DHTML Jigsaw Puzzle: IE4; Grabbing Pieces

The DHTML Lab Jigsaw Puzzle, Part II: IE4 cont'd
grabbing the puzzle pieces

Drag & Drop

We have discussed drag and drop concepts extensively in columns 6 and 7. Complete drag and drop code for Explorer 4 was developed in column 7. Here we will discuss only the modifications necessary for the puzzle.

As you will recall, the drag and drop code for Explorer took this form:

currentX = currentY = 0; // vars for tracking mouse position
whichEl = null;          // var for tracking dragged element
    
function grabEl() {
    // statements to execute when user grabs an element (mousedown)
    // go here
}
    
function moveEl() {
    // statements to execute when user drags an element (mousemove)
    // go here
}
    
function checkEl() {
    if (whichEl!=null) { return false } // don't select parts of document
                                        //  if drag is in progress
}
function dropEl() {
    // statements to execute when user drops an element (mousedown)
    // go here
}
document.onmousemove = moveEl;    // define which functions
document.onselectstart = checkEl; // should be called
document.onmousedown = grabEl;    // for which events
document.onmouseup = dropEl;
Grabbing the Puzzle or Puzzle Piece
We will modify the grabEl() function in two places. We gave all draggable
elements in the puzzle a draggable property. This applies to the
puzzle itself and all puzzle pieces that are created. The value of this
property is true or false, depending on whether the element should be
dragged or not. In the case of the full puzzle, the value is toggled by
the drag toggle button in the controls. Puzzle pieces cannot be dragged
if they are placed in their correct position in the puzzle. We will be
discussing routines to determine this further down. Since we are
concerned with a true value for the draggable property, we modify grabEl()
to search it through the element heirarchy that exists under the
mousedown:
function grabEl() {
    whichEl = event.srcElement;
    while (!whichEl.draggable) {         // identify element to be dragged
        whichEl = whichEl.parentElement; // by finding true value for draggable
        if (whichEl == null) { return }
    }
    
    if (whichEl != elPuzzle && whichEl != activeEl) { // exclude full puzzle
        whichEl.style.zIndex = activeEl.style.zIndex + 1;
        activeEl = whichEl;
    }
    
    whichEl.style.pixelLeft = whichEl.offsetLeft;
    whichEl.style.pixelTop = whichEl.offsetTop;
    
    currentX = (event.clientX + document.body.scrollLeft);
    currentY = (event.clientY + document.body.scrollTop); 
}
We also make a change in the code that places the dragged element on
top of other draggable elements. When the full puzzle is dragged, no
puzzle pieces are visible, as it can only be dragged in a solved
form. Therefore, the full puzzle element is excluded from the z-index
incrementation.
The user can now continue to attempt a solve by moving the piece.



Produced by Peter Belesis and
 
All Rights Reserved. Legal Notices.

Created: Nov. 13, 1997

Revised: Jan. 18, 1998
URL: https://www.webreference.com/dhtml/column9/puzzGrab.html