Extending native class

[NickIliev]

From @Serge-SDL on November 1, 2017 18:21

Hi!

I am triying to extend a native android class but I am facing some difficulties. I try to extend the nativescript-drawingpad plugin based on android-signaturepad native plugin to avoid some memory leaks.

My goal is to overwrite some methods in the native class "com.github.gcacace.signaturepad.views.SignaturePad".

my first step is to extend the nativescript plugin like this to use a CustomSignaturePad object intead of the com.github.gcacace.signaturepad.views.SignaturePad:

import { DrawingPad as DrawingPadVendor } from "nativescript-drawingpad";

export class DrawingPad extends DrawingPadVendor {
  public createNativeView() {
    const signaturePad = new CustomSignaturePad(this._context, null);
    if (this.penColor) {
      signaturePad.setPenColor(this.penColor.android);
    }
    if (this.penWidth) {
      signaturePad.setMinWidth(this.penWidth);
    }
    return signaturePad;
  }
}

next step is to create the new CustomSignaturePad class:

declare var com: any;

class CustomSignaturePad extends com.github.gcacace.signaturepad.views.SignaturePad {
    constructor(context, attrs) {
        super(context, attrs);
        return global.__native(this);
    }

    init() {
        return global.__native(this);
    }

    clear() {
        this.super.clear();
    }

    clearView() {
        this.super.clearView();
    }
}

here it the first typescript issue: super(context, attrs); cause a "call target does not contain any signatures" error. This error can be fixed with a little workaround described here microsoft/TypeScript#17032 . declartion is now:
class CustomSignaturePad extends (com.github.gcacace.signaturepad.views.SignaturePad as { new(context, attrs):any; }) { }

This code works (no error), but there is still 2 issues:

• clear() function, called from nativescript plugin is executed but clearView() function, called in the native clear() method is not used (if I log something it doesn't display anything). -> Is it possible to overwrite a method that is called in native androd class?
• I can't have access to android class private properties, I tried this.property and this.super.property but it doesn't works -> is it possible to access this properties?

Thank you for your help!
serge

Copied from original issue: NativeScript/NativeScript#5022

[petekanev]

Hey @Serge-SDL, NativeScript does not officially support extending of extended native classes in TypeScript - which appears to be the case with DrawingPad extending DrawingPadVendor.

This code works (no error), but there is still 2 issues:

• I am not sure I understand the workflow of the clear() you want to achieve. Are you referring to the call to https://github.com/gcacace/android-signaturepad/blob/f394b150842869cfc67a6372bfdd3e89d9f1e52c/signature-pad/src/main/java/com/github/gcacace/signaturepad/views/SignaturePad.java#L194? When you call this.super.clear(), that method will be called, and the super's clearView will be called.
• No, you cannot access private class properties outside of the class which declared said properties. Hence 'private'.

Anyway - I tend to agree with sean-perkins's point on the forums and think you should work directly with the plugin author.