DiscreteKnob

Discrete knob UI component

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Description

A discrete knob UI component represents an instrument control knob and allows an app user to select an option from a discrete set in an app. Use the DiscreteKnob object to modify the appearance and behavior of a discrete knob after you create it.

Creation

Create a discrete knob in an app using the uiknob function, specifying the knob style as "discrete".

Properties

Knob

Value — Value

element of Items | element of ItemsData

Value, specified as an element of the Items or ItemsData arrays. By default, Value is the first element in Items.

Specifying Value as an element of Items sets the knob selector to the corresponding label on the knob. If ItemsData is not empty, then Value must be set to an element of ItemsData, and the knob selector will point to the associated label.

Items — Knob options

{'Off','Low','Medium','High'} (default) | cell array of character vectors | string array | ...

Knob options, specified as a cell array of character vectors, string array, or 1-D categorical array. The array must contain at least two elements. The knob displays as many options as there are elements in the Items array. The options display in clockwise order. If you specify this property as a categorical array, MATLAB^® uses the values in the array, not the full set of categories.

Example: {'Off','Slow','Fast'}

Example: {'1','2','3','4'}

ItemsData — Data associated with each element of Items

empty array [] (default) | 1-by-n numeric array | 1-by-n cell array

Data associated with each element of the Items property value, specified as a 1-by-n numeric array or a 1-by-n cell array. It is valid to specify duplicate array elements in the ItemsData value.

When the number of array elements in the ItemsData and Items do not match:

• If the ItemsData value is empty, then all Items elements are presented to the app user.

• If the ItemsData value has more elements than the Items value, then all the Items elements are presented to the app user and MATLAB ignores the extra ItemsData elements.

• If the ItemsData value has fewer elements than the Items value (but greater than none), then the only Items elements presented to the app user are those that have a corresponding ItemsData value.

Example: {'One' 'Two' 'Three'}

Example: {10 20 30 40}

ValueIndex — Index of value in items

1 (default) | positive integer

Index of the component value in the list of items or item data, specified as a positive integer.

In most cases, you can use the Value property to query and update the component value. However, the ValueIndex property can be useful when both the Items and ItemsData properties are nonempty. In this case, you can use the ValueIndex property to query the element of Items that corresponds to the current value.

fig = uifigure;
k = uiknob(fig,"discrete", ...
    "Items",["Off","Slow","Fast"], ...
    "ItemsData",[0 20 50]);
idx = k.ValueIndex;

disp(k.Items(idx) + ": " + k.Value)

Off: 0

Font

FontName — Font name

system supported font name

Font name, specified as a system supported font name. The default font depends on the specific operating system and locale.

If the specified font is not available, then MATLAB uses the best match among the fonts available on the system where the app is running.

Example: 'Arial'

FontSize — Font size

positive number

Font size, specified as a positive number. The units of measurement are pixels. The default font size depends on the specific operating system and locale.

Example: 14

FontWeight — Font weight

'normal' (default) | 'bold'

Font weight, specified as one of these values:

• 'normal' — Default weight as defined by the particular font

• 'bold' — Thicker character outlines than 'normal'

Not all fonts have a bold font weight. For fonts that do not, specifying 'bold' results in the normal font weight.

FontAngle — Font angle

'normal' (default) | 'italic'

Font angle, specified as 'normal' or 'italic'. Not all fonts have an italic font angle. For fonts that do not, specifying 'italic' results in the normal font angle.

FontColor — Font color

[0 0 0] (default) | RGB triplet | hexadecimal color code | 'r' | 'g' | 'b' | ...

Font color, specified as an RGB triplet, a hexadecimal color code, or one of the options listed in the table.

RGB triplets and hexadecimal color codes are useful for specifying custom colors.

• An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1]; for example, [0.4 0.6 0.7].

• A hexadecimal color code is a character vector or a string scalar that starts with a hash symbol (#) followed by three or six hexadecimal digits, which can range from 0 to F. The values are not case sensitive. Thus, the color codes "#FF8800", "#ff8800", "#F80", and "#f80" are equivalent.

Alternatively, you can specify some common colors by name. This table lists the named color options, the equivalent RGB triplets, and hexadecimal color codes.

Color Name	Short Name	RGB Triplet	Hexadecimal Color Code	Appearance
"red"	"r"	[1 0 0]	"#FF0000"
"green"	"g"	[0 1 0]	"#00FF00"
"blue"	"b"	[0 0 1]	"#0000FF"
"cyan"	"c"	[0 1 1]	"#00FFFF"
"magenta"	"m"	[1 0 1]	"#FF00FF"
"yellow"	"y"	[1 1 0]	"#FFFF00"
"black"	"k"	[0 0 0]	"#000000"
"white"	"w"	[1 1 1]	"#FFFFFF"

Here are the RGB triplets and hexadecimal color codes for the default colors MATLAB uses in many types of plots.

RGB Triplet	Hexadecimal Color Code	Appearance
[0 0.4470 0.7410]	"#0072BD"
[0.8500 0.3250 0.0980]	"#D95319"
[0.9290 0.6940 0.1250]	"#EDB120"
[0.4940 0.1840 0.5560]	"#7E2F8E"
[0.4660 0.6740 0.1880]	"#77AC30"
[0.3010 0.7450 0.9330]	"#4DBEEE"
[0.6350 0.0780 0.1840]	"#A2142F"

Interactivity

Visible — State of visibility

'on' (default) | on/off logical value

State of visibility, specified as 'on' or 'off', or as numeric or logical 1 (true) or 0 (false). A value of 'on' is equivalent to true, and 'off' is equivalent to false. Thus, you can use the value of this property as a logical value. The value is stored as an on/off logical value of type matlab.lang.OnOffSwitchState.

• 'on' — Display the object.

• 'off' — Hide the object without deleting it. You still can access the properties of an invisible UI component.

To make your app start faster, set the Visible property to 'off' for all UI components that do not need to appear at startup.

Enable — Operational state

'on' (default) | on/off logical value

Operational state, specified as 'on' or 'off', or as numeric or logical 1 (true) or 0 (false). A value of 'on' is equivalent to true, and 'off' is equivalent to false. Thus, you can use the value of this property as a logical value. The value is stored as an on/off logical value of type matlab.lang.OnOffSwitchState.

• If you set this property to 'on', the app user can interact with the component.

• If you set this property to 'off', the component appears dimmed, indicating that the app user cannot interact with it, and that it will not trigger a callback.

Tooltip — Tooltip

'' (default) | character vector | cell array of character vectors | string array | 1-D categorical array

Tooltip, specified as a character vector, cell array of character vectors, string array, or 1-D categorical array. Use this property to display a message when the user hovers the pointer over the component at run time. The tooltip displays even when the component is disabled. To display multiple lines of text, specify a cell array of character vectors or a string array. Each element in the array becomes a separate line of text. If you specify this property as a categorical array, MATLAB uses the values in the array, not the full set of categories.

ContextMenu — Context menu

empty GraphicsPlaceholder array (default) | ContextMenu object

Context menu, specified as a ContextMenu object created using the uicontextmenu function. Use this property to display a context menu when you right-click on a component.

Position

Position — Location and size of knob

[100 100 60 60] (default) | [left bottom width height]

Location and size of the knob, excluding the state marks and labels, specified as the vector, [left bottom width height]. This table describes each element in the vector.

Element	Description
left	Distance from the inner left edge of the parent container to the outer left edge of the knob
bottom	Distance from the inner bottom edge of the parent container to the outer bottom edge of the knob
width	Distance between the right and left outer edges of the knob, excluding tick marks and labels
height	Distance between the top and bottom outer edges of the knob, excluding tick marks and labels

All measurements are in pixel units. Due to aspect ratio constraints, you cannot change the knob height and width independently of each other. To increase the knob size, set width and height using Position(3:4) = [width height].

The Position values are relative to the drawable area of the parent container. The drawable area is the area inside the borders of the container and does not include the area occupied by decorations such as a menu bar or title.

Example: [100 200 60 60]

InnerPosition — Inner location and size of knob

[100 100 60 60] (default) | [left bottom width height]

Inner location and size of the knob, excluding state marks and state labels, specified as the vector, [left bottom width height]. Position values are relative to the parent container. All measurements are in pixel units. This property value is identical to Position for knob components.

OuterPosition — Outer location and size of knob

[73 100 127 76] (default) | [left bottom width height]

This property is read-only.

Outer location and size of the knob, including state marks and labels, returned as the vector, [left bottom width height]. Position values are relative to the parent container. All measurements are in pixel units.

Layout — Layout options

empty LayoutOptions array (default) | GridLayoutOptions object

Layout options, specified as a GridLayoutOptions object. This property specifies options for components that are children of grid layout containers. If the component is not a child of a grid layout container (for example, it is a child of a figure or panel), then this property is empty and has no effect. However, if the component is a child of a grid layout container, you can place the component in the desired row and column of the grid by setting the Row and Column properties on the GridLayoutOptions object.

For example, this code places a discrete knob in the third row and second column of its parent grid.

g = uigridlayout([4 3]);
k = uiknob(g,'discrete');
k.Layout.Row = 3;
k.Layout.Column = 2;

To make the knob span multiple rows or columns, specify the Row or Column property as a two-element vector. For example, this knob spans columns 2 through 3:

k.Layout.Column = [2 3];

Callbacks

ValueChangedFcn — Value changed callback

'' (default) | function handle | cell array | character vector

Value changed callback, specified as one of these values:

• A function handle.

• A cell array in which the first element is a function handle. Subsequent elements in the cell array are the arguments to pass to the callback function.

• A character vector containing a valid MATLAB expression (not recommended). MATLAB evaluates this expression in the base workspace.

This callback executes when the user turns the knob selector in the app. The callback does not execute if the Value property changes programmatically.

This callback can access specific information about the user’s interaction with the knob. MATLAB passes this information in a ValueChangedData object as the second argument to your callback function. In App Designer, the argument is called event. You can query the object properties using dot notation. For example, event.PreviousValue returns the previous value of the knob. The ValueChangedData object is not available to callback functions specified as character vectors.

The following table lists the properties of the ValueChangedData object.

Property	Value
Value	Value of knob after the app user’s most recent interaction
PreviousValue	Value of knob before the app user’s most recent interaction
ValueIndex	Index of knob value in items after the app user’s most recent interaction
PreviousValueIndex	Index of knob value in items before the app user’s most recent interaction with it
Source	Component that executes the callback.
EventName	'ValueChanged'

For more information about writing callbacks, see Callbacks in App Designer.

CreateFcn — Creation function

'' (default) | function handle | cell array | character vector

Object creation function, specified as one of these values:

• Function handle.

• Cell array in which the first element is a function handle. Subsequent elements in the cell array are the arguments to pass to the callback function.

• Character vector containing a valid MATLAB expression (not recommended). MATLAB evaluates this expression in the base workspace.

For more information about specifying a callback as a function handle, cell array, or character vector, see Callbacks in App Designer.

This property specifies a callback function to execute when MATLAB creates the object. MATLAB initializes all property values before executing the CreateFcn callback. If you do not specify the CreateFcn property, then MATLAB executes a default creation function.

Setting the CreateFcn property on an existing component has no effect.

If you specify this property as a function handle or cell array, you can access the object that is being created using the first argument of the callback function. Otherwise, use the gcbo function to access the object.

DeleteFcn — Deletion function

'' (default) | function handle | cell array | character vector

Object deletion function, specified as one of these values:

• Function handle.

• Cell array in which the first element is a function handle. Subsequent elements in the cell array are the arguments to pass to the callback function.

• Character vector containing a valid MATLAB expression (not recommended). MATLAB evaluates this expression in the base workspace.

For more information about specifying a callback as a function handle, cell array, or character vector, see Callbacks in App Designer.

This property specifies a callback function to execute when MATLAB deletes the object. MATLAB executes the DeleteFcn callback before destroying the properties of the object. If you do not specify the DeleteFcn property, then MATLAB executes a default deletion function.

If you specify this property as a function handle or cell array, you can access the object that is being deleted using the first argument of the callback function. Otherwise, use the gcbo function to access the object.

Callback Execution Control

Interruptible — Callback interruption

'on' (default) | on/off logical value

Callback interruption, specified as 'on' or 'off', or as numeric or logical 1 (true) or 0 (false). A value of 'on' is equivalent to true, and 'off' is equivalent to false. Thus, you can use the value of this property as a logical value. The value is stored as an on/off logical value of type matlab.lang.OnOffSwitchState.

This property determines if a running callback can be interrupted. There are two callback states to consider:

• The running callback is the currently executing callback.

• The interrupting callback is a callback that tries to interrupt the running callback.

MATLAB determines callback interruption behavior whenever it executes a command that processes the callback queue. These commands include drawnow, figure, uifigure, getframe, waitfor, and pause.

If the running callback does not contain one of these commands, then no interruption occurs. MATLAB first finishes executing the running callback, and later executes the interrupting callback.

If the running callback does contain one of these commands, then the Interruptible property of the object that owns the running callback determines if the interruption occurs:

• If the value of Interruptible is 'off', then no interruption occurs. Instead, the BusyAction property of the object that owns the interrupting callback determines if the interrupting callback is discarded or added to the callback queue.

• If the value of Interruptible is 'on', then the interruption occurs. The next time MATLAB processes the callback queue, it stops the execution of the running callback and executes the interrupting callback. After the interrupting callback completes, MATLAB then resumes executing the running callback.

Note

Callback interruption and execution behave differently in these situations:

• If the interrupting callback is a DeleteFcn, CloseRequestFcn, or SizeChangedFcn callback, then the interruption occurs regardless of the Interruptible property value.

• If the running callback is currently executing the waitfor function, then the interruption occurs regardless of the Interruptible property value.

• If the interrupting callback is owned by a Timer object, then the callback executes according to schedule regardless of the Interruptible property value.

Note

When an interruption occurs, MATLAB does not save the state of properties or the display. For example, the object returned by the gca or gcf command might change when another callback executes.

BusyAction — Callback queuing

'queue' (default) | 'cancel'

Callback queuing, specified as 'queue' or 'cancel'. The BusyAction property determines how MATLAB handles the execution of interrupting callbacks. There are two callback states to consider:

• The running callback is the currently executing callback.

• The interrupting callback is a callback that tries to interrupt the running callback.

The BusyAction property determines callback queuing behavior only when both of these conditions are met:

• The running callback contains a command that processes the callback queue, such as drawnow, figure, uifigure, getframe, waitfor, or pause.

• The value of the Interruptible property of the object that owns the running callback is 'off'.

Under these conditions, the BusyAction property of the object that owns the interrupting callback determines how MATLAB handles the interrupting callback. These are possible values of the BusyAction property:

• 'queue' — Puts the interrupting callback in a queue to be processed after the running callback finishes execution.

• 'cancel' — Does not execute the interrupting callback.

BeingDeleted — Deletion status

on/off logical value

This property is read-only.

Deletion status, returned as an on/off logical value of type matlab.lang.OnOffSwitchState.

MATLAB sets the BeingDeleted property to 'on' when the DeleteFcn callback begins execution. The BeingDeleted property remains set to 'on' until the component object no longer exists.

Check the value of the BeingDeleted property to verify that the object is not about to be deleted before querying or modifying it.

Parent/Child

Parent — Parent container

Figure object (default) | Panel object | Tab object | ButtonGroup object | GridLayout object

Parent container, specified as a Figure object created using the uifigure function, or one of its child containers: Tab, Panel, ButtonGroup, or GridLayout. If no container is specified, MATLAB calls the uifigure function to create a new Figure object that serves as the parent container.

HandleVisibility — Visibility of object handle

'on' (default) | 'callback' | 'off'

Visibility of the object handle, specified as 'on', 'callback', or 'off'.

This property controls the visibility of the object in its parent's list of children. When an object is not visible in its parent's list of children, it is not returned by functions that obtain objects by searching the object hierarchy or querying properties. These functions include get, findobj, clf, and close. Objects are valid even if they are not visible. If you can access an object, you can set and get its properties, and pass it to any function that operates on objects.

HandleVisibility Value	Description
'on'	The object is always visible.
'callback'	The object is visible from within callbacks or functions invoked by callbacks, but not from within functions invoked from the command line. This option blocks access to the object at the command-line, but allows callback functions to access it.
'off'	The object is invisible at all times. This option is useful for preventing unintended changes to the UI by another function. Set the HandleVisibility to 'off' to temporarily hide the object during the execution of that function.

Identifiers

Type — Type of graphics object

'uidiscreteknob'

This property is read-only.

Type of graphics object, returned as 'uidiscreteknob'.

Tag — Object identifier

'' (default) | character vector | string scalar

Object identifier, specified as a character vector or string scalar. You can specify a unique Tag value to serve as an identifier for an object. When you need access to the object elsewhere in your code, you can use the findobj function to search for the object based on the Tag value.

UserData — User data

[] (default) | array

User data, specified as any MATLAB array. For example, you can specify a scalar, vector, matrix, cell array, character array, table, or structure. Use this property to store arbitrary data on an object.

If you are working in App Designer, create public or private properties in the app to share data instead of using the UserData property. For more information, see Share Data Within App Designer Apps.

Examples

Set and Access Property Values

Create a discrete knob.

fig = uifigure;
kb = uiknob(fig,'discrete');

Change the knob states. Associate specific data with the knob states by configuring ItemsData. In this case, ItemsData reflects temperatures in degrees Fahrenheit.

kb.Items = {'Cold', 'Warm', 'Hot'};
kb.ItemsData = {32, 80, 212};

Get the temperature associated with the current knob value.

degrees = kb.Value

degrees =

    32

Code Response to Changed Discrete Knob Setting

Create a discrete knob that performs an action after the app user turns it. Turning the knob updates the value of an edit field to reflect the app user's choice.

Copy and paste the following code into a file named displayknobvalue.m on your MATLAB path. This code creates a window containing a discrete knob and an edit field. It specifies a ValueChangedFcn callback to update the edit field when the knob is turned.

function displayKnobValue
% Create figure window

fig = uifigure('Position',[100 100 283 275]);

% Create the text field
txt = uieditfield(fig,'text',...
    'Position', [69 82 100 22]);

% Create the knob
kb = uiknob(fig,'discrete',...
    'Position',[89 142 60 60],...
    'ValueChangedFcn',@(kb,event) knobTurned(kb,txt));
end

% Code the knob callback function
function knobTurned(knob,txt)
txt.Value = knob.Value;
end

Run displayKnobValue, and then turn the knob. When you release the mouse button, the edit field is updated to reflect the new knob value.

Version History

Introduced in R2016a

R2023b: Access index of component value in list of items

Access the index of the component value in the list of items by using the ValueIndex property.