使用 Architecture Component 实现 MVVM 的正确姿势

class MvvmViewModel : ViewModel() {

    private val _billLiveData = MutableLiveData<Int>(0)

    val billLiveData: LiveData<Int>
        get() = _billLiveData

    fun pay() {
        _billLiveData.value = _billLiveData.value?.inc()
    }
}
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class MvvmActivity : AppCompatActivity() {
    private val viewModel by viewModels<MvvmViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_mvvm)
        btn.setOnClickListener {
            viewModel.pay()
        }
        viewModel.billLiveData.observe(this, Observer {
            Log.d("sample", "收到了 $it")
        })
    }
}
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// this 就是 LifecycleOwner，可以是 Activity 或者 Fragment
val viewModel = ViewModelProvider(this).get(MvvmViewModel::class.java) 
// 或者如果引入了 activity-ktx 或者 fragment-ktx 也可以这样用：
class MvvmActivity : AppCompatActivity() {
    private val viewModel by viewModels<MvvmViewModel>()
}
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/**
     * Returns the {@link ViewModelStore} associated with this activity
     * <p>
     * Overriding this method is no longer supported and this method will be made
     * <code>final</code> in a future version of ComponentActivity.
     *
     * @return a {@code ViewModelStore}
     * @throws IllegalStateException if called before the Activity is attached to the Application
     * instance i.e., before onCreate()
     */
    @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }
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/**
     * Returns the {@link ViewModelStore} associated with this Fragment
     * <p>
     * Overriding this method is no longer supported and this method will be made
     * <code>final</code> in a future version of Fragment.
     *
     * @return a {@code ViewModelStore}
     * @throws IllegalStateException if called before the Fragment is attached i.e., before
     * onAttach().
     */
    @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        if (mFragmentManager == null) {
            throw new IllegalStateException("Can't access ViewModels from detached fragment");
        }
        return mFragmentManager.getViewModelStore(this);
    }
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@MainThread
public inline fun <reified VM : ViewModel> ComponentActivity.viewModels(
        noinline factoryProducer: (() -> ViewModelProvider.Factory)? = null
): Lazy<VM> {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }

    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}

@MainThread
public inline fun <reified VM : ViewModel> Fragment.viewModels(
        noinline factoryProducer: (() -> ViewModelProvider.Factory)? = null
): Lazy<VM> {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }

    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}
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// 懒加载，只有在使用的时候才初始化，但是还是要保证在 Activity.onCreate() 或者
// Fragment.onAttach() 之后再使用
private val viewModel: MyViewModel by viewModels<MyViewModel>()
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object Repo {
    private val _userData = MutableLiveData<User>()
    val userData: LiveData<User> = _userData

    fun fetchData() {
        // 模拟延迟两秒返回数据
        Thread.sleep(2000)
        _userData.value = User(1234, "joe")
    }
}

data class User(val id: Int, val name: String)
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class MvvmViewModel : ViewModel() {
    // UI  层不关心 user 的 id, 只需要展示用户名即可, 这里做一次 map 转换
    val uiModel: LiveData<String> =
        Transformations.map(Repo.userData, Function { user -> user.name })

    fun fetchData() {
        Repo.fetchData()
    }
}
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class MvvmActivity : AppCompatActivity() {
    private val viewModel by viewModels<MvvmViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_mvvm)
        viewModel.uiModel.observe(this, Observer {
            Log.d("sample", it)
        })
        btn.setOnClickListener {
            viewModel.fetchData()
        }
    }
}
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object UserRepo {
    private val _userData = MutableLiveData<User>()
    val userData: LiveData<User> = _userData

    fun fetchData() {
        // 模拟延迟两秒返回数据
        Thread.sleep(2000)
        _userData.value = User(1234, "joe")
    }
}

data class User(val id: Int, val name: String)
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object BookRepo {

    fun getBooksOfUser(id: Int): LiveData<Book> {
        // 模拟延迟两秒返回数据
        Thread.sleep(2000)
        return MutableLiveData<Book>(Book("钢铁是怎样炼成的"))
    }
}

data class Book(val name: String)
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class MvvmViewModel : ViewModel() {
    // 这里监听了 user 的变化，当 user 更新之后去 BookRepo 中获取图书信息，并返回一个 LiveData
    val uiModel: LiveData<Book> =
        Transformations.switchMap(UserRepo.userData, Function { user -> BookRepo.getBooksOfUser(user.id) })

    fun fetchData() {
        UserRepo.fetchData()
    }
}
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class MvvmActivity : AppCompatActivity() {
    private val viewModel by viewModels<MvvmViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_mvvm)
        // UI 监听图书信息的变化
        viewModel.uiModel.observe(this, Observer {
            Log.d("sample", it.name)
        })
        btn.setOnClickListener {
            viewModel.fetchData()
        }
    }
}
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class MvvmViewModel : ViewModel() {

    private val local = LocalRepo.userData
    private val remote = RemoteRepo.userData

    // 共同监听本地和远程数据的变化
    private val _result = MediatorLiveData<String>().apply {
        addSource(local, Observer {
            // 本地缓存数据不为空就使用本地缓存
            if (it != null) {
                this.value = it
            }
        })
        addSource(remote, Observer {
            // 网络数据获取到了就使用网络数据
            this.value = it
        })
    }
    
    val result: LiveData<String> = _result
}
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class MvvmActivity : AppCompatActivity() {
    private val viewModel by viewModels<MvvmViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_mvvm)
        // UI 数据变化
        viewModel.result.observe(this, Observer {
            Log.d("sample", it)
        })
    }
}
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class MvvmViewModel : ViewModel() {
    
    // 当 View 对 uiModel 取消监听之后，这里 ViewModel 和 UserRepo/BookRepo 之间监听也会断开
    val uiModel: LiveData<Book> =
        Transformations.switchMap(
            UserRepo.userData,
            Function { user -> BookRepo.getBooksOfUser(user.id) })

    fun fetchData() {
        UserRepo.fetchData()
    }
}
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