Replacement for `AbsoluteLayout` from Vaadin 8 Framework in Vaadin 10 Flow?

问题

The AbsoluteLayout in Vaadin 8 (Framework) enables pixel-position-oriented placement of widgets within a layout. While not my first layout of choice, the AbsoluteLayout is suited to porting code from other UI-building platforms that use pixel-position-oriented layout.

Example code from the manual:

// A 400x250 pixels size layout
AbsoluteLayout layout = new AbsoluteLayout();
layout.setWidth("400px");
layout.setHeight("250px");

// A component with coordinates for its top-left corner
TextField text = new TextField("Somewhere someplace");
layout.addComponent(text, "left: 50px; top: 50px;");

I can see that the passed argument is simply CSS coding. But I am no HTML/CSS wizard, that’s why I am using Java-based Vaadin in the first place.

The migration guide for moving from Vaadin 8 (Framework) to Vaadin 10 (Flow) says in this list of components that the AbsoluteLayout from 8 is not included in 10, nor do they plan to add it in the future. But that page does offer this note about replacement for AbsoluteLayout:

Very easy to achieve the same in V10 using e.g. Div

• Can someone explain what this would mean in a Java-based Vaadin app? Perhaps an example?
• How might a person conveniently and routinely do pixel-positioning of widgets in a Vaadin 10 Flow app?

回答1:

As your request for an "Hello World" example app, I downloaded the Project Starter with Spring Starter from https://vaadin.com/start and combined Tatu's solution with your example usage code. You can find it at https://github.com/Peppe/absolute-layout-demo.

You can test it live with the following commands in terminal / command line:

https://github.com/Peppe/absolute-layout-demo.git
cd absolute-layout-demo
mvn spring-boot:run

I created a class called AbsoluteLayout, with it's entirety looking like this:

public class AbsoluteLayout extends Div {

    public AbsoluteLayout() {
        getElement().getStyle().set("position", "relative");
    }

    public void add(Component component, int top, int left) {
        add(component);
        component.getElement().getStyle().set("position", "absolute");
        component.getElement().getStyle().set("top", top + "px");
        component.getElement().getStyle().set("left", left + "px");
    }
}

Only change that I did, compared to what Tatu said, was to give the position relative to the parent layout. This makes the position of the children added to the layout relative to the layout, and not the body (or parent position relative in the DOM structure). Otherwise the component would be in top:50px, left:50px from browser corner.

Then the usage class looks like this:

@HtmlImport("styles/shared-styles.html")
@Route
public class MainView extends VerticalLayout {

    public MainView() {
        setClassName("main-layout");

        //Just to add some content on the page to test relative position
        for (int i = 0; i<5; i++){
            add(new Div(new Text("Hello")));
        }

        // A 400x250 pixels size layout
        AbsoluteLayout layout = new AbsoluteLayout();
        layout.setWidth("400px");
        layout.setHeight("250px");

        // A component with coordinates for its top-left corner
        TextField text = new TextField("Somewhere someplace");
        layout.add(text, 50, 50);
        add(layout);
    }
}

I added a few lines of text before the layout to add some rows of text, just to test out the position:relative mentioned above.

Hope this helps and gets you on the right path. As you notice, this "AbsoluteLayout" doesn't have really any code to it - it is just a div. You can do this same trick with any layout in your app if you want to place one element into a relative position.

回答2:

The simplest way in Java-based Vaadin app is to use Div as the layout and add components there.

For each component you want to position you need to apply CSS styles, there is Java API for that, i.e. component.getElement().getStyle().

It could be something like

public void setPosition(Component component, int x, int y) {
   component.getElement().getStyle().set("position","absolute");
   component.getElement().getStyle().set("top",y+"px");
   component.getElement().getStyle().set("left",x+"px");
} 

Probably you want to extend Div and the above method (that makes rudimentary AbsoluteLayout)

See also https://developer.mozilla.org/en-US/docs/Web/CSS/position

来源：https://stackoverflow.com/questions/51216311/replacement-for-absolutelayout-from-vaadin-8-framework-in-vaadin-10-flow