Error "TypeError: ... is not a function" when trying to use a component function in template

I have a component as following and I am trying to provide my svg viewBox dimension dynamically, injected from my bootstrap in main.ts.

import {Component} from 'angular2/core';
import {CircleComponent} from './circle.component';
import {Circles} from './circles.service';

@Component({
    selector: 'my-app',
    styleUrls: ['./app/app.component.css'],
    directives: [CircleComponent],
    providers: [Circles],    
    template: `
        <svg [attr.viewBox]="getViewBox()" preserveAspectRatio="xMidYMid meet">
            <svg:g my-circle *ngFor="#circle of circles.circles" [circle]="circle" />
        </svg>
    `
})
export class AppComponent{
    static parameters = [Circles, 'canvasWidth', 'canvasHeight'];

    constructor(circles: Circles, canvasWidth: Number, canvasHeight: Number) {
        this.circles = circles;
        this.width = canvasWidth;
        this.height = canvasHeight;
    }    

    function getViewBox(): String {
        return `0 0 ${this.width} ${this.height}`;
    }
}

My main.ts

import {provide}        from 'angular2/core';
import {bootstrap}      from 'angular2/platform/browser'
import {AppComponent}   from './app.component'

bootstrap(AppComponent, [
    provide('canvasWidth', {useValue: 900}),
    provide('canvasHeight', {useValue: 300})
]);

And the exception I get

EXCEPTION: TypeError: l_context.getViewBox is not a function in [getViewBox() in AppComponent@1:13]

I'm not sure it's a good way to do this thing, but this information is needed elsewhere in my circles.service. One thing I don't like though is that I cannot specify the Number type so I have to define static parameters = [Circles, 'canvasWidth', 'canvasHeight']; in AppComponent.