Can't Extend Native Classes with ES6

[NickIliev]

From @vbresults on November 6, 2017 11:22

I ran into this when setting a delegate in my project which is TypeScript compiled down to ES6. The problem would be that application.js expects a native delegate object while it's a JS function in ES6.

Copied from original issue: NativeScript/NativeScript#5038

[ginev]

@vbresults can we have a snippet that we can use to reproduce the case?

class Delegate extends UIResponder implements UIApplicationDelegate {
	public static ObjCProtocols = [UIApplicationDelegate];

	applicationDidReceiveRemoteNotification(application: UIApplication, userInfo: NSDictionary<string, any>) {
		// ...<snip>
	}

	// ...<snip>
}

const application = require("application");
application.ios.delegate = Delegate;

[tdermendjiev]

Hi @vbresults, are you getting any errors while building/running?

[ginev]

@vbresults - this looks like it should be working as expected. The application.ios property returns an instance of the IOSApplication class which we internally use to represent an iOS app. Its delegate property accepts a type, not an instance of it - so you basically need to set it to point to the class which instance you would like to use as a custom delegate.

This article might be helpful in resolving the case: https://docs.nativescript.org/core-concepts/application-lifecycle#ios-uiapplicationdelegate

I don't see you calling the start() method of the app. Can you share some more code here?

@tdermendjiev Yes, the error is that Delegate is not a native class but instead a JS function.

[tdermendjiev]

@vbresults it would be useful if you provide the exact error with the stack trace as well as the whole content of the js file.

@tdermendjiev It's triggered by this line in application.ios.js:

UIApplicationMain(0, null, null, iosApp && iosApp.delegate ? NSStringFromClass(iosApp.delegate) : NSStringFromClass(Responder));

NSStringFromClass expects a class but instead it's getting a JS function. My guess is that NS uses some sort of built-in __extends function which does not affect ES6+ native extends.

Make sure lib is set to es6 and dom in tsconfig.json.

[tdermendjiev]

@vbresults I managed to reproduce the issue and indeed its related to the __extends function implemented by the ios runtime. ES6 extends has a different behavior and it doesn't call runtime's __extends. That said, ES6 is not supported by both the iOS and Android runtimes. We don't plan supporting ES6 in the near future.

@tdermendjiev That's unfortunate. Native ES6 does not have some of the limitations that __extends does (specifically around inheritance and calling context); ES6 really helps to clean up my code.

[ginev]

@vbresults ES6 support will certainly become more demanded and we will have to support it at one point. By saying that we do not plan it for the near future our point is that we do not plan it for the upcoming two or three releases of NativeScript. If the demand for it increases, we will escalate its implementation.