Sets the callback function for a source.
The callback for a source is called from the source's dispatch function.
The exact type of func depends on the type of source; ie. you should not count on func being called with
data as its first parameter.
Typically, you won't use this function. Instead use functions specific to the type of source you are using.
| this |
the source |
| func |
a callback function |
| data |
the data to pass to callback function |
| notify |
a function to call when |