Cesium3DTileStyle.style is not reactive

[bkuster]

The Issue

If a Cesium3DTileStyle is created, its options are used to populate the readonly style property, which is described by the docs as follows:

Gets the object defining the style using the 3D Tiles Styling language.

This is not the case, since style.style is only evaluated at style creation. Thus the following fails, making cloning & persisting of styles tedious:

var style = new Cesium3DTileStyle({ show: true });
style.color = "rgb(125, 125, 125)";
var clone = new Cesium3DTileStyle(style.style);
expect(clone.color).to.equal(style.color);
// fails: expected undefined to equal "rgba(125, 125, 125)"

Proposed Solution

When setting an expression on the style, the getExpression function not only receives this.style and value but also key. The key is then set on this.style to reflect the current value. You can see an implementation of this solution here, I would gladly open a PR for this.

[hpinkos]

@bkuster I can't see any reason for not updating _style as the properties change. Please open that PR and we'd be happy to review it =) Thanks!