7.2 Class Button - SWT: The Standard Widget Toolkit
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7.2 Class Button7.2.1 Examples
7.2.2 Button Hierarchy
7.2.3 Button Styles
7.2.4 Button Events
Buttons are simple controls that are commonly found in desktop applications. Aside from their appearance, buttons are fundamentally different from labels: Unlike labels, buttons are active. They react when the user clicks on them and issue events.
7.2.5 Text and ImagesButtons support text and images using methods with the same signatures as Label. One interesting difference between strings in Button versus Label is that Button does not support SWT.WRAP or the linefeed character. This means that more than one line of text cannot be displayed in a button. Like labels, buttons can display either text or an image but not both at the same time (see the Text or Image but Not Both box earlier in this chapter for details).
Despite the fact that images are supported, buttons that display images are generally uncommon. Tool bars are almost always used instead (see Classes ToolBar and ToolItem). 7.2.6 AlignmentYou can horizontally align the button text or the image using one of the SWT .LEFT, SWT.CENTER, or SWT.RIGHT style bits, or the setAlignment() method.
Button alignment is something that is rarely used in programs. Normally, you should not override the default alignment of a button because the default matches the standard appearance for the operating system. If you change the alignment of the buttons in your application, it may look out of place. 7.2.7 Push ButtonsPush buttons are created using the style SWT.PUSH. They are often used within a program to launch an action. For example, programs often use push buttons to allow the user to move forward in a wizard dialog, choose Yes in a message box, or to get more details in an error dialog. When pushed, buttons draw in a pressed state. The user normally releases it at once, causing it to return to an unpressed state. If the mouse is released inside a button, an SWT.Selection event is issued. Application programs use this event to launch an action. Unlike other buttons, push buttons do not maintain their selected or unselected state between SWT.Selection events. They are considered to be always unselected. The following code fragment creates a push button and sets the text to the string "Ok".
Button button = new Button(parent, SWT.PUSH);
button.setText("Ok");
7.2.8 Check ButtonsCheck buttons are created with the style SWT.CHECK. They differ from push buttons in that they maintain a boolean selection state.[6] The user toggles the selection state of a check button using the mouse or the keyboard.
The appearance of a selected check button varies between platforms. For example, on some platforms, a check button draws an x when selected, whereas on others it draws a diamond. You can set the selection state of a check button using the setSelection() method.
The following code fragment creates a check button, sets the text to the string "Overwrite when typing", then selects the button, causing it to be checked.
Button button = new Button(parent, SWT.CHECK);
button.setText("Overwrite when typing");
button.setSelection(true);
Check buttons are mainly used to represent boolean state in a program. Very often, check buttons can be found in "preference" dialogs. Preference dialogs show the user the options available in a program and those that are selected. Although check buttons issue the SWT.Selection event when the user selects them, unlike push buttons, this event is generally not used to invoke an action. In the case of the preferences dialog, the action that accompanies the change in selection is performed when the dialog is closed (or when an apply push button in the dialog is pressed).[7]
When a group of check buttons appears together, selecting one check button in the group does not affect the selection state of others. 7.2.9 Radio ButtonsRadio buttons, created using the SWT.RADIO style, are common user interface elements. Like check buttons, they maintain a boolean state between selections. However, when multiple radio buttons have the same parent, only one button can be selected at a time. When the user selects another radio button, the previous radio button loses its selection state. This is called radio behavior.[8]
The following code creates three radio buttons that allow the user to configure the word wrap state of a hypothetical text editor.
Button button1 = new Button(parent, SWT.RADIO);
button1.setText("Don't wrap");
button1.setSelection(true);
Button button2 = new Button(parent, SWT.RADIO);
button2.setText("Wrap to window");
Button button3 = new Button(parent, SWT.RADIO);
button3.setText("Wrap to ruler");
Radio buttons typically represent enumerations in a program. For example, in the code fragment above, they could be used to indicate three possible word wrap states of a text editor. Although not necessarily the case, in the source code for the editor, there would probably be an enumeration for the three wrap states and a variable to keep track of the current state. If this were the case, the following code fragment could be used to initialize the state of each radio button. button1.setSelection(wrap == WRAP_NONE); button2.setSelection(wrap == WRAP_WINDOW); button3.setSelection(wrap == WRAP_RULER); If for some reason you need radio buttons that do not have radio behavior, the SWT.NO_RADIO_GROUP style, when specified on the parent, allows you to disable it. This style is almost never used because radio buttons without radio behavior are confusing to the user. 7.2.10 Toggle ButtonsToggle buttons are push buttons that remain pressed when selected. They are created with the style SWT.TOGGLE.
Button button = new Button(parent, SWT.TOGGLE);
button.setText("Play");
button.setSelection(true);
Toggle buttons should generally be avoided. Before tool bars were invented, toggle buttons and push buttons were sometimes used to implement tool bar behavior. Programs that do this today look dated. Because toggle buttons maintain their selection, every place you might use a toggle button, a check button can be used instead. Probably the only place where a toggle button might be useful is to model a physical button that is expected to stay pressed. For example, a toggle button would look better than a check button when implementing a software DVD player. 7.2.11 Arrow ButtonsArrow buttons are buttons that draw a small, arrow-shaped icon. The arrow shape is platform-specific and is not guaranteed to match the arrow part of a scroll bar, although it is often similar. Arrow buttons are created with the style SWT.ARROW. They can be made to point in one of four directions using the constants SWT.LEFT, SWT.RIGHT, SWT.UP, and SWT.DOWN. Button button = new Button(parent, SWT.ARROW); button.setAlignment(SWT.RIGHT); Arrow buttons are uncommon. Occasionally, they are used to allow users to navigate through data one page at a time. More often, operations such as "next page" and "previous page" are presented to the user as icons on a tool bar. 7.2.12 Button EventsSWT.Selection (SelectionEvent)The SWT.Selection event (typed event SelectionEvent) is sent when the user clicks on a button with the mouse or activates it with the keyboard. The space bar or <Enter> key usually activates a button (but this is not guaranteed on all platforms). The selection event for Button contains meaningful values in only the display, widget, and type fields. The following code fragment prints "Ok Pressed" when a button is selected.
button.addListener(SWT.Selection, new Listener() {
public void handleEvent(Event event) {
System.out.println("Ok Pressed");
}
});
Sometimes programmers are confused by the fact that SWT.Selection is sent whenever the user causes the state of a button to change. The rationale should be obvious with some reflection, but the result has subtle implications for radio buttons. Using SWT.Selection with Radio ButtonsFirst of all, SWT.Selection is rarely used with radio buttons. Like check buttons, radio buttons often appear in dialogs. On most operating systems, programs do not take action until the dialog is closed. Surprisingly, when the user selects a radio button in a group of radio buttons, two SWT.Selection events are sent. The previously selected radio button receives an SWT.Selection event indicating that the selection state has been changed to false. The newly selected radio button gets an SWT.Selection event to indicate that its selection state is now true. It sometimes helps to understand why there are two SWT.Selection events if you imagine for a moment that radio buttons did not support radio behavior. Hypothetically, the first event would be generated when the user clicked to "turn off" the first radio button, and the second event would be generated when the user clicked to "turn on" the second button. If you want to perform an action when the user selects a radio button, you almost always need to get the selection before performing the action. The following code fragment uses getSelection() to ensure that the arriving message is printed once each time the user selects a radio button.
Listener listener = new Listener() {
public void handleEvent(Event event) {
Button button = (Button) event.widget;
if (!button.getSelection()) return;
System.out.println(
"Arriving " + button.getText());
}
};
Button land = new Button(shell, SWT.RADIO);
land.setText("By Land");
land.addListener(SWT.Selection, listener);
Button sea = new Button(shell, SWT.RADIO);
sea.setText("By Sea");
sea.addListener(SWT.Selection, listener);
sea.setSelection(true);
Figure 7.1 shows this code fragment running within Shell. Figure 7.1. Using SWT.Selection with two radio buttons.
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