MVVM

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Todo-mvvm-databinding
  • Model: 内存，本地，网络数据的实现与获取
  • View: 对应Activity,Fragment,负责View的绘制，与用户交互，在xml中编写databinding对model层数据的引用绑定，ViewModelHolder<VH>作为非UI的Fragment持有ViewModel，并绑定到Activity的生命周期
  • ViewModel: databinding框架绑定ViewModel，ViewModel持有View中引用的变量observable fields，以及数据管理类TasksRepository，数据或属性发生变化时，databinding自动更新UI

  • 优缺点

    • 低耦合，ViewModel的逻辑代码可重用性，解决MVP中，View，Presenter相互持有问题，响应界面操作无需由View传递，数据变换也无需Presenter调用View实现,开发人员可以专注于业务逻辑和数据的开发（ViewModel），设计人员可以专注于页面设计。可测试,界面素来是比较难于测试的，而现在测试可以针对ViewModel来写
    • ViewModel存在Model的依赖，databinding不方便调试
      代码分析
      TasksActivity,绑定TasksFragment以及ViewModelHolder(持有ViewModel的非UI的Fragment,绑定到Activity的生命周期)

    • 获取TasksViewModel,并绑定到Activity

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      private TasksViewModel findOrCreateViewModel() {
             @SuppressWarnings("unchecked")
             ViewModelHolder<TasksViewModel> retainedViewModel =
                     (ViewModelHolder<TasksViewModel>) getSupportFragmentManager()
                             .findFragmentByTag(TASKS_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.
                 TasksViewModel viewModel = new TasksViewModel(
                         Injection.provideTasksRepository(getApplicationContext()),
                         getApplicationContext());
                 // and bind it to this Activity's lifecycle using the Fragment Manager.
                 ActivityUtils.addFragmentToActivity(
                         getSupportFragmentManager(),
                         ViewModelHolder.createContainer(viewModel),
                         TASKS_VIEWMODEL_TAG);
                 return viewModel;
             }
         }

    • 绑定Fragment和View,设置Activity的跳转

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       public interface TasksNavigator {

          void addNewTask();
      }

       mViewModel = findOrCreateViewModel();
       mViewModel.setNavigator(this);

       // Link View and ViewModel
       tasksFragment.setViewModel(mViewModel);
        
      TasksViewModel
      void setNavigator(TasksNavigator navigator) {
              mNavigator = navigator;
      }

    • 在onDestory()回收设置的跳转接口

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       @Override
       protected void onDestroy() {
           mViewModel.onActivityDestroyed();
           super.onDestroy();
       }
          
      TasksViewModel
      void onActivityDestroyed() {
           // Clear references to avoid potential memory leaks.
           mNavigator = null;
      }

    • TasksFragment,持有TasksViewModel,TasksFragBinding，TasksAdapter中持有TaskItemViewModel

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      @Override
      public void onResume() {
          super.onResume();
          mTasksViewModel.start();
      }

      @Nullable
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                  Bundle savedInstanceState) {
           mTasksFragBinding = TasksFragBinding.inflate(inflater, container, false);

           mTasksFragBinding.setView(this);

           mTasksFragBinding.setViewmodel(mTasksViewModel);

           setHasOptionsMenu(true);

           View root = mTasksFragBinding.getRoot();

           return root;
      }

    • 设置SnackBar text属性的监听回调，属性改变时显示SnackBar，在onDestory()中移除回调

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       private void setupSnackbar() {
              mSnackbarCallback = new Observable.OnPropertyChangedCallback() {
                  @Override
                  public void onPropertyChanged(Observable observable, int i) {
                      SnackbarUtils.showSnackbar(getView(), mTasksViewModel.getSnackbarText());
                  }
              };
              mTasksViewModel.snackbarText.addOnPropertyChangedCallback(mSnackbarCallback);
          }
          
      @Override
      public void onDestroy() {
           mListAdapter.onDestroy();
           if (mSnackbarCallback != null) {
                  mTasksViewModel.snackbarText.removeOnPropertyChangedCallback(mSnackbarCallback);
           }
           super.onDestroy();
       }

    • 单个任务的抽象类TaskViewModel,暴露Task的观察者，设置标题，描述，Task，以及SnackBarText

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       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();
          }
      }

    • TasksItemViewModel,持有Navigator的弱引用，在task_item.xml中,绑定点击事件android:onClick="@{() -> viewmodel.taskClicked()}"，以及Task激活状态android:checked="@={viewmodel.completed}"，标题android:text="@{viewmodel.titleForList}"

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       public class TaskItemViewModel extends TaskViewModel {

          // This navigator is s wrapped in a WeakReference to avoid leaks because it has references to an
          // activity. There's no straightforward way to clear it for each item in a list adapter.
          @Nullable
          private WeakReference<TaskItemNavigator> mNavigator;

          public TaskItemViewModel(Context context, TasksRepository tasksRepository) {
              super(context, tasksRepository);
          }

          public void setNavigator(TaskItemNavigator navigator) {
              mNavigator = new WeakReference<>(navigator);
          }

          /**
           * Called by the Data Binding library when the row is clicked.
           */
          public void taskClicked() {
              String taskId = getTaskId();
              if (taskId == null) {
                  // Click happened before task was loaded, no-op.
                  return;
              }
              if (mNavigator != null && mNavigator.get() != null) {
                  mNavigator.get().openTaskDetails(taskId);
              }
          }
      }

    • TasksViewModel,主要是任务列表的数据加载，UI更新

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      public class TasksViewModel extends BaseObservable {

          // These observable fields will update Views automatically
          public final ObservableList<Task> items = new ObservableArrayList<>();

          public final ObservableBoolean dataLoading = new ObservableBoolean(false);

          public final ObservableField<String> currentFilteringLabel = new ObservableField<>();

          public final ObservableField<String> noTasksLabel = new ObservableField<>();

          public final ObservableField<Drawable> noTaskIconRes = new ObservableField<>();

          public final ObservableBoolean tasksAddViewVisible = new ObservableBoolean();

          final ObservableField<String> snackbarText = new ObservableField<>();

          private TasksFilterType mCurrentFiltering = TasksFilterType.ALL_TASKS;

          private final TasksRepository mTasksRepository;

          private final ObservableBoolean mIsDataLoadingError = new ObservableBoolean(false);

          private Context mContext; // To avoid leaks, this must be an Application Context.

          private TasksNavigator mNavigator;

          public TasksViewModel(
                  TasksRepository repository,
                  Context context) {
              mContext = context.getApplicationContext(); // Force use of Application Context.
              mTasksRepository = repository;

              // Set initial state
              setFiltering(TasksFilterType.ALL_TASKS);
      }

    • tasks_frag.xml中绑定TasksFragment,TasksViewModel

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      <data>

             <import type="android.view.View" />

             <variable
                 name="view"
                 type="com.example.android.architecture.blueprints.todoapp.tasks.TasksFragment" />

             <variable
                 name="viewmodel"
                 type="com.example.android.architecture.blueprints.todoapp.tasks.TasksViewModel" />

         </data>

    • 自定义设置可滑动子View

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      ublic class ScrollChildSwipeRefreshLayout extends SwipeRefreshLayout {

          private View mScrollUpChild;

          public ScrollChildSwipeRefreshLayout(Context context) {
              super(context);
          }

          public ScrollChildSwipeRefreshLayout(Context context, AttributeSet attrs) {
              super(context, attrs);
          }

          @Override
          public boolean canChildScrollUp() {
              if (mScrollUpChild != null) {
                   return mScrollUpChild.canScrollVertically(-1);
      //            return ViewCompat.canScrollVertically(mScrollUpChild, -1);
              }
              return super.canChildScrollUp();
          }

          public void setScrollUpChild(View view) {
              mScrollUpChild = view;
          }
      }

    • ScrollChildSwipeRefreshLayout刷新时加载方法android:onRefresh="@{viewmodel}"，自定义setter

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      public class SwipeRefreshLayoutDataBinding {

      @BindingAdapter("android:onRefresh")
      public static void setSwipeRefreshLayoutOnRefreshListener(ScrollChildSwipeRefreshLayout view,
                  final TasksViewModel viewModel) {
              view.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
                  @Override
                  public void onRefresh() {
                      viewModel.loadTasks(true);
                  }
              });
          }
      }

    • app:items="@{viewmodel.items}"替换ListView的items

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      public class TasksListBindings {
          @SuppressWarnings("unchecked")
          @BindingAdapter("app:items")
          public static void setItems(ListView listView, List<Task> items) {
              TasksFragment.TasksAdapter adapter = (TasksFragment.TasksAdapter) listView.getAdapter();
              if (adapter != null)
              {
                  adapter.replaceData(items);
              }
          }
      }

    • TasksAapter，TasksItemBinding绑定ViewModel,ListView优化策略达到重复利用Binding，在item的snackbarText发生改变时，改变TasksViewModel的snackbarText,然后显示SnackBar的提示信息

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      public static class TasksAdapter extends BaseAdapter {

              @Nullable private TaskItemNavigator mTaskItemNavigator;

              private final TasksViewModel mTasksViewModel;

              private List<Task> mTasks;

              private TasksRepository mTasksRepository;

              public TasksAdapter(List<Task> tasks, TasksActivity taskItemNavigator,
                                  TasksRepository tasksRepository,
                                  TasksViewModel tasksViewModel) {
                  mTaskItemNavigator = taskItemNavigator;
                  mTasksRepository = tasksRepository;
                  mTasksViewModel = tasksViewModel;
                  setList(tasks);

              }

              public void onDestroy() {
                  mTaskItemNavigator = null;
              }

              public void replaceData(List<Task> tasks) {
                  setList(tasks);
              }

              @Override
              public int getCount() {
                  return mTasks != null ? mTasks.size() : 0;
              }

              @Override
              public Task getItem(int i) {
                  return mTasks.get(i);
              }

              @Override
              public long getItemId(int i) {
                  return i;
              }

              @Override
              public View getView(int i, View view, ViewGroup viewGroup) {
                  Task task = getItem(i);
                  TaskItemBinding binding;
                  if (view == null) {
                      // Inflate
                      LayoutInflater inflater = LayoutInflater.from(viewGroup.getContext());

                      // Create the binding
                      binding = TaskItemBinding.inflate(inflater, viewGroup, false);
                  } else {
                      // Recycling view
                      binding = DataBindingUtil.getBinding(view);
                  }

                  final TaskItemViewModel viewmodel = new TaskItemViewModel(
                          viewGroup.getContext().getApplicationContext(),
                          mTasksRepository
                  );

                  viewmodel.setNavigator(mTaskItemNavigator);

                  binding.setViewmodel(viewmodel);
                  // To save on PropertyChangedCallbacks, wire the item's snackbar text observable to the
                  // fragment's.
                  viewmodel.snackbarText.addOnPropertyChangedCallback(
                          new Observable.OnPropertyChangedCallback() {
                      @Override
                      public void onPropertyChanged(Observable observable, int i) {
                          mTasksViewModel.snackbarText.set(viewmodel.getSnackbarText());
                      }
                  });
                  viewmodel.setTask(task);

                  return binding.getRoot();
              }


              private void setList(List<Task> tasks) {
                  mTasks = tasks;
                  notifyDataSetChanged();
              }
          }
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