在左上角对齐ScrollPane的困难

Question

问题是，如何将JScrollPane与resizeble JPane对齐，使其与左上角而不是中心对齐？

这感觉就像是一项明显的任务，但我花了好几个小时尝试所有不同的布局和属性。

可以表达挫折感，因为我只能提出两种选择：

1. 使用尊重儿童最大首选尺寸的布局FlowLayout为例。副作用 - ScrollPane开始表现得像是Panel：



1. 使用可伸展布局，例如BorderLayout，并将元素放入BorderLayout.CENTER（BorderLayout.PAGE_START导致 1。）。我失去了对Panel的位置的控制权，它在中心：



当窗口很小时，滚动按预期工作：



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是否可以同时拥有这两个世界：JScrollPane不会延伸beyound最大首选大小，但不会丢失 Scroll ？

如果有人需要源代码：

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTabbedPane;

public class MainPanelTest extends JFrame {


    public static void main(String[] args)  {
        new MainPanelTest().setVisible(true);
    }

    public MainPanelTest()  {
        super();
        this.setLayout(new BorderLayout());

        setupGUI();
    }

    private void setupGUI() {
        JTabbedPane tabbedPane = new JTabbedPane();
        tabbedPane.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
        this.add(tabbedPane, BorderLayout.CENTER);

        JComponent filesPanel = setupFilesPanel();
        tabbedPane.addTab("Files", filesPanel);

        JPanel secondPanel = new JPanel();
        tabbedPane.addTab("Placeholder", secondPanel);

    }

    private JComponent setupFilesPanel() {
        JPanel filesPanel = new JPanel();
        filesPanel.setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        c.anchor = GridBagConstraints.NORTHWEST;
        for(int i=0; i<15; i++) {
            c.gridy = i;
            filesPanel.add(new JLabel("Test row " + i), c);
        }       

        JScrollPane scrollFilesPane = new JScrollPane(filesPanel);
        scrollFilesPane.setMaximumSize(scrollFilesPane.getPreferredSize());
        return scrollFilesPane;
    }

}

Answer 1

GridBagLayout将空间划分为组件所在的逻辑单元格。仅当逻辑单元格大于其组件时，anchor才有意义。如果同一行/列中的组件需要更大的大小或者存在额外空间且关联的weightx或weighty值不为零，则可能发生这种情况。由于您所需的行为与额外空间有关，因此您必须相应地设置权重值。唯一列的weightx不为零，但最后一行的weighty非零，只占用其下方的整个空间：

GridBagLayout gridBagLayout = new GridBagLayout();
JPanel filesPanel = new JPanel(gridBagLayout);
GridBagConstraints c = new GridBagConstraints();
c.gridx = GridBagConstraints.REMAINDER;
c.anchor = GridBagConstraints.NORTHWEST;

c.weightx = 1; // for the column
for(int i=0; i<15; i++)
    filesPanel.add(new JLabel("Test row " + i), c);

// now change the last row:
c.weighty = 1;
gridBagLayout.setConstraints(
  filesPanel.getComponent(filesPanel.getComponentCount()-1), c);

---

或者你可以不修改单元格，但是通过添加一个空的额外行和列来消耗整个布局，消耗额外的空间，这将有效地将原始行和列移动到左上角：

// add all visible components which should not grow
GridBagLayout gridBagLayout = new GridBagLayout();
JPanel filesPanel = new JPanel(gridBagLayout);
GridBagConstraints c = new GridBagConstraints();
c.gridx = GridBagConstraints.REMAINDER;
for(int i=0; i<15; i++)
    filesPanel.add(new JLabel("Test row " + i), c);

// add an extra row consuming vertical extra space
int nRows=filesPanel.getComponentCount();
gridBagLayout.rowHeights=new int[nRows+1];
gridBagLayout.rowWeights=new double[nRows+1];
gridBagLayout.rowWeights[nRows]=1;
// add an extra column consuming extra horizontal space
gridBagLayout.columnWidths=new int[] { 0, 0 };
gridBagLayout.columnWeights=new double[] { 0, 1 };

Answer 2

看看这是否是你想要实现的目标：

private JComponent setupFilesPanel() {
    JPanel filesPanel = new JPanel();
    filesPanel.setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();
    c.anchor = GridBagConstraints.NORTHWEST;
    for(int i=0; i<15; i++) {
        c.gridy = i;
        filesPanel.add(new JLabel("Test row " + i), c);
    }       

    // ---- Add this
    JPanel newPanel = new JPanel(new GridBagLayout());
    c.weightx = 1;
    c.weighty = 1;
    newPanel.setBackground(Color.yellow);
    newPanel.add(filesPanel, c);
    // ----

    // ---- change the panel you pass to JScrollPanel constructor:
    JScrollPane scrollFilesPane = new JScrollPane(newPanel); // <--------- newPanel
    scrollFilesPane.setMaximumSize(scrollFilesPane.getPreferredSize());
    return scrollFilesPane;
}

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