Tag Archives: animate

A C# Developer’s Guide to: ReactJS – Part 2 – Moving Controls

Following on from my previous post, I’m going to extend our ReactJS application by adding some boxes, and allowing the user to re-arrange them on the screen.

Concepts

There are two key concepts to consider when working with React, and we’ll cover one of them in this post; that is: state.

React has a special property of each component known as state. If you use that to bind any of the UI to, then React will refresh that component when the state changes.

Moving a UI Element

Okay – that sounds great, but how would we do this in practice?

Imagine that you have a HTML box drawn on the screen; you might have something like this:

<div style="height:100px; width:200px; background:#0000FF" />

We can draw a box. If you use this code, then your box will be on the top left of the screen; so we can tell it not to be by specifying the left and top; let’s try defining a CSS style:

<div style="left:10px; top:20px;height:100px; width:200px; background:#0000FF" />

With React, we can set those values to a value derived from the state; for example:

render() {
    const top = this.state.newY;
    const left = this.state.newX;

    const myStyle = {
        height: '100px',
        width: '200px', 
        top: `calc(${top}px)`,
        left: `calc(${left}px)`, 
        borderStyle: 'solid',
        borderColor: 'blue', 
        position: 'absolute',
        zIndex: 1,
        background: 'blue'
    };

    return ( 
        <div> 
            <div style={myStyle}>source</div>
        </div>
    );
}

What this means is that every time I change the state values, the values in the style will update.

To illustrate this, if we write a React application like this (only the timeout function is new):

render() {
    const top = this.state.newY;
    const left = this.state.newX;
    const myStyle = {
        height: '100px',
        width: '200px', 
        top: `calc(${top}px)`,
        left: `calc(${left}px)`, 
        borderStyle: 'solid',
        borderColor: 'blue', 
        position: 'absolute',
        zIndex: 1,
        background: 'blue'
    };

    setTimeout(() => {
        this.setState({newX: this.state.newX + 1, newY: this.state.newY + 1});
    }, 50);

    return ( 
        <div> 
            <div style={myStyle}></div>
        </div>
    );
}

We can make out box move diagonally across the screen:

Screenshot included because you couldn’t possibly imaging what a blue rectangle looks like.

Start the app using VS Code

A quick note on VS Code. If you select Terminal -> New from the menu, you can run the React application directly from the IDE; and the best part is that if there’s something already running on the port, VS Code will just give you a new one:

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