无法在页面加载之前动态添加命令按钮

Question

我可以在加载页面后动态地向页面添加命令按钮，但不能在页面加载之前添加。

在下面的代码中，单击标有添加按钮的命令按钮。视图操作失败，因为执行rootview.findcomponent时，panelGrid为null。

<f:metadata>
    <f:viewAction action="#{modStepMBean.updateClientViewAction()}"/>
</f:metadata>
<h:head>
    <title>Start Node Input</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
</h:head>
<h:body>

    <h:form id="picNumForm" styleClass="form"  prependId="false">
        <p:panelGrid id="MediaBtnGrid"   columns="7">


        </p:panelGrid>
    </h:form>

    <h:form>
        <p:commandButton value="Add Buttons" action="#{modStepMBean.updateClientViewAction()}">

        </p:commandButton>
    </h:form>

托管bean

@SessionScoped
@RolesAllowed({"Users"})
@Named
public class ModStepMBean implements Serializable {


private UIComponent buttonGrid;


private void btnCount(Integer mediaCount) {
    FacesContext ctx = FacesContext.getCurrentInstance();
    UIViewRoot rootView = ctx.getViewRoot();
    buttonGrid = (PanelGrid) rootView.findComponent("picNumForm:MediaBtnGrid");
    buttonGrid.getChildren().clear();
    for (int i = 0; i < mediaCount; i++) {
        addMediaBut(i + 1);
    }
}

private void addMediaBut(Integer num) {
    String strNum = Integer.toString(num);
    CommandButton button = new CommandButton();
    button.setValue("Media" + strNum);
    button.setId("MediaBut" + strNum);
    FacesContext facesCtx = FacesContext.getCurrentInstance();
    ELContext elContext = facesCtx.getELContext();
    Application app = facesCtx.getApplication();
    ExpressionFactory elFactory = app.getExpressionFactory();
    MethodExpression methodExpression = null;
    methodExpression = elFactory.createMethodExpression(elContext, "#{modStepMBean.selPic(" + strNum + ")}", null, new Class[]{});
    button.setActionExpression(methodExpression);
    buttonGrid.getChildren().add(button);
}


public void updateClientViewAction()
{
    btnCount(2);
       RequestContext ctx = org.primefaces.context.RequestContext.getCurrentInstance();
            ctx.update("picNumForm");

}

这种方法有误吗？如果是这样，那么正确的方法是什么？我错过了什么？感谢