Synchronize Data Models—Binding and Triggers - JavaFX: Developing Rich Internet Applications - page 85

Here are two nearly identical functions, the only difference being that moveTo-
Bound() is preceded with the bound keyword, whereas moveToUnBound() is not.
This subtle difference does affect how the variables cell1 and cell2 are evalu-
ated. First, a change in the value of the arguments ( r and c set to 5) causes both
functions to be re-invoked (twice) resulting in new and updated cell1 and cell2
instances. However, when the value of translate is changed, the behavior of
the bound and unbound functions diverge. The moveToUnBound() function is
unaware of any change to the translate variable and is consequently not re-
invoked, whereas moveToBound() is re-invoked because the bound function can
detect the change in translate . Here's the output of this script:
cell1: row=0, col=0
cell2: row=0, col=0
cell1: row=5, col=5
cell2: row=5, col=5
cell1: row=5, col=5
cell2: row=12, col=12
An important point regarding bound functions is that the function body is no dif-
ferent than the previously discussed bound block expression (with all of its limi-
tations). The last expression—typically the only expression—inside the function
body is the bound function's return value. Finally, bound functions may be
invoked outside the context of a bind expression. Calling a bound function in this
way is no different than calling a regular, plain old function.
Triggers
JavaFX includes a mechanism that facilitates the catching and handling of data
modification events. By adding a trigger to a variable, you associate a block of code
that will be executed every time the variable is modified. A trigger is formally intro-
duced to a variable declaration by appending a phrase starting with the keywords on
replace . Although strongly discouraged, a trigger, in its most rudimentary form,
can be used to mimic the behavior of bind . For example, the following statements:
var x : String;
var y = bind x;
can be re-written using triggers in a nearly equivalent manner:
var x : String on replace {
y = x;
}
var y : String;