NativeScript/packages/core/fps-meter/fps-native.ios.ts

import * as definition from './fps-native';

@NativeClass
class FrameHandlerImpl extends NSObject {
	private _owner: WeakRef<FPSCallback>;

	public static initWithOwner(owner: WeakRef<FPSCallback>): FrameHandlerImpl {
		const handler = <FrameHandlerImpl>FrameHandlerImpl.new();
		handler._owner = owner;

		return handler;
	}

	public handleFrame(sender: CADisplayLink): void {
		const owner = this._owner?.deref();
		if (owner) {
			owner._handleFrame(sender);
		}
	}

	public static ObjCExposedMethods = {
		handleFrame: { returns: interop.types.void, params: [CADisplayLink] },
	};
}

export class FPSCallback implements definition.FPSCallback {
	public running: boolean;
	private onFrame: Function;
	private displayLink: CADisplayLink;
	private impl: FrameHandlerImpl;

	constructor(onFrame: (currentTimeMillis: number) => void) {
		this.onFrame = onFrame;

		this.impl = FrameHandlerImpl.initWithOwner(new WeakRef(this));

		this.displayLink = CADisplayLink.displayLinkWithTargetSelector(this.impl, 'handleFrame');
		this.displayLink.paused = true;
		this.displayLink.addToRunLoopForMode(NSRunLoop.currentRunLoop, NSDefaultRunLoopMode);
		// UIScrollView (including in UIITableView) will run a loop in UITrackingRunLoopMode during scrolling.
		// If we do not add the CADisplayLink in this mode, it would appear paused during scrolling.
		this.displayLink.addToRunLoopForMode(NSRunLoop.currentRunLoop, UITrackingRunLoopMode);
	}

	public start() {
		if (this.running) {
			return;
		}

		this.running = true;
		this.displayLink.paused = false;
	}

	public stop() {
		if (!this.running) {
			return;
		}

		this.displayLink.paused = true;
		this.running = false;
	}

	public _handleFrame(sender: CADisplayLink) {
		if (!this.running) {
			return;
		}

		// timestamp is CFTimeInterval, which is in seconds, the onFrame callback expects millis, so multiply by 1000
		this.onFrame(sender.timestamp * 1000);
	}
}