Vaadin DataProvider：如何避免“自动获取”？

Question

至少有两个用例：

• 除非用户提供筛选器参数，否则显示网格数据是没有意义的
• 返回@preserveonrefresh标记视图时，不应将显示的数据替换为当前数据。（更新中有进一步阐述）

作为Vaadin 14+的新手，我不知道如何实现这一点。每次显示我的GridView时，都会查询DataProvider的count和fetch回调。调用来自网格的DataCommunicator。

        at org.vaadin.example.GridView.fetch(GridView.java:46)
        at org.vaadin.example.GridView.lambda$new$c4b2c115$1(GridView.java:23)
        at com.vaadin.flow.data.provider.CallbackDataProvider.fetchFromBackEnd(CallbackDataProvider.java:137)
        at com.vaadin.flow.data.provider.AbstractBackEndDataProvider.fetch(AbstractBackEndDataProvider.java:61)
        at com.vaadin.flow.data.provider.DataCommunicator.fetchFromProvider(DataCommunicator.java:362)
        at com.vaadin.flow.data.provider.DataCommunicator.activate(DataCommunicator.java:647)
        at com.vaadin.flow.data.provider.DataCommunicator.collectKeysToFlush(DataCommunicator.java:589)
        at com.vaadin.flow.data.provider.DataCommunicator.flush(DataCommunicator.java:461)
        at com.vaadin.flow.data.provider.DataCommunicator.lambda$requestFlush$2f364bb9$1(DataCommunicator.java:425)
        at com.vaadin.flow.internal.StateTree.lambda$runExecutionsBeforeClientResponse$2(StateTree.java:390)
        at [java.util.stream] omitted
        at com.vaadin.flow.internal.StateTree.runExecutionsBeforeClientResponse(StateTree.java:387)
        at com.vaadin.flow.server.communication.UidlWriter.encodeChanges(UidlWriter.java:411)
        at com.vaadin.flow.server.communication.UidlWriter.createUidl(UidlWriter.java:187)
        at com.vaadin.flow.server.communication.UidlRequestHandler.writeUidl(UidlRequestHandler.java:122)
        at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:91)
        at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40)
        at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1547)
        at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:247)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)

@Route(value = "grid", layout = MainView.class)
public class GridView extends VerticalLayout {
    public GridView() {
        final Grid<Person> g = new Grid(Person.class);
        g.setColumns("name");
        g.setDataProvider(DataProvider.fromCallbacks(q -> fetch(q), q -> count(q)));
        add(g);
        // filter omitted
        final Button refresh = new Button("refresh");
        refresh.addClickListener(e -> {
            System.out.println("refresh clicked");
            g.getDataProvider().refreshAll();
        });
        add(refresh);

        add(new TextField("State check"));
    }

    // fake DataProvider
    private int count(Query<Person, Void> q) { return 3; }

    private Stream<Person> fetch(Query<Person, Void> q) {
        q.getLimit(); //vaadin checks these have been called
        q.getOffset(); //vaadin checks these have been called
        System.out.println("fetching again");
        new Exception().printStackTrace(); //figure out who called
        return Arrays.asList(new Person("1"), new Person("2"), new Person("3")).stream();
    }
}

我的MainView用于在GridView和EmptyView之间切换

@PreserveOnRefresh
public class MainView extends AppLayout {

    private Component emptyBView;
    private Component gridBView;

    public MainView() {
        final Button emptyB = new Button("Btn empty");
        emptyB.addClickListener(e -> {
            if (emptyBView == null) { emptyBView = new EmptyView();}
            setContent(emptyBView);
        });
        addToNavbar(emptyB);

        final Button gridB = new Button("Btn grid");
        gridB.addClickListener(e -> {
            if (gridBView == null) gridBView = new GridView();
            setContent(gridBView);
        });
        addToNavbar(gridB);
    }
}

MainView是一个applayout，用于将applayout的内容从gridview切换到emptyview。

用例2是：当返回到GridView时，GridView的状态应该与以前完全相同（这与TextField很好地配合）。

1. 打开gridview->网格不应填充数据
2. 输入筛选器参数（代码中未显示）
3. 单击“刷新”填充网格
4. 在textfield“statecheck”
中输入“蜘蛛侠”
5. 切换到emptyview
6. 在实际应用程序中：在emptyview和其他视图中执行某些操作
7. 返回gridview->网格不应该重新加载数据，它应该保持原样-就像textfield仍然显示“蜘蛛侠”一样，网格应该显示与以前相同的数据而不重新加载它。

Answer 1

对于情况1：在回调检查中，如果有筛选器参数，则返回一个空集。使用新的V17+API，其外观如下所示：

grid.setItems(query -> {
    if(filterParameters.isEmpty()) {
      // Return an empty stream
    } else {
      // Fetch from backend
    }
});

您可以在以下文档中阅读更多内容:https://vaadin.com/docs/latest/flow/binding-data/data-provider(V19)或https://vaadin.com/docs/V14/flow/binding-data/tutorial-flow-data-provider(V14)

我需要更多关于你目前正在做什么的信息来帮助解决案例2。您是如何构造视图的，您的代码是什么样子的？带有“原因”的完整堆栈跟踪也会有所帮助。