViewModel、LiveData
Android Jetpack之LifeCycle
一、官方推荐的Jetpack架构
ViewModel是介于View（视图）和Model（数据模型）之间的中间层，能够使视图和数据分离，又能提供视图和数据之间的通信。
LiveData是一个能够在ViewModel中数据发生变化时通知页面刷新UI线程的组件库。
Room是在SQLLite上提供了一层封装。（Android采用SQLLite作为数据库存储）
二、添加依赖
implementation "androidx.activity:activity-ktx:1.2.0"
implementation "androidx.fragment:fragment-ktx:1.3.0"
下面的案例，只是简洁的代码片段，大致介绍一下，让大家了解这个架构。具体详细代码及深度原理，请大家自行查阅其他深度讲解文章。
三、创建Repository
StudentRepository
class StudentRepository {
var studentDao: StudentDao? = null
constructor(context: Context) {
val database = MyDatabase.getInstance(context)
studentDao = database!!.getStudentDao()
}
//LiveData集合
fun getAllStudentsLive(): LiveData<List<Student>> {
return studentDao!!.getAllStudentsLive()
}
fun deleteAllStudents() {
studentDao!!.deleteAllStudents()
}
fun queryAll(): List<Student>? {
return studentDao!!.queryAll()
}
fun insertStudent(vararg student: Student?) {
studentDao!!.insertStudent(*student)
}
fun deleteStudent(vararg student: Student) {
studentDao!!.deleteStudent(*student)
}
fun updateStudent(vararg student: Student) {
studentDao!!.updateStudent(*student)
}
}StudentDao
@Dao
interface StudentDao {
@Insert
fun insertStudent(vararg students: Student?)
@Delete
fun deleteStudent(vararg students: Student?)
@Update
fun updateStudent(vararg students: Student?)
@Query("SELECT * FROM student")
fun getAllStudent(): List<Student?>?
@Query("SELECT * FROM student WHERE id = :id")
fun getStudentById(id: Int): List<Student?>?
}Repository类中是关于Dao的操作，getAllStudentsLive()方法返回的是一个LiveData集合。
四、创建ViewModel
StudentViewModel
class StudentViewModel(application: Application) : AndroidViewModel(application) {
//初始化Repository对象
private val repository by lazy { StudentRepository(application) }
//定义LiveData集合
private var liveDataStudent: LiveData<List<Student>>? = null
init {
liveDataStudent = repository.getAllStudentsLive()
}
fun getAllStudentsLive(): LiveData<List<Student>>? {
viewModelScope.launch(Dispatchers.Default) {
liveDataStudent = repository.getAllStudentsLive()
}
return liveDataStudent
}
private fun insertStudent(vararg student: Student) {
viewModelScope.launch(Dispatchers.Default) {
repository.insertStudent(*student)
}
}
private fun deleteStudent(vararg student: Student) {
viewModelScope.launch(Dispatchers.Default) {
repository.deleteStudent(*student)
}
}
private fun updateStudent(vararg student: Student) {
viewModelScope.launch(Dispatchers.Default) {
repository.updateStudent(*student)
}
}
private fun deleteAllStudents() {
viewModelScope.launch(Dispatchers.Default) {
repository.deleteAllStudents()
}
}
//以下为DataBinding布局调用的方法-------------------
fun mInsert() {
val s1 = Student("zhangsan", 26)
val s2 = Student("lisi", 18)
insertStudent(s1, s2)
}
fun mClear() {
deleteAllStudents()
}
fun mDelete() {
var s1 = Student(3)
deleteStudent(s1)
}
fun mUpdate() {
val s1 = Student(2, "wanger", 21)
updateStudent(s1)
}
}初始化数据库Repository，创建一个livedata集合的属性，并提供对其操作的方法。
五、activity中使用
mainActivity
class MainActivity : AppCompatActivity() {
private var adapter: StudentRecyclerViewAdapter? = null
private var listStudent: ArrayList<Student> = ArrayList()
private var viewModel: StudentViewModel? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//databinding
val binding = DataBindingUtil.
setContentView<ActivityMainBinding>(this, R.layout.activity_main)
//viewmodel
viewModel = ViewModelProvider(this)[StudentViewModel::class.java]
binding.studentViewModel = viewModel
binding.lifecycleOwner = this
binding.recycleView.layoutManager = LinearLayoutManager(this)
adapter = StudentRecyclerViewAdapter(listStudent)
binding.recycleView.adapter = adapter
//livedata、room
viewModel!!.getAllStudentsLive()!!.
observe(this,androidx.lifecycle.Observer {
adapter!!.students = it
adapter!!.notifyDataSetChanged()
})
}
}上面案例中，通过ActivityMainBinding获取对应控件，添加对应的逻辑，其次使用getAllStudentsLive方法获取到对应的LiveData，并通过observe监听LiveData集合。
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