MVVM框架 todo-mvvm-databinding 项目例子。
MVP框架 todo-mvp 项目例子。
目的
分析和学习官方mvvm框架的设计模式和数据绑定在其中的具体用法,制作一套符合当前公司业务场景的mvvm框架。其中也分析一下数据源在项目中的设计以及框架中单元测试的实施。
设计模式
MVVM框架中的`ViewModel`相比MVP框架中的`Presenter`起着类似的作用。两种框架结构的不同之处在于View分别与`ViewModel`或`Presenter`进行通信:
- 当MVVM框架中数据发生变化时会影响`ViewModel`的改变,`View`会自动更新库或框架。你不能直接通过`ViewModel`来修改`View`。
- 在这个项目中,你可以使用布局文件将`ViewModel`中的变量绑定到特定的UI元素上(如`TextView`或`ImageView`)。数据绑定可以确保View和ViewModel保持双向同步,如下图所示。
项目结构
项目相对比较简单,有以下几个模块
- tasks 任务首页模块
- taskdetail 任务详情模块
- addedittask 添加任务模块
- statistics 静态计数模块
- data 数据模块
- util 工具包
源码分析
在这里不将每个模块具体剖析,用`taskdetail`作为代表模块分析mvvm框架的设计。
在TaskDetailActivity中获取Fragment和ViewModel对象
来看`TaskActivity`的`onCreate()`方法, 通过`findOrCreateViewFragment()`和`findOrCreateViewModel()`来获取Fragment和ViewModel对象。
@Override protected void onCreate(Bundle savedInstanceState) { ... TaskDetailFragment taskDetailFragment = findOrCreateViewFragment(); mTaskViewModel = findOrCreateViewModel(); mTaskViewModel.setNavigator(this); // Link View and ViewModel taskDetailFragment.setViewModel(mTaskViewModel); }
@NonNull private TaskDetailViewModel findOrCreateViewModel() { // In a configuration change we might have a ViewModel present. It's retained using the // Fragment Manager. @SuppressWarnings("unchecked") ViewModelHolder<TaskDetailViewModel> retainedViewModel = (ViewModelHolder<TaskDetailViewModel>) getSupportFragmentManager() .findFragmentByTag(TASKDETAIL_VIEWMODEL_TAG); if (retainedViewModel != null && retainedViewModel.getViewmodel() != null) { // If the model was retained, return it. return retainedViewModel.getViewmodel(); } else { // There is no ViewModel yet, create it. TaskDetailViewModel viewModel = new TaskDetailViewModel( getApplicationContext(), Injection.provideTasksRepository(getApplicationContext())); // and bind it to this Activity's lifecycle using the Fragment Manager. ActivityUtils.addFragmentToActivity( getSupportFragmentManager(), ViewModelHolder.createContainer(viewModel), TASKDETAIL_VIEWMODEL_TAG); return viewModel; } } @NonNull private TaskDetailFragment findOrCreateViewFragment() { // Get the requested task id String taskId = getIntent().getStringExtra(EXTRA_TASK_ID); TaskDetailFragment taskDetailFragment = (TaskDetailFragment) getSupportFragmentManager() .findFragmentById(R.id.contentFrame); if (taskDetailFragment == null) { taskDetailFragment = TaskDetailFragment.newInstance(taskId); ActivityUtils.addFragmentToActivity(getSupportFragmentManager(), taskDetailFragment, R.id.contentFrame); } return taskDetailFragment; }
Fragment中的数据绑定
在`TaskDetailFagment`中的`onCreateView()`方法做了数据绑定,用`setViewmodel()`将`ViewModel`类和XML布局绑定到一起。
@Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View view = inflater.inflate(R.layout.taskdetail_frag, container, false); TaskdetailFragBinding viewDataBinding = TaskdetailFragBinding.bind(view); viewDataBinding.setViewmodel(mViewModel); setHasOptionsMenu(true); return view; }
我们再来看一下布局数据绑定片段
<layout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto"> <data> <import type="android.view.View" /> <variable name="viewmodel" type="TaskDetailViewModel" /> </data> </layout>
ViewModel 实现
先来看一下子类`TaskDetailViewModel`实现,该类继承了`TaskViewModel`并提供了`deleteTask`和`startEditTask`两个对外方法供`Fragment`调用。这里的设计和MVP结构的P层是类似的,对外提供业务方法,内部实现具体业务。`mTaskDetailNavigator`接口是在`TaskDetailActivity`做具体实现的。
public class TaskDetailViewModel extends TaskViewModel { @Nullable private TaskDetailNavigator mTaskDetailNavigator; public TaskDetailViewModel(Context context, TasksRepository tasksRepository) { super(context, tasksRepository); } public void setNavigator(TaskDetailNavigator taskDetailNavigator) { mTaskDetailNavigator = taskDetailNavigator; } public void onActivityDestroyed() { // Clear references to avoid potential memory leaks. mTaskDetailNavigator = null; } /** * Can be called by the Data Binding Library or the delete menu item. */ public void deleteTask() { super.deleteTask(); if (mTaskDetailNavigator != null) { mTaskDetailNavigator.onTaskDeleted(); } } public void startEditTask() { if (mTaskDetailNavigator != null) { mTaskDetailNavigator.onStartEditTask(); } } }
再来看父类`TaskViewModel`的实现。代码比较多,主要包含三个部分。
- 属性变量定义,为了实现双向绑定所以`new`的是`ObservableField`对象
- 数据相关方法,例如`setTask()`和`getCompleted()`等这些
- 通用业务操作方法,基本的封装
public abstract class TaskViewModel extends BaseObservable implements TasksDataSource.GetTaskCallback { public final ObservableField<String> snackbarText = new ObservableField<>(); public final ObservableField<String> title = new ObservableField<>(); public final ObservableField<String> description = new ObservableField<>(); private final ObservableField<Task> mTaskObservable = new ObservableField<>(); private final TasksRepository mTasksRepository; private final Context mContext; private boolean mIsDataLoading; public TaskViewModel(Context context, TasksRepository tasksRepository) { mContext = context.getApplicationContext(); // Force use of Application Context. mTasksRepository = tasksRepository; // Exposed observables depend on the mTaskObservable observable: mTaskObservable.addOnPropertyChangedCallback(new OnPropertyChangedCallback() { @Override public void onPropertyChanged(Observable observable, int i) { Task task = mTaskObservable.get(); if (task != null) { title.set(task.getTitle()); description.set(task.getDescription()); } else { title.set(mContext.getString(R.string.no_data)); description.set(mContext.getString(R.string.no_data_description)); } } }); } public void start(String taskId) { if (taskId != null) { mIsDataLoading = true; mTasksRepository.getTask(taskId, this); } } public void setTask(Task task) { mTaskObservable.set(task); } // "completed" is two-way bound, so in order to intercept the new value, use a @Bindable // annotation and process it in the setter. @Bindable public boolean getCompleted() { Task task = mTaskObservable.get(); return task != null && task.isCompleted(); } public void setCompleted(boolean completed) { if (mIsDataLoading) { return; } Task task = mTaskObservable.get(); // Notify repository and user if (completed) { mTasksRepository.completeTask(task); snackbarText.set(mContext.getResources().getString(R.string.task_marked_complete)); } else { mTasksRepository.activateTask(task); snackbarText.set(mContext.getResources().getString(R.string.task_marked_active)); } } @Bindable public boolean isDataAvailable() { return mTaskObservable.get() != null; } @Bindable public boolean isDataLoading() { return mIsDataLoading; } // This could be an observable, but we save a call to Task.getTitleForList() if not needed. @Bindable public String getTitleForList() { if (mTaskObservable.get() == null) { return "No data"; } return mTaskObservable.get().getTitleForList(); } @Override public void onTaskLoaded(Task task) { mTaskObservable.set(task); mIsDataLoading = false; notifyChange(); // For the @Bindable properties } @Override public void onDataNotAvailable() { mTaskObservable.set(null); mIsDataLoading = false; } public void deleteTask() { if (mTaskObservable.get() != null) { mTasksRepository.deleteTask(mTaskObservable.get().getId()); } } public void onRefresh() { if (mTaskObservable.get() != null) { start(mTaskObservable.get().getId()); } } public String getSnackbarText() { return snackbarText.get(); } @Nullable protected String getTaskId() { return mTaskObservable.get().getId(); } }
总结
相对分析来看,这个项目代码比较简单没有什么难度,也有点失望。本来想看看官方项目有没有什么比较好的对`ViewModel`封装的方法,结果每个`Activity`都复制了一遍`findOrCreateViewFragment()`和`findOrCreateViewModel()`这两个方法,在`Fragment`中对数据绑定也没有做进一步的封装处理,心累...果然是基础框架教程,框架封装的事还是自己来搞吧。