JavaFX node.lookup（）仅为用FXMLLoader加载的父元素中的某些元素返回null

Question

我从FXML加载了一个父级，将其添加到场景/舞台中的窗格中并显示，然后立即查找组件。lookup（）对某些返回null，但对其他返回non-null。在什么情况下它会这样做？

下面是加载和查找代码：

    rootUi = FXMLLoader.load(getClass().getResource("PracticeScreen.fxml"));
    // added to Parent within stage and setVisible(true)
    analysisGroup = (Pane)rootUi.lookup("#analysisGroup");       // null
    stickiesPane = (Pane)rootUi.lookup("#stickiesPane");         // null
    scoreScroll = (ScrollPane)rootUi.lookup("#scoreScrollPane"); // GOOD
    tintLayer = rootUi.lookup("#tintLayer");                     // GOOD
    scoreImageView = (ImageView)rootUi.lookup("#scoreImage");    // null
    StackPane scoreRenderStack = (StackPane)rootUi.lookup("#scoreRenderStack"); // null
    StackPane scoreScrollStack = (StackPane)rootUi.lookup("#scoreScrollStack"); // null

scoreScrollPane返回OK，但它的所有子级返回NULL。

已加载FXML：

  <?import javafx.geometry.Insets?>
  <?import javafx.scene.control.Label?>
  <?import javafx.scene.control.ScrollPane?>
  <?import javafx.scene.control.ToggleButton?>
  <?import javafx.scene.effect.DropShadow?>
  <?import javafx.scene.effect.Glow?>
  <?import javafx.scene.image.ImageView?>
  <?import javafx.scene.layout.*?>
  <?import javafx.scene.shape.Rectangle?>
  <?import javafx.scene.text.Font?>
  <BorderPane style="-fx-background-color: white;" xmlns="http://javafx.com/javafx/8.0.65" xmlns:fx="http://javafx.com/fxml/1">
     <center>
        <StackPane id="scoreScreenStack" style="-fx-background-color: blue;" BorderPane.alignment="CENTER">
           <children>
              <ScrollPane id="scoreScrollPane" fitToHeight="true" hbarPolicy="ALWAYS" vbarPolicy="NEVER">
                 <content>
                    <StackPane id="scoreScrollStack">
                       <children>
                          <StackPane id="scoreRenderStack" alignment="CENTER_LEFT" StackPane.alignment="CENTER_LEFT">
                             <children>
                                <ImageView id="scoreImage" pickOnBounds="true" preserveRatio="true" StackPane.alignment="CENTER_LEFT" />
                                <Pane id="analysisGroup" />
                                <Pane id="stickiesPane" minHeight="200.0" minWidth="200.0" />
                                <Rectangle id="playbackCursor" arcHeight="5.0" arcWidth="5.0" fill="DODGERBLUE" height="150.0" stroke="BLACK" strokeType="INSIDE" strokeWidth="0.0" visible="false" width="2.0">
                                   <effect>
                                      <DropShadow color="DODGERBLUE" />
                                   </effect>
                                </Rectangle>
                             </children>
                          </StackPane>
                       </children>
                    </StackPane>
                 </content>
              </ScrollPane>
              <BorderPane pickOnBounds="false">
                 <top>
                    <StackPane BorderPane.alignment="CENTER">
                       <children>
                          <BorderPane>
                             <left>
                                <Label id="recordingIndicator" text="Listening..." textFill="#00a7ff" BorderPane.alignment="CENTER">
                                   <font>
                                      <Font size="18.0" />
                                   </font>
                                   <effect>
                                      <Glow level="0.86" />
                                   </effect>
                                   <BorderPane.margin>
                                      <Insets />
                                   </BorderPane.margin>
                                   <padding>
                                      <Insets bottom="8.0" left="8.0" right="8.0" top="8.0" />
                                   </padding>
                                </Label>
                             </left>
                             <right>
                                <HBox BorderPane.alignment="CENTER">
                                   <children>
                                      <ToggleButton id="finalPerformance" mnemonicParsing="false" text="Final Performance" />
                                   </children>
                                   <BorderPane.margin>
                                      <Insets bottom="8.0" left="8.0" right="8.0" top="8.0" />
                                   </BorderPane.margin>
                                </HBox>
                             </right>
                          </BorderPane>
                       </children>
                    </StackPane>
                 </top>
              </BorderPane>
              <Pane id="tintLayer" opacity="0.0" pickOnBounds="false" style="-fx-background-color: #00a7ff;" visible="false" />
           </children>
        </StackPane>
     </center>
  </BorderPane>

Answer 1

在将CSS应用到场景之前，查找不能保证正常工作。这通常发生在第一次布局过程中（即场景图第一次呈现到屏幕上），在某些情况下甚至可能发生在之后。通常不推荐使用查找作为获取场景图中定义的节点的机制。

引用FXML中定义的控件的推荐方法是使用controller类。将FXML中的所有id属性更改为fx:id属性，并向根元素添加一个controller类属性：

<BorderPane style="-fx-background-color: white;" xmlns="http://javafx.com/javafx/8.0.65" xmlns:fx="http://javafx.com/fxml/1"
    fx:controller="com.my.company.PracticeScreenController">
     <center>
        <StackPane fx:id="scoreScreenStack" style="-fx-background-color: blue;" BorderPane.alignment="CENTER">
           <children>
              <ScrollPane fx:id="scoreScrollPane" fitToHeight="true" hbarPolicy="ALWAYS" vbarPolicy="NEVER">
                 <content>
                    <StackPane fx:id="scoreScrollStack">

                        <!-- etc -->

                    </StackPane>
                 </content>
              </ScrollPane>
           </children>
        </StackPane>
    </center>
</BorderPane>

现在定义一个具有相应@fxml注释字段的控制器类。您可以访问initialize()方法中的这些字段（注意，在调用构造函数时，它们尚未初始化）：

package com.my.company ;

// imports...

public class PracticeScreenController {

    @FXML
    private StackPane scoreScreenStack ;
    @FXML
    private ScrollPane scoreScrollPane ;
    @FXML
    private ScrollPane scoreScrollStack ;

    // etc...

    public void initialize() {
        // configure controls here, as needed...
    }
}

如果迫切需要从加载FXML文件的地方（而不是在控制器中）检索对FXML文件中定义的节点的引用，则可以通过fxmlloader的命名空间这样做。再次强调，这不是推荐的方法，您确实应该使用一个controller类来访问这些，但这确实提供了另一种解决方案。

使用上面的FXML中显示的相同fx:id属性，您可以执行以下操作：

FXMLLoader loader =  new FXMLLoader(getClass().getResource("PracticeScreen.fxml"));
rootUi = loader.load();

Map<String, Object> namespace = loader.getNamespace();

// added to Parent within stage and setVisible(true)
analysisGroup = (Pane)namespace.get("analysisGroup");
stickiesPane = (Pane)namespace.get("stickiesPane");
scoreScroll = (ScrollPane)namespace.get("scoreScrollPane"); 
tintLayer = (Pane)namespace.get("tintLayer");                     
scoreImageView = (ImageView)namespace.get("scoreImage");
StackPane scoreRenderStack = (StackPane)namespace.get("scoreRenderStack");
StackPane scoreScrollStack = (StackPane)namespace.get("scoreScrollStack");