Another 'Cannot mutate bound sequence' Question

Question

Hi Everyone,
The below code throws a "Cannot mutate bound sequence" when the line (id:'a') is pressed first. However it runs fine when lines 'b' or 'c' are pressed first. But the output shows the correct line 'a' has been deleted when pressed first. Can anyone suggest a way around this or shed some light on what is happening?
Im still new to javafx and have been reading up on 'bind'. It looks like there are a few known issues out there so hopefully this is one and there is a easy fix.
package bindingerror;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.Group;
import javafx.scene.shape.Line;
class Wall extends Line{
     override var onMousePressed = function(e):Void {
            println(WallArray);
            delete e.node as Wall from WallArray;
            println(WallArray);
var WallArray = [
                Wall{id:'a' startX: 10 startY: 20 endX: 90 endY: 20 strokeWidth:10}
                Wall{id:'b' startX: 10 startY: 50 endX: 90 endY: 50 strokeWidth:10}
                Wall{id:'c' startX: 10 startY: 80 endX: 90 endY: 80 strokeWidth:10}
var Group = Group{content: bind for(x in [0..(sizeof WallArray - 1)]) {WallArray[x]}};
Stage {
        scene: Scene {
                   width:   100
                   height:  100
                   content: Group{content: bind Group
                                 }//Group
                      }//Scene
}//Stage

Answer 1

No wonder. Has you have seen yourself, binding is a delicate domain, particularly when used with a sequence and/or a function. That's why I won't answer your question but instead criticize your code... :-) No, seriously, I will try and answer (but the inconsistent behavior is still a bit puzzling), if I can, but since you say you are new to JavaFX, I think some remarks can be beneficial as well.
Generic remark: I am not known to follow blindly the Sun Java coding rules (I have my own internal set of rules I apply across languages, when possible), but I still think using a capital initial for class names, and a lowercase initial for variable names is a useful convention. Particularly when you name a variable the same as a built in class name... The compiler sorts things out, but the reader is confused... :-)
Other remark: I don't know if you come from the Java world. Knowing Java to do JavaFX isn't mandatory, but useful, since JavaFX routinely uses Java classes. I suppose most JavaFX coders have at least some Java background.
That said, I am surprised to see so many newbies to JavaFX using the old Java way of iterating over a sequence, by index.
Let's show a more idiomatic JavaFX. First, you can rewrite your loop as:
for (x in [0 ..< sizeof WallArray]) WallArray[x];The ..< sequence notation is often overlooked/forgotten, I find it more readable than using -1.
It can still be written:
for (w in WallArray) w;But you create a new sequence, why not just use the original one?
var Group = Group { content: bind WallArray };Ah, I magically dispelled the error message! But I modify WallArray, I don't know if that's the intended behavior. Ah, yes, since the mouse pressed function delete items from WallArray. The problem might lie there: I think the Group reference items in WallArray, and you delete items there, so its references hangs out in the void. Or something like that...
Last remark: unless you are simulating something more complex, you don't need to put a group in a group.
So, here is my version of your code:
class Wall extends Line {
     override var onMousePressed = function (e: MouseEvent): Void {
        print(walls); print(" => ");
        delete e.node as Wall from walls;
        println(walls);
var walls = [
    Wall { id:'a' startX: 10 startY: 20 endX: 90 endY: 20 strokeWidth: 10 }
    Wall { id:'b' startX: 10 startY: 50 endX: 90 endY: 50 strokeWidth: 10 }
    Wall { id:'c' startX: 10 startY: 80 endX: 90 endY: 80 strokeWidth: 10 }
//~ var Group = Group { content: bind walls };
Stage {
    scene: Scene {
        width:   100
        height:  100
        content: bind walls
    }//Scene
}//Stage