dcg-viewport-tracker

(function() {
    'use strict';
    const ID_PREFIX = 'vp_tracker';
    const FOLDER_ID = ID_PREFIX + '_folder';
    // change this to false to make expressions visible
    const HIDDEN_EXPRESSIONS = true;
    // Github: https://github.com/SlimRunner
    // Twitter: @runner_slim

    let state = Calc.getState();
    let newExprs = [
        String.raw`X_{vpmin}`,
        String.raw`Y_{vpmin}`,
        String.raw`X_{vpmax}`,
        String.raw`Y_{vpmax}`
    ].map((expval, i) => {
        return {
            id: `${ID_PREFIX}_${i}`,
            latex: `${expval}=0`,
            folderId: FOLDER_ID,
            type: 'expression'
        };
    });

    newExprs.unshift({
        collapsed: true,
        secret: HIDDEN_EXPRESSIONS,
        id: FOLDER_ID,
        type: 'folder'
    });

    state.expressions.list.push(...newExprs);
    Calc.setState(state, {allowUndo: true});

    Calc.observe('graphpaperBounds',() => {
        // this happens each time the GraphPaper is updated in Desmos
        let mCoords = Calc.graphpaperBounds.mathCoordinates;

        Calc.setExpression({
            id: `${ID_PREFIX}_0`,
            latex: `X_{vpmin}=${mCoords.left}`
        });
        Calc.setExpression({
            id: `${ID_PREFIX}_1`,
            latex: `Y_{vpmin}=${mCoords.top}`
        });
        Calc.setExpression({
            id: `${ID_PREFIX}_2`,
            latex: `X_{vpmax}=${mCoords.right}`
        });
        Calc.setExpression({
            id: `${ID_PREFIX}_3`,
            latex: `Y_{vpmax}=${mCoords.bottom}`
        });

    });
}());