The functions domElement.getClientWidth() and domElement.getClientHeight() are returning a value of 0

When trying to extract the size of a GWT element with a specific CssClass using standard methods, I am faced with the issue that it returns 0;

    Canvas canvas = Canvas.createIfSupported();
    if(canvas == null){
        /*alert code*/
    }
    Element e = canvas.getElement();
    canvas.getElement().setClassName(canvasElement);
    Context2d context = canvas.getContext2d();

    int clientWidth = e.getClientWidth();
    /*draw logic there*/

The value of clientWidth is 0.

Here is the defined CSS class:

    .canvasElement{
        position: absolute;
        left:10px;
        top:10px;
        width:40px;
        height:30px;
    } 

At what point in time will the element size be calculated? How can I capture that moment?

Answer №1

The information will only be available once the item is displayed on the screen. This requires the item to be a) part of the DOM, and b) visible (i.e. not set to display:none;). Keep in mind that setting it to visibility:hidden; allows for layout changes without being visible.

Consider the CSS rules you intend to apply. If there's another rule like .parent .canvasElement, the browser can't determine whether to apply it until the element is present in the DOM.

For instance, if a parent element resizes its children and then a canvas is added to that parent, the width may change. Measuring before and after attaching could lead to different results.

Specifically with your code snippet, Canvas is a subclass of Widget, implementing HasAttachHandlers. You can add an attach event handler to accurately measure every time it is attached to a context (or just measure the first time and remove the handler afterward).

Answer №2

My understanding of how GWT works is limited, but based on my experience, an element needs to be present in the DOM structure before we can retrieve its client width/height. From looking at your code, it seems like the canvas element may not have been added to the DOM when you were attempting to access its clientWidth().

Answer №3

It is necessary to specify the width and height of the canvas in the HTML file. For example:

<canvas id="example" width="200" height="200">
This message will only display if your browser does not support HTML5 Canvas.
</canvas>

The Html Canvas element requires you to define WIDTH and HEIGHT in the HTML document, rather than allowing you to solely specify them in CSS.

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