Category Archives: HTML5

CORS Errors while playing with the HTML Canvas

In this post, I wrote about how you might draw a graph using the HTML canvas; my next step was to make that more efficient; however, upon trying to import a Javascript module into my script:

import animatedGraphics from './animatedGraphics';

I started getting this error:

Access to script at ‘file:///C:/repos/webanimations/animated-columns-optimised/columns.js’ from origin ‘null’ has been blocked by CORS policy: Cross origin requests are only supported for protocol schemes: http, data, chrome, chrome-extension, https.

After a bit of digging, I uncovered a feature of ES6 that prevents opening this directly in a browser.

Hosting the site locally

The issue is caused because the file is being opened directly; so there seemed to be a couple of ways around this: one is to disable the security in Chrome, although try as I might, I couldn’t manage to get it to give up the ghost: I tried various combinations around the –disable-web-security flag of Chrome.

The second option is to host the site locally. For a brief moment I considered using something like IIS Express; but fortunately, I came across this tool that hosts a site locally for you.

It can be installed as an npm package:

npm install --global http-server

Once installed, you just navigate to the relevant directory, and type http-server:

C:\repos\webanimations\animated-columns-optimised>http-server
Starting up http-server, serving ./
Available on:
  http://192.168.1.79:8080
  http://127.0.0.1:8080
  http://172.17.230.225:8080
Hit CTRL-C to stop the server

You can then navigate to your specific page; for example:

http://127.0.0.1:8080/columns

And no more CORS error (doesn’t quite work yet, but that’s a whole different story).

Drawing Custom Graphs in HTML and Javascript

While recently playing with the HTML Canvas again, it occurred to me that this power could be used for evil (or statistics are it is commonly known these days).

In this post, I’m going to draw an animated chart using the HTML Canvas and Javascript.

Let’s start with the HTML:

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" type="text/css" href="test.css">
    <script src="test.js"></script>
</head>
<body onload="doDrawing()">
    <canvas id="canvas">
    </canvas>
</body>
</html>

Again, as with my previous post (linked above), I’ll remind you that this will not cope with screen resizing. The CSS, again, isn’t much to write home about:

* { 
    margin:0; 
    padding:0; 
}
canvas {  
    display: block;    
}

Javascript

Finally, we come to the Javascript: this is a bit more involved, so I’ll break it down into the individual functions; we’ll start with the top level one:

const doDrawing = () => {
    var c = document.getElementById("canvas");
    
    c.width = window.innerWidth;
    c.height = window.innerHeight;
    var ctx = c.getContext("2d");    
    drawGraphTop(50, 5, 500);
    drawColumn(100, 500, 4, 'January');
    drawColumn(150, 350, 5, 'February');
    drawColumn(200, 150, 6, 'March');
}

All we’re doing here is getting a handle to the canvas, setting the height and width so that we fill the screen, and then calling some functions to do the drawing. The helper method to get the context is trivial, but for completeness:

function getContext() {
    var c = document.getElementById("canvas");    
    var ctx = c.getContext("2d");        
    return ctx;
}

There’s not much to explain here, we’re just getting the context from the canvas and returning it – thereby saving 3 lines of code each time we do that. The drawGraphTop function is a little more interesting:

const drawGraphTop = (top, interval, width) => {
    let ctx = getContext();
    ctx.beginPath();
    ctx.rect(1, top, width, 2);
    let stage = width / interval;
    for (let i = 0; i <= interval; i++) {
        ctx.rect(i * stage, top, 2, 5);
    }
    ctx.stroke();
}

The idea here is that we display a flat, horizontal line across the top of the screen, with markers. There is no line method on the canvas context, so a flat rectangle is the best we can do.

The more complex method is the drawColumn method (technically, it isn’t actually drawing columnns – but they still feel like columns – at least, more than rows):

const drawColumn = (top, target, speed, label) => {
    let ctx = getContext();
    let x = 10;    
    let directionHorizontal = speed;

    let intervalHandle = setInterval(() => {
        const buffer = 15;
        const height = 30;
        let showText = false;
        ctx.beginPath();
        ctx.clearRect(1, top - 1, target + buffer + Math.abs(directionHorizontal) + 2, height + 2);

        if (directionHorizontal === 1 && x <= target + buffer) {
            
        } else if (directionHorizontal > 0 && x >= target + buffer) {
            directionHorizontal = -1;
        } else if (directionHorizontal < 0 && x <= target) {
            clearInterval(intervalHandle);
            directionHorizontal = 0;
            showText = true;            
        }

        x += directionHorizontal;
        ctx.rect(1, top, x, height);
        ctx.stroke();

        if (showText) {                        
            ctx.fillText(label, 10, top + 20);
        }
    }, 1);
    
}

There is quite a lot to this; let’s focus on the interval; we take a handle to the interval, so that we can cancel it when we’ve finished drawing.

The first thing we do with the context is call beginPath – this allows us to group a series of updates into a single screen update; then we’re clearing an area, just wide of, the size of the rectangle.

We then have a conditional check – if we’re heading right, and have yet to reach the target (which has a small buffer appended for the purpose of animation), then this drops through to the code below; if we’ve reached that then we change direction; and when we’re back to the actual target, we cancel the update and set a flag to make the text appear.

Subsequently, we draw the (horizontal) column and, if we’ve finished, display the text.

Output and Caveats

It’s worth bearing in mind that the way this code is structured is probably not the best for performance – ideally, you would have a sort of game loop and draw, and then update the entire screen in one go.

Code

You can find the code for this here.

Playing with the HTML Canvas Again

I’ve previously written about how you can write games using tools like React. I’ve also written about creating a basic game in Html and Javascript, and things like rotating shapes.

In this post, I’m going to demonstrate a sort of Etch A Sketch type program.

The HTML is very simple here:

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" type="text/css" href="test.css">
    <script src="test.js"></script>
</head>
<body onload="doDrawing()">
    <canvas id="canvas">
    </canvas>
</body>
</html>

Basically just bringing in a Javascript file, CSS and putting a canvas on the screen. I’m also calling a function onload – it’s worth bearing in mind that, as it goes, this won’t resize should you change the size of the browser. If you want that behaviour, then have a look at one of my previous posts mentioned above.

The CSS is even simpler:

* { 
    margin:0; 
    padding:0; 
}
canvas {  
    display: block;    
}

All we’re doing here is removing the default margin, and stopping scroll bars from appearing.

Javascript

The Javascript is where everything is happening; let’s start with some variables:

let x = 10;
let y = 10;
let directionHorizontal = 1;
let directionVertical = 0;

The four variables determine the position that we want to draw in, and which way we’re heading. We can now render this to the screen like this:

const doDrawing = () => {
    var c = document.getElementById("canvas");
    
    c.width = window.innerWidth;
    c.height = window.innerHeight;

    var ctx = c.getContext("2d");
    setInterval(() => {
        ctx.fillRect(x, y, 1, 1);
        x += directionHorizontal;
        y += directionVertical;
    }, 10);
}

The canvas width and height are the first things to note: when I started playing with this, I made the mistake of trying to set this in CSS; if you do, it actually doesn’t change the size of the canvas, but stretches it across the screen; this was the only way that I could get the canvas to display full screen without that stretching effect (if you know another / better way, please say so in the comments!)

Next we get the context from the canvas – this allows us to render to it, and then we simply set-up an interval, and draw a rectangle 1px x 1px each iteration.

Summary

That it – as with previous posts, there’s not a whole lot to using the HTML canvas, but I do like to re-experiment every so often.

Force React Components to sit Side-by-Side

For anyone that knows me, they’ll know that UI and UX is not exactly my strong suit. If I had my way, we’d all just use console applications like this:

>placeorder /productcode coffee /quantity 1

Unfortunately, the trend seems to have gone a different way, and now we have CSS. CSS is a brilliant idea; however, you need to be able to visualise what you want your stuff to look like first.

Anyway, onto this post. If you know anything about CSS or Bootstrap, then you’ve probably already read everything that could interest you about this post!

In my latest project, I have a search box, and I wanted to line up the controls on the screen like this:

Search         [Search Text]        [Search Button]

I’m using React, so the original code looked like this (more or less):

<label>Search</label>
<input type='text'/>
<SimpleButton buttonAction={props.searchAction} buttonLabel="Search" />            

And it rendered like this:

Search         [Search Text]        
[Search Button]

My first gambit was to define a CSS style (so I can now put “Front End Developer” on my CV):

.rowLine {
    display:flex; 
    flex-direction:row;
}

And I changed the HTML to look like this:

<div className='rowLine'>
    <label>Search</label>
    <input type='text'/>
    <SimpleButton buttonAction={props.searchAction} buttonLabel="Search" />            
</div>

That worked, and I copied most of it from here. So now I’ve updated my CV to “Senior Front End Developer”.

It then occurred to me that, as good as this looks, there’s probably something in Bootstrap, and if there is, then my web page can look like the rest of the internet. It turns out I was right:

<div className="form-group row">
    <label htmlFor="searchText" className="col-sm-2 col-form-label">Search</label>
    <div className="col-sm-8">
        <input id="searchText" className="form-control" type='text'
                    placeholder="e.g. Goats" />
    </div>
    <div className="col-sm-2">
        <SimpleButton buttonAction={props.searchAction} buttonLabel="Search" />            
    </div>
</div>        

You may notice that React has its own “for” property, called “htmlFor”.

Disclaimer / Apology

As usual, please take everything you read in my blog with a healthy dose of salt. If you are a front end developer then please take solace in the fact that, despite me being facetious, I really need frameworks like Bootstrap, because I would never think to change the colour of a button, or to align the sizes.

This was all so much simpler in the days of Turbo Pascal / Turbo C, where you would draw your buttons using the ANSI character set!

References

https://stackoverflow.com/questions/39702130/line-two-divs-side-by-side-with-css-and-react

https://stackoverflow.com/questions/39187722/error-ts2339-property-for-does-not-exist-on-type-htmlpropshtmllabelelement

https://www.tutorialrepublic.com/twitter-bootstrap-tutorial/bootstrap-forms.php

Building a list with Asp.Net Core

I’ve recently been working with Asp.Net Core to build some functionality, involving building a list of values. Typically, with Asp.Net Core using Razor, you have a form that may look something like this:

@using (Html.BeginForm("MyAction", "ControllerName", FormMethod.Post)
{
    @Html.AntiForgeryToken()
    <div class="form-group">
        @Html.LabelFor(model => model.MyValue)
        @Html.TextBoxFor(model => model.MyValue)
    </div>

    <div class="form-group">
        <button type="submit">Submit</button>
    </div>

This works really well in 90% of cases, where you want the user to enter a value and submit. This is your average CRUD application; however, what happens if, for some reason, you need to manipulate one of these values? Let’s say, for example, that you want to submit a list of values.

For the sake of simplicity, we’ll say that the controller accepts a csv, but we want to build this up before submission. You can’t simply call a controller method for two reasons: the first is that the controller will reload the page; and the second that you don’t have anywhere to put the value on the server. If this was, say, a method to create an entry in the DB, the DB entry, by definition, couldn’t exist until after the submission.

This all means that you would need to build this list on the client.

A solution

Let’s start with a very simple little feature of Html Helpers – the hidden field:

@Html.HiddenFor(model => model.MyList)

This means that we can store the value being submitted to the user, without showing it to the user.

We’ll now need to display the data being added. An easy way to do this is a very simple table (you can load existing values into the table for edit scenarios):

    <div>
        <table id="listTable">
            <tbody>
                @if ((Model?.ValueList ?? null) != null)
                {
                    @foreach (var v in Model.ValueList)
                    {
                        <tr>
                            <td>@v</td>
                        </tr>
                    }
                }
            </tbody>
        </table>
    </div>    

Pay particular attention to the Table Id and the fact that the conditional check is inside the tbody tag. Now let’s allow the user to add a new piece of data:

    <div class="form-group">
        @Html.LabelFor(model => model.NewValue)
        @Html.TextBoxFor(model => model.NewValue)
    </div>
    <div>
        <button type="button" id="add-value">Add Value</button>
    </div>

Okay, so now we have a button and a field to add the value; we also have a method of displaying those values. We’ll need a little bit of Javascript (JQuery in this case) to append to our list:

@section Scripts {
        $('#add-value').click(() => {

            const hiddenList = $('#MyList');
            const newValue = $('#NewValue');

            if (!hiddenList.val()) {
                hiddenList.val(newValue.val());
            } else {
                hiddenList.val(hiddenList.val() + ',' + newValue.val());
            }
            
            $('#listTable > tbody:last-child').append('<tr><td>' + newValue.val() + '</td></tr>');            
        });

On the button click, we get the hidden list and the new value, we then simply add the new value to the list. Finally, we manipulate the table in order to display the new value. If you F12 the page, you’ll notice that the Razor engine replaces the Html Helpers with controls that have Ids the same as the fields that they are displaying (note that if the field name contains a “.”, for example: MyClass.MyField, the Id would be MyClass_MyField).

When you now submit this, you’ll see that the hidden field contains the correct list of values.

References

https://stackoverflow.com/questions/16174465/how-do-i-update-a-model-value-in-javascript-in-a-razor-view/16174926

https://stackoverflow.com/questions/171027/add-table-row-in-jquery

https://stackoverflow.com/questions/36317362/how-to-add-an-item-to-a-list-in-a-viewmodel-using-razor-and-net-core

Using Blazor Components

Imagine that you’re writing a Blazor application – maybe it’s similar to this one. Now, imagine that you have a large chunk of HTML in your main view. You might think: I wish I was using React, then I could separate this into its own component.

You can also do this in Blazor. Here’s how.

Components in Blazor

Let’s start with moving your code. The first step is to cut your HTML and paste it into a new Razor Component:

The format of your new component, from scratch, will be:

&lt;h3&gt;Component Name&lt;/h3&gt;

@code {

}

Your existing code should go beneath, or instead of:

&lt;h3&gt;Component Name&lt;h3&gt;

Parameters

The @code section allows you to put all kinds of crazy C# code in a code behind type model – so you probably don’t want to use that, except for passing parameters; for example:

@code {
    [Parameter]
    private string MyParameter { get; set; }
}

This allows you to pass a string into your component; for example (in your main view):

&lt;MyComponent MyParameter=&quot;test&quot; /&gt;

Complex Parameters

So far so good. But what if you need a complex type? You could, for example, pass a View Model into your component:

[Parameter]
private MyViewModel MyViewModel { get; set; }

You can pass this into the component as though it were a primitive type:

&lt;MyComponent MyViewModel=&quot;@MyViewModel&quot; /&gt;

This means that you can lift and shift the code with no changes.

Using External Namespaces

As with standard C#, you can access anything within the current namespace. Should you need any classes that are not in your current namespace, you can declare them at the top of the file, like this:

@using MVVMShirt

&lt;h3&gt;My Component&lt;/h3&gt;

Summary

Blazor is still in its infancy, but hopefully, adding actual code to these @code sections will become as frowned upon as code-behind.

Using View Models in Blazor

Being new to Blazor (and Razor), the first thing that tripped me up was that the view seemed divorced from the rest of the application. In fact, this is actually quite a nice design, as it forces the use of DI.

For example, say you wanted to create a View Model for your view, you could register that ViewModel in the Startup:

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddTransient<MyViewModel, MyViewModel>();
        }

Note here that you don’t need an interface. If you’re only creating an interface for the purpose of this then that abstraction provides no benefit. That isn’t to say there may not be a reason for having an interface, but if you have one and this is the only place it’s used, you probably should reconsider.

The views in Razor / Blazor (at the time of writing) are *.razor files. In order to resolve the dependency inside the view, you would use the following syntax:

@page "/"
@inject ViewModels.MyViewModel MyViewModel

(Note that @page “/” is only in this snippet to orientate the code.)

You can call initialisation in the view model using something like:

@code {

    protected override async Task OnInitAsync()
    {
        await MyViewModel.Init();
    }    
}

And, within your HTML, you can reference the view model like this:

<div>@MyViewModel.MyData</div>

Magic. Hopefully more to come on Blazor soon.

Adding a New Screen to the React Template Project

In this post I started looking into ReactJS. Following getting the sample project running, I decided that I’ve try adding a new screen. Since it didn’t go as smoothly as I expected, I’ve documented my adventures.

The target of this post is to create a new screen, using the sample project inside Visual Studio.

Step 1

Create a brand new project for React:

If you run this out of the box (if you can’t because of missing packages then see this article), you’ll get a screen that looks like this:

Step 2

Add a new tsx file to the components:

Here’s some code to add into this new file:

import * as React from 'react';
import { RouteComponentProps } from 'react-router';
 
 
export class NewScreen extends React.Component<RouteComponentProps<{}>, {}> {
    public render() {
        return <div>
            <h1>New Screen Test</h1>
        </div>;
    }
}
 

The Javascript as HTML above is one of the things that makes ReactJS an appealing framework. Combine that with Typescript, and you get a very XAML feel to the whole web application.

Step 3

Add a link to the Navigation Screen (NavMenu.tsx):

<div className='navbar-collapse collapse'>
    <ul className='nav navbar-nav'>
        <li>
            <NavLink to={ '/' } exact activeClassName='active'>
                <span className='glyphicon glyphicon-home'></span> Home
            </NavLink>
        </li>
        <li>
            <NavLink to={ '/counter' } activeClassName='active'>
                <span className='glyphicon glyphicon-education'></span> Counter
            </NavLink>
        </li>
        <li>
            <NavLink to={ '/fetchdata' } activeClassName='active'>
                <span className='glyphicon glyphicon-th-list'></span> Fetch data
            </NavLink>
        </li>
        <li>
            <NavLink to={'/newscreen'} activeClassName='active'>
                <span className='glyphicon glyphicon-th-list'></span> New screen
            </NavLink>
        </li>
 
    </ul>
</div>

If you run this now, you’ll see the navigation entry, but clicking on it will give you a blank screen. It is just that scenario that motivated this post!

Step 4

Finally, the routes.tsx file needs updating so that it knows which screen to load when:

import * as React from 'react';
import { Route } from 'react-router-dom';
import { Layout } from './components/Layout';
import { Home } from './components/Home';
import { FetchData } from './components/FetchData';
import { Counter } from './components/Counter';
import { NewScreen } from './components/NewScreen';
 
export const routes = <Layout>
    <Route exact path='/' component={ Home } />
    <Route path='/counter' component={ Counter } />
    <Route path='/fetchdata' component={FetchData} />
    <Route path='/newscreen' component={NewScreen} />
</Layout>;

Playing multiple videos simultaneously using HTML5

I was interested to see how playing multiple videos simultaneously affected the performance of a web page. This probably seems a little like a time machine back to 1995 – I’ll soon be posting about flashing red text and Dreamweaver!

However, playing short videos with only a few frames can be a way to draw attention to a particular part of the page; for example, if you’re browsing a clothing catalogue and one of the models moves when you hover over.

It’s worth pointing out that the entire page will be local, and so I have no network considerations whatsoever. This isn’t, however, about having three Netflix streams running at the same time – it’s short, and small videos.

The video that I’m using came from here. I’m creating an effect whereby you hover over an image of a space ship and it explodes.

MP4

It’s worth bearing in mind that most video formats are not supported by most browsers. MP4 is the exception, so it looks like this is the way to go for video encoding. I used this tool to convert the file.

HTML

Here’s the HTML for a single icon:

<head>
  <script src="HoverAnimate.js"></script>
</head>
<body onload="loaded()">
  <div id="ship1" style="background-image: url('assets/titan.png'); background-repeat: no-repeat; background-size: 50px 50px; width: 50px; height: 50px">
    <video  width="50" height="50" id="ship1-video" 
            style="visibility: hidden">
      <source src="assets/explosion.mp4" type="video/mp4" />
    </video>
  </div>
</body>

Clearly this could be neater if the CSS was separated, but essentially what we have is a div element with a scaled background image, which contains a video (currently hidden). The next thing is the Javascript that plays the video:

function loaded() {
    var imgs = document.getElementsByTagName("div");
    [].forEach.call(imgs, function (item) {
        if (item.style.backgroundImage !== "") {
            item.addEventListener('mouseover', hoverImg, false);
        }
    });
   
}

function hoverImg(e) {      
    var vid = document.getElementById(this.id + "-video");
    vid.onended = function() {        
        vid.style.visibility = 'hidden';
    }
        
    vid.style.visibility = 'visible';
    vid.play();
}

This iterates through all the div elements and, for those that has a background image, hooks up a hover event. I’ve also assumed that the div element will be named using the format “{imagename}-video”.

Conclusion

I tried this with seven videos simultaneously, and didn’t see any jerking of the animations. Whether this would stand up under networked conditions, it’s hard to say, but with the video locally available, performance is fine.

References

https://www.w3schools.com/html/html_media.asp

http://www.online-convert.com/

https://www.w3schools.com/html/html5_video.asp

http://www.dreamincode.net/forums/topic/281583-video-plays-on-mouse-over-but-not-with-multiple-videos/

https://stackoverflow.com/questions/10881678/html-play-a-video-inside-an-image

Create CSS effect to “Shine” a button border

Imagine that you have an HTML button or element on a page and you would like an effect where the border shines all around the perimeter. this provides an excellent example of an effect of the entire element shining, and this post will largely be based on that code.

Animations

CSS has the concept of an animation, to define it, use the following syntax:

.growOnHover:hover:after {
    animation: growAnimation 1s;
}

Here is the HTML referencing this:

<a href="#" class="growOnHover">Grow</a>

Tge “growAnimation” refers to a KeyFrame:

@keyframes growAnimation {
    from {width: 100px; height: 100px;}
    to {width: 110px; height: 110px;}
}

The effect

The effect that I want is for a light to run around the circumference of the button when it’s hovered over. In this case, instead of animating from .. to, we can specify at which stage a particular section of the animation kicks in.

.borderShine:after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 5;
  height: 5;
  opacity: 0;  

  border-radius: 1;

  background: rgba(255, 255, 255, 10);
}

.borderShine:hover:after {
  animation: shineAnimation 2s 1;  
}

@keyframes shineAnimation {
  0%   {left: 0; top: 0; width: 2; height: 2; opacity: 0}
  10%  {width: 100; height: 2}  
  20%  {left: 98; top: 0; width: 2; height: 2}
  25%  {opacity: 1;}
  30%  {height: 100}
  40%  {left: 98; top: 98; height: 2}
  50%  {left: 0; top: 98; width: 100}
  55%  {opacity: 1;}
  60%  {left: 0; top: 98; width: 2; height: 2}
  70%  {left: 0; top: 0; width: 2; height: 100}
  80%  {left: 0; top: 0; width: 2; height: 2}
  100% {opacity: 0;}
}

There are a few useful things to remember here:

  • The animation is a transition between the state that the screen is currently in, and the state that you want it to be in; so, for example, the opacity set to 1 at 25% will cause the white bar to gradually appear over the steps between the two. The reason that I’ve set opacity twice here is to prevent it from transitioning back too soon.
  • All the figures above are absolute (as my buttons are 100 x 100).

References

http://jsfiddle.net/AntonTrollback/nqQc7/

https://css-tricks.com/useful-nth-child-recipies/

https://css-tricks.com/using-multi-step-animations-transitions/