Swing布局 - SpringLayout

简介

SpringLayout是弹性布局，适用于表单的展示。弹性针对的是该布局中组件的高度、宽度会随着Container的变化自动变化。
弹性距离在SpringLayout中使用javax.swing.Spring接口来表示，该接口主要提供以下方法：

• getMinimumValue : 最小值
• getPreferredValue : 偏好值
• getMaximumValue : 最大值
• getValue : 当前值

往使用了Spring布局的Container中添加Component后，需要指定元素的坐标 (x, y)，高度与宽度。与绝对布局不同的是，Spring布局指定坐标，高宽等信息不是用代表像素的int值，而是需要传递一个代表弹性距离的Spring对象。

使用方式

常用API

// 创建固定长度的Spring对象
Spring x = Spring.constant(10);
// 为Container指定使用SpringLayout布局
JPanel panel = new JPanel(new SpringLayout());
// 从container中获取SpringLayout
SpringLayout layout = (SpringLayout)panel.getLayout();
// 获取组件的Constraints对象
panel.add(new JLabel());
SpringLayout.Constraints constraints = layout.getConstraints(panel.getComponent(0));
// constraint对象常用操作
Spring width = constraints.getWidth();
constraints.setX(x);
// 两个Spring对象求和
x = Spring.sum(x, width);
// 两个Spring对象求最大值
Spring maxWidth = Spring.max(width1, width2);
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对于表单布局，jdk提供了工具类直接帮你完成Constraints 的设置。

JPanel panel = new JPanel(new SpringLayout());
String[] labels = {"用户名", "密码", "手机号"};
for (String label : labels) {
	// 文本右对齐
	JLabel label = new JLabel(label, SwingConstants.TRAILING);
	panel.add(label);
	JTextField textField = new JTextField(30);
	label.setLabelFor(textField);
    panel.add(textField);
}
// jdk提供的工具类，完成表单使用Spring布局
SpringUtilities.makeCompactGrid(dialogPanel, labels.length, 2, 10, 10, 10, 10);
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SpringUtilities类定义如下：

public class SpringUtilities {
    public static void makeCompactGrid(Container parent, int rows, int cols, int initialX, int initialY, int xPad, int yPad) {
        SpringLayout layout = (SpringLayout) parent.getLayout();
        // 设置元素的x坐标和宽度
        Spring x = Spring.constant(initialX);
        for (int c = 0; c < cols; c++) {
            // 取每一行该列的宽度，求最大值，计算得出该列的宽度
            Spring width = Spring.constant(0);
            for (int r = 0; r < rows; r++) {
                width = Spring.max(width, getConstraintsForCell(r, c, parent, cols).getWidth());
            }
            // 设置该列每个元素的x坐标和宽度
            for (int r = 0; r < rows; r++) {
                SpringLayout.Constraints constraints = getConstraintsForCell(r, c, parent, cols);
                constraints.setX(x);
                constraints.setWidth(width);
            }
            // 下一列的x坐标 = 当前x + 当前列宽 + xPad
            x = Spring.sum(x, Spring.sum(width, Spring.constant(xPad)));
        }
        // 设置元素的y坐标和高度
        Spring y = Spring.constant(initialY);
        for (int r = 0; r < rows; r++) {
            Spring height = Spring.constant(0);
            for (int c = 0; c < cols; c++) {
                height = Spring.max(height, getConstraintsForCell(r, c, parent, cols).getHeight());
            }
            for (int c = 0; c < cols; c++) {
                SpringLayout.Constraints constraints = getConstraintsForCell(r, c, parent, cols);
                constraints.setY(y);
                constraints.setHeight(height);
            }
            y = Spring.sum(y, Spring.sum(height, Spring.constant(yPad)));
        }
        // 设置父组件的约束，子组件设置的initialX和initialY相当于设置WEST、NORTH，还需要手动设置SOUTH、EAST的间距
        SpringLayout.Constraints pConstraints = layout.getConstraints(parent);
        pConstraints.setConstraint(SpringLayout.EAST, x);
        pConstraints.setConstraint(SpringLayout.SOUTH, y);
    }

    private static SpringLayout.Constraints getConstraintsForCell(int row, int col, Container parent, int cols) {
        SpringLayout layout = (SpringLayout)parent.getLayout();
        Component c = parent.getComponent(row * cols + col);
        return layout.getConstraints(c);
    }
}
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