MAD，现代安卓开发技术：Android 领域开发方式的重大变革～（2）

<!--result_profile.xml-->
<LinearLayout ... >
    <TextView android:id="@+id/name" />
</LinearLayout>

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle) {
        super.onCreate(savedInstanceState)
        val binding = ResultProfileBinding.inflate(layoutInflater)
        setContentView(binding.root)
        binding.name.text = "Hello world"
    }
}

<layout ...>
    <data>
        <import type="android.view.View"/>
        <variable
            name="viewModel" type="com.example.splash.ViewModel" />
    </data>
    <LinearLayout ...>
        <TextView
            ...
            android:text="@{viewModel.userName}"
            android:visibility="@{viewModel.age >= 18 ? View.VISIBLE : View.GONE}"/>
    </LinearLayout>
</layout>

val binding = ResultProfileBinding.inflate(layoutInflater)
binding.viewModel = viewModel

class MyLifecycleObserver(
    private val lifecycle: Lifecycle
) : LifecycleObserver {
    ...
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun init() {
        enabled = checkStatus()
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun start() {
        if (enabled) {
            connect()
        }
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun stop() {
        if (connected) {
            disconnect()
        }
    }
}

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle) {
        ...
        MyLifecycleObserver(lifecycle).also { lifecycle.addObserver(it) }
    }
}

class MyLifecycleObserver(...) : LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun init() {
        checkStatus { result ->
            if (result) {
                enable()
            }
        }
    }
    fun enable() {
        enabled = true
        // 初始化完毕的时候确保只有在 STARTED 及以后的状态下执行连接
        if (lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            if (!connected) {
                connect()
            }
        }
    }
    ...
}

class StockLiveData(symbol: String) : LiveData<BigDecimal>() {
    private val stockManager = StockManager(symbol)
    private val listener = { price: BigDecimal ->
        // 将请求到的数据发射出去
        value = price
    }
    // 画面活动状态下才请求
    override fun onActive() {
        stockManager.requestPriceUpdates(listener)
    }
    // 非活动状态下移除请求
    override fun onInactive() {
        stockManager.removeUpdates(listener)
    }
}
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        // 注册观察
        StockLiveData("Tesla").run { observe(this@MainActivity, Observer { ... })}
    }
}

@Entity
class Movie() : BaseObservable() {
    @PrimaryKey(autoGenerate = true)
    var id = 0
    @ColumnInfo(name = "movie_name", defaultValue = "Harry Potter")
    lateinit var name: String
    ...
}

@Dao
interface MovieDao {
    @Insert
    fun insert(vararg movies: Movie?): LongArray?
    @Delete
    fun delete(movie: Movie?): Int
    @Update
    fun update(vararg movies: Movie?): Int
    @get:Query("SELECT * FROM movie")
    val allMovies: LiveData<List<Movie?>?>
}

@Database(entities = [Movie::class], version = 1)
abstract class MovieDataBase : RoomDatabase() {
    abstract fun movieDao(): MovieDao
    companion object {
        @Volatile
        private var sInstance: MovieDataBase? = null
        private const val DATA_BASE_NAME = "jetpack_movie.db"
        @JvmStatic
        fun getInstance(context: Context): MovieDataBase? {
            if (sInstance == null) {
                synchronized(MovieDataBase::class.java) {
                    if (sInstance == null) {
                        sInstance = createInstance(context)
                    }
                }
            }
            return sInstance
        }
        private fun createInstance(context: Context): MovieDataBase {
            return Room.databaseBuilder(context.applicationContext,
                    MovieDataBase::class.java, DATA_BASE_NAME).build()
        }
    }
}

class MovieViewModel(application: Application) : AndroidViewModel(application) {
    private val mediatorLiveData = MediatorLiveData<List<Movie?>?>()
    private val db: MovieDataBase?
    init {
        db = MovieDataBase.getInstance(application)
        if (db != null) {
            mediatorLiveData.addSource(db.movieDao().allMovies) { movieList ->
                if (db.databaseCreated.value != null) {
                    mediatorLiveData.postValue(movieList)
                }
            }
        };
    }
    fun getMovieList(owner: LifecycleOwner?, observer: Observer<List<Movie?>?>?) {
        if (owner != null && observer != null)
            mediatorLiveData.observe(owner, observer)
    }
}

class PersonContextModel(application: Application) : AndroidViewModel(application) {
    val personLiveData = MutableLiveData<Person>()
    val personInWork: Unit
        get() {
            val testPerson = Person(30, "Ellison")
            personLiveData.postValue(testPerson)
        }
}
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        ...
        val model = ViewModelProvider(this).get(
            PersonContextModel::class.java
        )
        model.personLiveData.observe(this, Observer { person: Person ->
            binding.name.setText(person.name)
            binding.age.setText(person.age.toString())
        })
        binding.get.setOnClickListener({ view -> model.personInWork })
    }
}

    private void setupCamera(PreviewView previewView) {
        ListenableFuture<ProcessCameraProvider> cameraProviderFuture =
                ProcessCameraProvider.getInstance(this);
        cameraProviderFuture.addListener(() -> {
            try {
                mCameraProvider = cameraProviderFuture.get();
                bindPreview(mCameraProvider, previewView);
            } catch (ExecutionException | InterruptedException e) {
                e.printStackTrace();
            }
        }, ContextCompat.getMainExecutor(this));
    }
    private void bindPreview(@NonNull ProcessCameraProvider cameraProvider,
                             PreviewView previewView) {
        mPreview = new Preview.Builder().build();
        mCamera = cameraProvider.bindToLifecycle(this,
                CameraSelector.DEFAULT_BACK_CAMERA, mPreview);
        mPreview.setSurfaceProvider(previewView.getSurfaceProvider());
    }