Function Handle Syntax :: Handle Graphics Objects (Graphics)

Graphics

Function Handle Syntax

In Handle Graphics, functions that you want to use as function handle callbacks must define at least two input arguments in the function definition:

The handle of the object generating the callback (the source of the event)

The event data structure (can be empty for some callbacks)

MATLAB passes these two arguments implicitly whenever the callback executes. For example, consider the following statements, which are contained in a single M-file.

function myGui
% Create a figure and specify a callback
figure('WindowButtonDownFcn',@myCallback)
            .
            .
            .
% Callback subfunction defines two input arguments
function myCallback(src,eventdata)
            .
            .
            .

The first statement creates a figure and assigns a function handle to its WindowButtondownFcn property (created by using the @ symbol before the function name). This function handle points to the subfunction myCallback. The definition of myCallback must specify the two required input arguments in its function definition line.

Passing Additional Input Arguments

You can define the callback function to accept additional input arguments by adding them to the function definition. For example,

function myCallback(src,eventdata,arg1,arg2)

When using additional arguments for the callback function, you must set the value of the property to a cell array (i.e., enclose the function handle and arguments in curly braces). For example,

figure('WindowButtonDownFcn',{@myCallback,arg1,arg2})

Defining Callbacks as a Cell Array of Strings -- Special Case

Defining a callback as a cell array of strings is a special case because MATLAB treats it differently from a simple string. Setting a callback property to a string causes MATLAB to evaluate that string in the base workspace when the callback is invoked. However, setting a callback to a cell array of strings requires the following:

The cell array must contain the name of an M-file that is on the MATLAB path as the first string element.

The M-file callback must define at least two arguments (the handle of the callback object and an empty matrix).

Any additional strings in the cell array are passed to the M-file callback as arguments.

For example,

figure('WindowButtonDownFcn',{myCallback,arg1})

requires you to define a function M-file that uses three arguments,

function myCallback(src,eventdata,arg1)

Function Handle Callbacks Why Use Function Handle Callbacks