## class Fl_Button

### Class Hierarchy

    Fl_Widget
       |
       +----Fl_Button
               |
               +----Fl_Check_Button, Fl_Light_Button, Fl_Repeat_Button,
                    Fl_Return_Button, Fl_Round_Button, Fl_Toggle_Button

### Include Files

    #include <FL/Fl_Button.H>

### Description

Buttons generate callbacks when they are clicked by the user. You control exactly when and how by changing the values for `type()` and `when()`.

Buttons can also generate callbacks in response to `FL_SHORTCUT` events. The button can either have an explicit `shortcut()` value or a letter shortcut can be indicated in the `label()` with an '&' character before it. For the label shortcut it does not matter if Alt is held down, but if you have an input field in the same window, the user will have to hold down the Alt key so that the input field does not eat the event first as an `FL_KEYBOARD` event.

### Methods

Fl_Button ~Fl_Button | clear down_box | set setonly | shortcut type | value when

#### Fl_Button::Fl_Button(int x, int y, int w, int h, const char *label = 0)

The constructor creates the button using the position, size, and label.

#### Fl_Button::~Fl_Button(void)

The destructor removes the button.

#### int Fl_Button::clear()

Same as `value(0)`.

#### Fl_Boxtype Fl_Button::down_box() const
void Fl_Button::down_box(Fl_Boxtype bt)

The first form returns the current down box type, which is drawn when `value()` is non-zero.

The second form sets the down box type. The default value of 0 causes FLTK to figure out the correct matching down version of `box()` .

#### int Fl_Button::set()

Same as `value(1)`.

#### void Fl_Button::setonly()

Turns on this button and turns off all other radio buttons in the group (calling `value(1)` or `set()` does not do this).

#### ulong Fl_Button::shortcut() const
void Fl_Button::shortcut(ulong key)

The first form returns the current shortcut key for the button.

The second form sets the shortcut key to `key`. Setting this overrides the use of '&' in the `label()`. The value is a bitwise OR of a key and a set of shift flags, for example `FL_ALT | 'a'` , `FL_ALT | (FL_F + 10)`, or just `'a'`. A value of 0 disables the shortcut.

The key can be any value returned by `Fl::event_key()`, but will usually be an ASCII letter. Use a lower-case letter unless you require the shift key to be held down.

The shift flags can be any set of values accepted by `Fl::event_state()`. If the bit is on that shift key must be pushed. Meta, Alt, Ctrl, and Shift must be off if they are not in the shift flags (zero for the other bits indicates a "don't care" setting).

#### uchar Fl_Button::type() const
void Fl_Button::type(uchar t)

The first form of `type()` returns the current button type, which can be one of:

- `0`: The value is unchanged.
- `FL_TOGGLE_BUTTON`: The value is inverted.
- `FL_RADIO_BUTTON`: The value is set to 1, and all other buttons in the current group with `type() == FL_RADIO_BUTTON` are set to zero.

The second form sets the button type to `t`.

#### char Fl_Button::value() const
int Fl_Button::value(int)

The first form returns the current value (0 or 1). The second form sets the current value.

#### Fl_When Fl_Widget::when() const
void Fl_Widget::when(Fl_When w)

Controls when callbacks are done. The following values are useful, the default value is `FL_WHEN_RELEASE`:

- `0`: The callback is not done, instead changed() is turned on.
- `FL_WHEN_RELEASE`: The callback is done after the user successfully clicks the button, or when a shortcut is typed.
- `FL_WHEN_CHANGED` : The callback is done each time the value() changes (when the user pushes and releases the button, and as the mouse is dragged around in and out of the button).