[移动开发]Android—Jetpack教程（五）

前言

在上一篇中，对Jetpack里面的Room进行了初步的使用，但每次操作数据后需要额外查询一次。在本篇中，将会对Room以及前面所学的进行一个整合。

话不多说，直接开始！

1、Room+ViewModel+LiveData

如图所示

这是官方推荐我们使用的架构，

• ViewModel和Repository交互
• Repository与对应的数据库、网络请求交互

在上一篇的基础上，对应Room数据库相关的暂且不动。

因为数据库和Repository相互关联，而Repository又与ViewModel相互关联，因此先来看看Repository究竟长啥样？

1.1 StudentRepository

class StudentRepository {

    var studentDao: StudentDao? = null

    constructor(context: Context) {
        val database = MyDatabase.getInstance(context)
        studentDao = database!!.getStudentDao()
    }

    fun insertStudent(vararg student: Student?) {
        studentDao!!.insertStudent(*student)
    }

    fun deleteStudent(vararg student: Student) {
        studentDao!!.deleteStudent(*student)
    }

    fun updateStudent(vararg student: Student) {
        studentDao!!.updateStudent(*student)
    }


    fun deleteAllStudents() {
        studentDao!!.deleteAllStudents()
    }

    fun getAllStudentsLive(): LiveData<List<Student>> {
        return studentDao!!.getAllStudentsLive()
    }

    fun queryAll(): List<Student>? {
        return studentDao!!.queryAll()
    }
}

我们可以看到，之前关于Dao的所有操作都转移至Repository，也比较简单。

不过注意的是：getAllStudentsLive()方法返回的是LiveData<List<Student>>，LiveData集合！

接下来看看ViewModel?

1.2 StudentViewModel

class StudentViewModel(application: Application) : AndroidViewModel(application) {

	//初始化对应的Repository对象
    private val repository by lazy { StudentRepository(application) }
       
	//定义对应得到LiveData集合
    private var liveDataStudent: LiveData<List<Student>>? = null
	
    init {
        liveDataStudent = repository.getAllStudentsLive()
        Log.d("StudentViewModel", "init liveDataStudent ${liveDataStudent?.value?.size}")
    }
    
    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()
        }
    }

    fun getAllStudentsLive(): LiveData<List<Student>>? {
        viewModelScope.launch(Dispatchers.Default) {
            liveDataStudent = repository.getAllStudentsLive()
        }
        return liveDataStudent
    }
    
	//以下为DataBinding布局调用的方法-------------------
    fun mInsert() {
        val s1 = Student("hqk", 26)
        val s2 = Student("Rose", 18)
        insertStudent(s1, s2)
    }
    fun mClear() {
        deleteAllStudents()
    }
    fun mDelete() {
        var s1 = Student(3)
        deleteStudent(s1)
    }
    fun mUpdate() {
        val s1 = Student(2, "Jason", 21)
        updateStudent(s1)
    }
}

我们也能看到，这里的ViewModel与repository相互关联，而对应的布局通过DataBinding又与ViewModel相互绑定！

注意这里在ViewModel里面使用了viewModelScope，因此需要添加如下依赖

    implementation "androidx.activity:activity-ktx:1.2.0"
    implementation "androidx.fragment:fragment-ktx:1.3.0"

接下来看看如何使用：

1.3 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)
//        setContentView(R.layout.activity_main)
        val binding =
            DataBindingUtil.setContentView<ActivityMainBinding>(this, R.layout.activity_main)


        viewModel = ViewModelProvider(this)[StudentViewModel::class.java]

        binding.studentViewModel = viewModel
        binding.lifecycleOwner = this

//        val recycleView = findViewById<RecyclerView>(R.id.recycleView)
        binding.recycleView.layoutManager = LinearLayoutManager(this)
        adapter = StudentRecyclerViewAdapter(listStudent)
        binding.recycleView.adapter = adapter

		//监听对应的LiveData,当数据库发生数据改变时，将会主动回调通知！
        viewModel!!.getAllStudentsLive()!!.observe(this, androidx.lifecycle.Observer {
            Log.d("StudentViewModel", "main  it ${it.size}")
            adapter!!.students = it
            adapter!!.notifyDataSetChanged()
        })

    }
}

这里很简单，就是通过ActivityMainBinding获取对应控件然后添加对应的逻辑，其次使用getAllStudentsLive方法获取到对应的LiveData，并通过observe监听LiveData集合。

Item对应的DataBinding为ItemBinding，相信经过第三篇DataBinding的讲解，读者应该轻易掌握了，这里就不贴对应代码了。

最后来看看运行效果吧

这里我们看到，每操作一次，数据自动加载，无需手动查询！

结束语

好了，本篇优化到这就结束了！在下一篇中，将会讲解Room对应的升级和预填充！