Wicket：隐藏组件而不必添加所有子组件

Question

I've been doing a project in Wicket, and I often find myself in an annoying situation. Let's say I have a piece of markup that I only show when some condition applies, like the following example:

<div wicket:id="myContainer">
    <div wicket:id="label1"></div>
    <div wicket:id="label2"></div>
    <div wicket:id="label3"></div>
    <div wicket:id="label4"></div>
</div>

and in my Java code:

WebMarkupContainer myContainer = new WebMarkupContainer("myContainer");
add(myContainer);

if(myDataObject != null){
    myContainer.add(new Label("label1", myDataObject.getData1());
    myContainer.add(new Label("label2", myDataObject.getData2());
    myContainer.add(new Label("label3", myDataObject.getData3());
    myContainer.add(new Label("label4", myDataObject.getData4());
} else{
    //HAVING TO DO THIS IS ABSURD!
   myContainer.add(new Label("label1", "");
   myContainer.add(new Label("label2", "");
   myContainer.add(new Label("label3", "");
   myContainer.add(new Label("label4", "");
   myContainer.setVisible(false);
}

As you can see, I'm forced to add dummy components to the container even in the cases where I'm not gonna show it, otherwise Wicket will throw an exception, saying I have components in the markup that I haven't added in code.

To me, this is ridiculous, having to instantiate extra components that I'm not gonna show is wasteful, time-consuming and makes the code less readable unnecessarily.

I'm hoping that it's just my ignorance of Wicket and that someone can tell me a method that allows me to "discard a component and all children".

Answer 1

You could improve you code a lot, so it becomes less painful :)

This is a nice place to use the CompoundPropertyModel .

Also, just control the visibility in the onConfigure() of the container.

WebMarkupContainer myContainer = new WebMarkupContainer("myContainer", new CompoundPropertyModel(myDataObject))
{
    public void onConfigure()
    {
        setVisible( this.getModelObject() != null);
    }
}

add(myContainer);

myContainer.add(new Label("data1"));
myContainer.add(new Label("data2"));
myContainer.add(new Label("data3"));
myContainer.add(new Label("data4"));

---

Alternative

The way you use your WebMarkupContainer suggests you could also use a Panel there to build a nice display component for this "myDataObject"

Java

public class MyDataObjectPanel extends Panel
{
    public MyDataObjectPanel(String id, IModel<MyDataObject> myDataObjectModel)
    {
        super(id,  new CompoundPropertyModel(myDataObjectModel);
        this.add(new Label("data1"));
        this.add(new Label("data2"));
        this.add(new Label("data3"));
        this.add(new Label("data4"));
    }

    public void onConfigure()
    {
        setVisible( this.getModelObject() != null);
    }

}

HTML

<wicket:panel>
    <div wicket:id="data1"></div>
    <div wicket:id="data2"></div>
    <div wicket:id="data3"></div>
    <div wicket:id="data4"></div>
</wicket:panel>

Then all places where you want to display it, just use this panel.

---

I would advise against this (because it makes you code more complicated and I don't think you'll win a lot of performance), but if you really care for not adding elements to the wicket-component-tree, you could the next code. It decides whether to display the specialized component or not and replaces the actual component:

 public class MainComponent { @Override protected void onConfigure() { MyDataObject obj = getDataObjectFromSomeModel(); if (obj == null) { this.addOrReplace( new EmptyPanel("dataPanelId").setVisible(false)); } else { this.addOrReplace( new MyDataObjectPanel("dataPanelId")); } } } 

Answer 2

I don't have time to try it right now but I'm pretty sure that

} else{
  myContainer.setVisible(false);
}

should work. Wicket won't complain that children are missing if the parent component is not rendered at all.

Answer 3

Don't assume that the cost is high.

Invisible components don't execute many methods, with the most important methods being onRender() and onComponentTag(). Models don't execute getObject(). The cost visible from the point of view html generation is zero. Additional info: https://ci.apache.org/projects/wicket/guide/7.x/guide/componentLifecycle.html

My knowledge is checked against current version 7.5.0

I think Components (invisible) must exist, this is normal Wicket philosophy. Invisible Components are lightweight.

BTW I'm not sure Your myDataObject is wicket Model. I agree with recommendation in doc: use models .

Code will be cleaner without such 'if' - I prefer not using branched instantiation - but with correct model.

Avoid too much "active" code in constructor, for example don't get values, let be conditionally (lazy) fetched via Model. Catching exceptions thrown in Wicket constructors is ugly. But application can catch exceptions in rendering phase very well.

And in last words, remember, these are two policies to make component hidden, one static, the second dynamic: https://ci.apache.org/projects/wicket/guide/7.x/guide/keepControl.html#keepControl_1