Swing Hacks

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Chapter 1, Basic JComponents

#10 Building a Drop-Down Menu Button

HACK

With the code so far, you can show the pop-up window. To close it, you

must listen for ancestor events to find out when something above the drop-

down in the component tree has changed. They all just call

hidePopup( )

to

safely turn it off:

public void ancestorAdded(AncestorEvent event){

hidePopup( );

}

public void ancestorRemoved(AncestorEvent event){

hidePopup( );

}

public void ancestorMoved(AncestorEvent event){

if (event.getSource( ) != popup) {

hidePopup( );

}

}

public void hidePopup( ) {

if(popup != null && popup.isVisible( )) {

popup.setVisible(false);

}

}

Adding a Color Selection Panel

With the DropDownComponent finished, you can finally build something with

it. For this hack, I’ve chosen a color selector. This is a small widget that lets

the user pick one of 12 standard colors without having to open up a full

color chooser. Most word processors and spreadsheets have a component

like this, so there’s no reason for Swing not to have one, too.

ColorSelectionPanel, shown in Example 1-22, is just a JPanel with a 4 × 3

grid of buttons. Each button represents one of the most common 10 colors,

plus black and white. When a color button is clicked, it will call

selectColor( ) to fire off a color selection event.

Example 1-22. A color selection panel to be used in the drop-down component

public class ColorSelectionPanel extends JPanel {

public ColorSelectionPanel( ) {

GridBagLayout gbl = new GridBagLayout( );

GridBagConstraints c = new GridBagConstraints( );

setLayout(gbl);

// reusable listener for each button

ActionListener color_listener = new ActionListener( ) {

public void actionPerformed(ActionEvent evt) {

selectColor(((JButton)evt.getSource()).getBackground( ));

}

};

Building a Drop-Down Menu Button #10

Chapter 1, Basic JComponents

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47

HACK

ColorSelectionPanel uses a custom JButton called ColorButton (shown in

Example 1-23). It has no text and a small size so that you can fit 12 of them

inside the drop-down window. The button’s background comes from the

color it represents, and the button draws its own border, so there is no need

to draw a grid.

// set up the standard 12 colors

Color[] colors = new Color[12];

colors[0] = Color.white;

colors[1] = Color.black;

colors[2] = Color.blue;

colors[3] = Color.cyan;

colors[4] = Color.gray;

colors[5] = Color.green;

colors[6] = Color.lightGray;

colors[7] = Color.magenta;

colors[8] = Color.orange;

colors[9] = Color.pink;

colors[10] = Color.red;

colors[11] = Color.yellow;

// lay out the grid

c.gridheight = 1;

c.gridwidth = 1;

c.fill = c.NONE;

c.weightx = 1.0;

c.weighty = 1.0;

for(int i=0; i<3; i++) {

for(int j=0; j<4; j++) {

c.gridx=j;

c.gridy=i;

JButton button = new ColorButton(colors[j+i*4]);

gbl.setConstraints(button,c);

add(button);

button.addActionListener(color_listener);

}

}

}

// fire off a selectedColor property event

protected Color selectedColor = Color.black;

public void selectColor(Color newColor) {

Color oldColor = selectedColor;

selectedColor = newColor;

firePropertyChange("selectedColor",oldColor, newColor);

}

}

Example 1-22. A color selection panel to be used in the drop-down component (continued)

48

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Chapter 1, Basic JComponents

#10 Building a Drop-Down Menu Button

HACK

To put the color selector together, you just need to pack the

ColorSelectionPanel and a status button into a DropDownComponent. You also

need to add a property change listener to detect when the user has selected a

new color and then hide the pop up. This is all handled by Example 1-24.

After building the

DropDownComponent and putting it in a standard JFrame,

your color selector will look like Figure 1-28.

Example 1-23. Custom JButton for color selection

public class ColorButton extends JButton {

public ColorButton(Color col) {

super( );

this.setText("");

Dimension dim = new Dimension(15,15);

this.setSize(dim);

this.setPreferredSize(dim);

this.setMinimumSize(dim);

this.setBorderPainted(true);

this.setBackground(col);

}

}

Example 1-24. Assembling a working color selection widget

public class DropDownTest extends JPanel {

public static void main(String[] args) {

final JButton status = new JButton("Color");

final JPanel panel = new ColorSelectionPanel( );

final DropDownComponent dropdown = new DropDownComponent(status,panel);

panel.addPropertyChangeListener("selectedColor",

new PropertyChangeListener( ) {

public void propertyChange(PropertyChangeEvent evt) {

dropdown.hidePopup( );

status.setBackground((Color)evt.getNewValue( ));

}

});

JFrame frame = new JFrame("Drop Down Test");

frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);

frame.getContentPane( ).setLayout(new BorderLayout( ));

frame.getContentPane( ).add("North",dropdown);

frame.getContentPane( ).add("Center",new JLabel("Drop Down Test"));

frame.pack( );

frame.setSize(300,300);

frame.show( );

}

}