ngIf for vanilla JS + NativeScript

README.md

What ?

This is the equivalent of using ngIf in angular + {N}, but for vanilla JS + {N}

Why ?

As opposed to using visibility attribute, this will not add the component specified in the template to the view, so the hidden component's loaded event won't be triggered until the component is shown.

Render.js

const {CustomLayoutView, Property, View, layout} = require("tns-core-modules/ui/layouts/layout-base");

class Render extends CustomLayoutView {
	constructor() {
		super();
		this.viewAdded = false;
	}

	refresh() {
		if (this.when) {
			this._addView(this.template);
			this.viewAdded = true;
		}
		else if (this.viewAdded) {
			this._removeView(this.template);
			this.viewAdded = false;
		}
	}

	onLayout(left, top, right, bottom) {
		if (this.when) {
			View.layoutChild(this, this.template, 0, 0, right - left, bottom - top);
		}
	}

	onMeasure(widthMeasureSpec, heightMeasureSpec) {
		if (this.when) {
			const result = View.measureChild(this, this.template, widthMeasureSpec, heightMeasureSpec);

			const width = layout.getMeasureSpecSize(widthMeasureSpec);
			const widthMode = layout.getMeasureSpecMode(widthMeasureSpec);

			const height = layout.getMeasureSpecSize(heightMeasureSpec);
			const heightMode = layout.getMeasureSpecMode(heightMeasureSpec);

			const widthAndState = View.resolveSizeAndState(result.measuredWidth, width, widthMode, 0);
			const heightAndState = View.resolveSizeAndState(result.measuredHeight, height, heightMode, 0);

			this.setMeasuredDimension(widthAndState, heightAndState);
		}
		else {
			this.setMeasuredDimension(0, 0);
		}
}
}

const when = new Property({
	name: "when",
	affectsLayout: true,
	valueChanged: function(target) {
		target.refresh();
	}
});
when.register(Render);


module.exports = {
	Render: Render
};

SomeView.xml

<Page xmlns:r="path/to/Render"> <!-- file on disk should be "path/to/Render.js" -->

  <r:Render when="{{ someCondition === true }}">
    <r:Render.template>
      <Label text="This will not be added to the view until 'someCondition' is true" />
    </r:Render.template>
  </r:Render>

</Page>

Kudos!
This is useful. Thank you! :D