One of the most useful feature in JavaFX is its Data Binding and Triggers functions. According to the official tutorial, Data binding is the ability to create an immediate and direct relationship between tow variables, This allow two or more variables can automatically change when the other or others change. At the same time, A Replace Trigger is a block of code that is attached to a variable, which when the variable changes, the code is automatically executed. So let's begin here.
Here is an example :
var x = 0;
def y = bind x;
x=1;
println(y); //y now equals 1
x = 333;
println(y); // y now is 333;
That is a extreme sample example to show the use of 'bind' keyword, You can know in that example, y is binded to x, and its value is changed with x, that's the basic use of bind. Note that y is declared as a def because we don't want any thing change y by assigning a value to it.
An other example:
var myStreet= " No.2 Phoenix Garden"
var myCity = "Xiamen"
def address = bind Address {
street : myStreet;
city : myCity;
};
println({address.street});
myStreet = "No.33 NoWhere"
println({address.street});
In this example the result of execute is :
No.2 Phoenix Garden
No.33 NoWhere
So, the conclusion is we can bind a variable with the value of a bound expression, which can be a basic type, an object, the outcome of a function, or the outcome of an expression. But in this example we must note that the changing of myStreet actually causes a new Address object to be created and then re-assigned to the address variable. If you don't want this happen, you can change the definition like this:
def address = bind Address{
street: bind myStreet;
city : myCity;
}
By this, you can change street without create a new object, and the bind in def line can be omited.
In next step, let us look at Binding and Functions.
Here is an example :
var scale = 1.0;
bound function makePoint(xPos : Number, yPos : Nubmer) : Point {
Point {
x: xPos * scale;
y: yPos * scale;
}
}
class Point {
var x : Number;
var y : Number;
}
var myX = 5.0;
var myY = 9.0;
def pt = bind makePoint(myX, myY);
println(pt.x);
myX *=2;
println(pt.x);
scale = 2.0;
println(pt.x);
At the head of this code, we announce a bound function by 'bound' keyword, and then we use our own myX and myY number to create a Point by that bound function, note we use bind keyword before the invocation of makePoint function, so pt binded to the outcome of makePoint function.
The result of executing is :
5.0
10.0
20.0
But if we remove the bound keyword from makePoint function(it became a non-bound function), the result would be:
5.0
10.0
10.0
The different between these two situation is a bound function will get re-invoked when every variable related to the function changed while the non-bound function only do that when one of their arguments change. And we MUST NOTE that bound function and bind keyword is work together.
In next step, let us look at Binding with Sequences:
Here is an example:
var q1 = [1..10];
def q2 = bind for (item in q1) item *8;
printSeqs();
insert 11 into q1;
function printSeqs() {
println("First Sequence:");
for( i in q1) { println(i) ; }
println("Second Sequence:");
for( i in q2) { println(i) ; }
}
You can guessed the result is first sequence is 1 to 10 with step of 1 and the second sequence is 8 to 80 with step of 8.
In this example, we bind two sequence by placing the bind before the for. For more understanding, we can try to change one or some items of q1, you can find the corresponding items q2 will automatically changed and others will not be affect, like we do in the example we insert a item 11 into the end of q1 and q2 automatically generated a item 88 at the end of itself.
Finally, let us look at the Replace Triggers.
It is arbitrary blocks of code that attach to variables and execute whenever the variable's value changes. Here is an example:
var test = "test" on replace oldValue {
println(" value has changed, old value is { oldValue}");
}
test = "no";
The trigger will fires two times, first is the initialized to "test" and the next is fires When the code "test= "no"; " executed, the program will report value has changed. NOTE that the oldValue store the value of test before the trigger was invoked.
想像一下，我剛才說了一句話，那句話是：「想像一下，我剛才說了一句話，那句話是：「想像一下，我剛才說了一句話，那句話是：……….」」，如此下去，就好像站在兩面平行擺設的鏡子中間，鏡子中的影像不斷的重複。再舉個例子，寫完一封信想要匿名保密，就署名「知名不具」。回信的人寫：「知 知名不具 具」。之後再回信的時候就變成：知知知名不具具具，加上括號可能比較清楚：(知(知(知名不具)具)具)。
1883年，一位法國的數學家 Edouard Lucas 教授在歐洲的一份雜誌上介紹了一個相當吸引人的難題──迷人的智力遊戲。這個遊戲名為河內塔 (Tower of Hanoi)，它源自古印度神廟中的一段故事 (也有一說是 Lucas 教授為增加此遊戲之神秘色彩而捏造的)。傳說在古老的印度，有一座神廟，據說它是宇宙的中心。在廟宇中放置了一塊上面插有三根長木釘的木板，在其中的一根木釘上，從上至下被放置了64片直徑由小至大的圓環形金屬片。古印度教的天神指示祂的僧侶們將64片的金屬片移至三根木釘中的其中一根上。規定在每次的移動中，只能搬移一片金屬片，並且在過程中必須保持金屬片由上至下是直徑由小至大的次序，也就是說不論在那一根木釘上，圓環形的金屬片都是直徑較小的被放在上層。直到有一天，僧侶們能將64片的金屬片依規則從指定的木釘上全部移動至另一根木釘上，那麼，世界末日即隨之來到，世間的一切終將被毀滅，萬物都將至極樂世界。
