Thursday, February 15, 2018

Re: Understanding JsInterop

On Thursday, February 15, 2018 at 10:21:59 AM UTC+1, Scott Shumway wrote:

I am trying to get a DOMRect from an Element. I can do this with JSNI, but have been unable to make a JsInterop version. Here's what I have:

public final class DOMRect extends JavaScriptObject
 // returns DOMRect or null
 public static native DOMRect getBoundingClientRect(Element element)
 return element.getBoundingClientRect && element.getBoundingClientRect();

 protected DOMRect()

 public final native double getX()
 return this.x;

 // etc.

I can make a JsType for DOMRect, no problem. The issue I'm having is with getBoundingClientRect(). This function may not exist which is why there is a null check.

You'll want a @JsProperty for the method so you can check whether it's null or not; and then either declare it also as a @JsMethod in parallel to call it, or have the @JsProperty return a @JsFunction (or then cast to an elemental2.dom.Element).
And you can hide the details the same way you did with JSNI, with a @JsOverlay:

@JsType(isNative=true, namespace=JsPackage.GLOBAL, name="Element")
class Element {
@JsProperty(name="getBoundingClientRect") private native Object getGetBoundingClientRect();
@JsMethod(name="getBoundingClientRect") private native DOMRect callGetBoundingClientRect();

public DOMRect getBoundingClientRect() {
return getGetBoundingClientRect() == null ? null : callGetBoundingClientRect();


@JsType(isNative=true, namespace=JsPackage.GLOBAL, name="Element")
class Element {
@JsProperty(name="getBoundingClientRect") public native GetBoundingClientRect getBoundingClientRect();

interface GetBoundingClientRect {
public DOMRect invoke();

If I make an interface for getBoundingClientRect, I'm not allowed to cast Element to implement it.

How so‽
You cannot use JavaScriptObject#cast(), but you should be able to use a standard Java cast.
You might have to first cast to Object to please javac:

((HasGetBoundingClientRect) (Object) elt).getBoundingClientRect()

I can't extend Element, either. With JSNI, I am allowed to compile Javascript into Javascript, but I can't with JsInterop. It seems like a lot of trouble for 1 line of Javascript.

How can I do this? Thanks in advance...

getBoundingClientRect is supported in (literally) all browsers, so don't bother and just call the method:,

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