Some tricks for working with SVG in JavaScript

Scalable vector graphics (SVG) is a part of the document object model (DOM) and thus can be modified just like any other DOM node from JavaScript. But SVG has some pitfalls like having its own coordinate system and different style attributes which can be a headache. What follows is a non comprehensive list of hints and tricks which I found helpful while working with SVG.

Coordinate system

From screen coordinates to SVG

function screenToSVG(svg, x, y) { // svg is the svg DOM node
  var pt = svg.createSVGPoint();
  pt.x = x;
  pt.y = y;
  var cursorPt = pt.matrixTransform(svg.getScreenCTM().inverse());
  return {x: Math.floor(cursorPt.x), y: Math.floor(cursorPt.y)}
}

From SVG coordinates to screen

function svgToScreen(element) {
  var rect = element.getBoundingClientRect();
  return {x: rect.left, y: rect.top, width: rect.width, height: rect.height};
}

Zooming and panning

Getting the view box

function viewBox(svg) {
    var box = svg.getAttribute('viewBox');
    return {x: parseInt(box.split(' ')[0], 10), y: parseInt(box.split(' ')[1], 10), width: parseInt(box.split(' ')[2], 10), height: parseInt(box.split(' ')[3], 10)};
};

Zooming using the view box

function zoom(svg, initialBox, factor) {
  svg.setAttribute('viewBox', initialBox.x + ' ' + initialBox.y + ' ' + initialBox.width / factor + ' ' + initialBox.height / factor);
}

function zoomFactor(svg) {
  var height = parseInt(svg.getAttribute('height').substring(0, svg.getAttribute('height').length - 2), 10);
  return 1.0 * viewBox(svg).height / height;
}

Panning (with zoom factor support)

function pan(svg, panX, panY) {
  var pos = viewBox(svg);
  var factor = zoomFactor(svg);
  svg.setAttribute('viewBox', (pos.x - factor * panX) + ' ' + (pos.y - factor * panY) + ' ' + pos.width + ' ' + pos.height);
}

Misc

Embedding HTML

function svgEmbedHTML(width, height, html) {
    var svg = document.createElementNS("http://www.w3.org/2000/svg", "foreignObject");
    svg.setAttribute('width', '' + width);
    svg.setAttribute('height', '' + height);
    var body = document.createElementNS('http://www.w3.org/1999/xhtml', 'body');
    body.style.background = 'none';
    svg.appendChild(body);
    body.appendChild(html);
    return svg;
}

Making an invisible rectangular click/touch area

function addTouchBackground(svgRoot) {
    var rect = svgRect(0, 0, '100%', '100%');
    rect.style.fillOpacity = 0.01;
    root.appendChild(rect);
}

Using groups as layers

This one needs an explanation. The render order of the svg children depends on the order in the DOM: the last one in the DOM is rendered last and thus shows above all others. If you want to have certain elements below or above others I found it helpful to use groups in svg and add to them.

function svgGroup(id) {
    var group = document.createElementNS('http://www.w3.org/2000/svg', 'g');
    if (id) {
        group.setAttribute('id', id);
    }
    return group;
}

// and later on:
document.getElementById(id).appendChild(yourElement);
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Lessons learned developing hybrid web apps (using Apache Cordova)

In the past year we started exploring a new (at leat for us) terrain: hybrid web apps. We already developed mobile web apps and native apps but this year we took a first step into the combination of both worlds. Here are some lessons learned so far.

Just develop a web app

after all the hybrid app is a (mobile) web app at its core, encapsulating the native interactions helped us testing in a browser and iterating much faster. Also clean architecture supports to defer decisions of the environment to the last possible moment.

Chrome remote debugging is a boon

The tools provided by Chrome for remote debugging on Android web views and browser are really great. You can even see and control the remote UI. The app has some redraw problems when the debugger is connected but overall it works great.

Versioning is really important

Developing web apps the user always has the latest version. But since our app can run offline and is installed as a normal Android app you have to have versions. These versions must be visible by the user, so he can tell you what version he runs.

Android app update fails silently

Sometimes updating our app only worked in parts. It seemed that the web view cached some files and didn’t update others. The problem: the updater told the user everything went smoothly. Need to investigate that further…

Cordova plugins helped to speed up

Talking to bluetooth devices? checked. Saving lots of data in a local sqlite? Plugins got you covered. Writing and reading local files? No problemo. There are some great plugins out there covering your needs without going native for yourself.

JavaScript isn’t as bad as you think

Working with JavaScript needs some discipline. But using a clean architecture approach and using our beloved event bus to flatten and exposing all handlers and callbacks makes it a breeze to work with UIs and logic.

SVG is great

Our apps uses a complex visualization which can be edited, changed, moved and zoomed by the user. SVG really helps here and works great with CSS and JavaScript.

Use log files

When your app runs on a mobile device without a connection (to the internet) you need to get information from the device to you. Just a console won’t cut it. You need log files to record the actions and errors the user provokes.

Accessibility is harder than you think

Modern design trends sometimes make it hard to get a good accessibility. Common problems are low contrast, using only icons on buttons, indiscernible touch targets, color as information bearer and touch targets that are too small.

These are just the first lessons we learned tackling hybrid development but we are sure there are more to come.

Internationalization of a React application with react-intl

For the internationalization of a React application I have recently used the seemingly popular react-intl package by Yahoo.

The basic usage is simple. To resolve a message use the FormattedMessage tag in the render method of a React component:

import {FormattedMessage} from "react-intl";

class Greeting extends React.Component {
  render() {
    return (
      <div>
        <FormattedMessage id="greeting.message"
            defaultMessage={"Hello, world!"}/>
      </div>
    );
  }
}

Injecting the “intl” property

If you have a text in your application that can’t be simply resolved with a FormattedMessage tag, because you need it as a string variable in your code, you have to inject the intl property into your React component and then resolve the message via the formatMessage method on the intl property.

To inject this property you have to wrap the component class via the injectIntl() function and then re-assign the wrapped class to the original class identifier:

import {intlShape, injectIntl} from "react-intl";

class SearchField extends React.Component {
  render() {
    const intl = this.props.intl;
    const placeholder = intl.formatMessage({
        id: "search.field.placeholder",
        defaultMessage: "Search"
      });
    return (<input type="search" name="query"
               placeholder={placeholder}/>);
  }
}
SearchField.propTypes = {
    intl: intlShape.isRequired
};
SearchField = injectIntl(SearchField);

Preserving references to components

In one of the components I had captured a reference to a child component with the React ref attribute:

ref={(component) => this.searchInput = component}

After wrapping the parent component class via injectIntl() as described above in order to internationalize it, the internal reference stopped working. It took me a while to figure out how to fix it, since it’s not directly mentioned in the documentation. You have to pass the “withRef: true” option to the injectIntl() call:

SearchForm = injectIntl(SearchForm, {withRef: true});

Here’s a complete example:

import {intlShape, injectIntl} from "react-intl";

class SearchForm extends React.Component {
  render() {
    const intl = this.props.intl;
    const placeholder = intl.formatMessage({
        id: "search.field.placeholder",
        defaultMessage: "Search"
      });
    return (
      <form>
        <input type="search" name="query"
               placeholder={placeholder}
               ref={(c) => this.searchInput = c}/>
      </form>
    );
  }
}
SearchForm.propTypes = {
  intl: intlShape.isRequired
};
SearchForm = injectIntl(SearchForm,
                        {withRef: true});

Conclusion

Although react-intl appears to be one of the more mature internationalization packages for React, the overall experience isn’t too great. Unfortunately, you have to litter the code of your components with dependency injection boilerplate code, and the documentation is lacking.

Arbitrary 2D curves with Highcharts

Highcharts is a versatile JavaScript charting library for the web. The library supports all kinds of charts: scatter plots, line charts, area chart, bar charts, pie charts and more.

A data series of type line or spline connects the points in the order of the x-axis when rendered. It is possible to invert the axes by setting the property inverted: true in the chart options.

var chart = $('#chart').highcharts({
  chart: {
    inverted: true
  },
  series: [{
    name: 'Example',
    type: 'line',
    data: [
      {x: 10, y: 50},
      {x: 20, y: 56.5},
      {x: 30, y: 46.5},
      // ...
    ]
  }]
});

line-chart-inverted

Connecting points in an arbitrary order

Connecting the points in an arbitrary order, however, is not supported by default. I couldn’t find a Highcharts plugin which supports this either, so I implemented a solution by modifying the getGraphPath function of the series object:

var series = $('#chart').highcharts().series[0];
var oldGetGraphPath = series.getGraphPath;
Object.getPrototypeOf(series).getGraphPath = function(points, nullsAsZeroes, connectCliffs) {
  points = points || this.points;
  points.sort(function(a, b) {
    return a.sortIndex - b.sortIndex;
  });
  return oldGetGraphPath.call(this, points, nullsAsZeroes, connectCliffs);
};

The new function sorts the points by a new property called sortIndex before the line path of the chart gets rendered. This new property must be assigned to each point object of the series data:

series.setData([
  {x: 10, y: 50, sortIndex: 1},
  {x: 20, y: 56.5, sortIndex: 2},
  {x: 30, y: 46.5, sortIndex: 3},
  // ...
], true);

Now we can render charts with points connected in an arbitrary order like this:

A line chart with points connected in arbitrary order

A line chart with points connected in arbitrary order

Modern developer #3: Framework independent JavaScript architecture

Usually small JavaScript projects start with simple wiring of callbacks onto DOM elements. This works fine when it the project is in its initial state. But in a short time it gets out of hand. Now we have spaghetti wiring and callback hell. Often at this point we try to get help by looking at adopting a framework, hoping to that its coded best practices draw us out of the mud. But now our project is tied to the new framework.
In search of another, framework independent way I stumbled upon scalable architecture by Nicholas Zakas.
It starts by defining modules as independent units. This means:

  • separate JavaScript and DOM elements from the rest of the application
  • Modules must not reference other modules
  • Modules may not register callbacks or even reference DOM elements outside their DOM tree
  • To communicate with the outside world, modules can only call the sandbox

The sandbox is a central hub. We use a pub/sub system:

sandbox.publish({type: 'event_type', data: {}});

sandbox.subscribe('event_type', this.callback.bind(this));

Besides being an event bus, the sandbox is responsible for access control and provides the modules with a consistent interface.
Modules are started and stopped (in case of misbehaving) in the application core. You could also use the core as an anti corruption layer for third party libraries.
This architecture gives a frame for implementation. But implementing it raises other questions:

  • how do the modules update their state?
  • where do we call the backend?

Handling state

A global model would reside or be referenced by the application core. In addition every module has its own model. Updates are always done in application event handlers, not directly in the DOM event handlers.
Let me illustrate. Say we have a module with keeps track of selected entries:

function Module(sandbox) {
  this.sandbox = sandbox;
  this.selectedEntries = [];
}

Traditionally our DOM event handler would update our model:

button.on('click', function(e) {
  this.selectedEntries.push(entry);
});

A better way would be to publish an application event, subscribe the module to this event and handle it in the application event handler:

this.sandbox.subscribe('entry_selected', this.entrySelected.bind(this));

Module.prototype.entrySelected = function(event) {
  this.selectedEntries.push(event.entry);
};

button.on('click', function(e) {
  this.sandbox.publish({type: 'entry_selected', entry: entry});
});

Other modules can now listen on selecting entries. The module itself does not need to know who selected the entry. All the internal communication of selection is visible. This makes it possible to use event sourcing.

Calling the backend

No module should directly call the backend. For this a special module called extension is used. Extensions encapsulate cross cutting concerns and shield communication with other systems.

Summary

This architecture keeps UI parts together with their corresponding code, flattens callbacks and centralizes the communication with the help of application events and encapsulates outside communication. On top of that it is simple and small.

ECMAScript 6 is coming

ECMAScript is the standardized specification of the scripting language that is the basis for JavaScript implementations in web browsers.

ECMAScript 5, which was released in 2009, is in widespread use today. Six years later in 2015 ECMAScript 6 was released, which introduced a lot of new features, especially additions to the syntax.

However, browser support for ES 6 needed some time to catch up.
If you look at the compatibility table for ES 6 support in modern browsers you can see that the current versions of the main browsers now support ES 6. This means that soon you will be able to write ES 6 code without the necessity of a ES6-to-ES5 cross-compiler like Babel.

The two features of ES 6 that will change the way JavaScript code will be written are in my opinion: the new class syntax and the new lambda syntax with lexical scope for ‘this’.

Class syntax

Up to now, if you wanted to define a new type with a constructor and methods on it, you would write something like this:

var MyClass = function(a, b) {
	this.a = a;
	this.b = b;
};
MyClass.prototype.methodA = function() {
	// ...
};
MyClass.prototype.methodB = function() {
	// ...
};
MyClass.prototype.methodC = function() {
	// ...
};

ES 6 introduces syntax support for this pattern with the class keyword, which makes class definitions shorter and more similar to class definitions in other object-oriented programming languages:

class MyClass {
	constructor(a, b) {
		this.a = a;
		this.b = b;
	}
	methodA() {
		// ...
	}
	methodB() {
		// ...
	}
	methodC() {
		// ...
	}
}

Arrow functions and lexical this

The biggest annoyance in JavaScript were the scoping rules of this. You always had to remember to rebind this if you wanted to use it inside an anonymous function:

var self = this;
this.numbers.forEach(function(n) {
    self.doSomething(n);
});

An alternative was to use the bind method on an anonymous function:

this.numbers.forEach(function(n) {
    this.doSomething(n);
}.bind(this));

ES 6 introduces a new syntax for lambdas via the “fat arrow” syntax:

this.numbers.forEach((n) => this.doSomething(n));

These arrow functions do not only preserve the binding of this but are also shorter. Especially if the function body consists of only a single expression. In this case the curly braces and and the return keyword can be omitted:

// ES 5
numbers.filter(function(n) { return isPrime(n); });
// ES 6
numbers.filter((n) => isPrime(n));

Wide-spread support for ECMAScript 6 is just around the corner, it’s time to familiarize yourself with it if you haven’t done so yet. A nice overview of the new features can be found at es6-features.org.

Recap of the Schneide Dev Brunch 2016-04-10

brunch64-borderedLast sunday, we held another Schneide Dev Brunch, a regular brunch on the second sunday of every other (even) month, only that all attendees want to talk about software development and various other topics. In case you miss the recap article about the february brunch: It didn’t happen. We all took a break, but are on track again. So if you bring a software-related topic along with your food, everyone has something to share. We were quite a lot of developers this time, so we had enough stuff to talk about. As usual, a lot of topics and chatter were exchanged. This recapitulation tries to highlight the main topics of the brunch, but cannot reiterate everything that was spoken. If you were there, you probably find this list inconclusive:

Why software development conferences?

We began with a curious question: Why are there even conferences about software development? You can read most of the content for free on the internet and even watch the talks afterwards. So why attend one for a lot of money? We discussed the topic a bit and came up with an analysis:
There are (at least) four different interested groups in a conference:

  • The organizer or commercial host is mostly interested in a positive revenue. As long as there’s a possibility for some net gain, somebody will host a conference. The actual topic is a secondary matter for them (this might explain some of the weirder conferences out there, like the boring conference).
  • The developers that really attend a conference are a small subset of all developers. They all have their own personal motives to pay money and invest time and inconviences to be there in person. Some might rely on the quality filter of a conference board, some are looking forward to meet their peers in an annual ritual. There might be those that can learn best if somebody talk-feeds them the topic. Whatever reason, a lot of developers enjoy participating at conferences. If it happens to be paid by the employer and booked as worktime, who would not?
  • Then there are the speakers. They have the additional burden to convince a committee of their topic, prepare a talk of high quality and be able to perform on stage (something that is harder than it looks). The speakers seek reputation and credible proof of expertise. His resume will probably profit, too.
  • And at last, the companies that sponsor the conference, maintain a booth with big roll-ups and smiling employees and give their developers a chance to attend are in the game to represent, to recruit and build their brand. A lot of traditional marketing effort goes into trade fairs, so why not treat the developer market like any other and be present in the developer fairs?

We can conclude that software development conferences can provide value for every associated stakeholder. As long as this sentence holds true, conferences will be held.
The question didn’t came out of the blue: one of our attendees got accepted as a speaker on the Karlsruher Entwicklertag 2016 and wanted to learn about the different expectations he needs to address. He will give his talk on the next Dev Brunch to practice the flow and to pass the hardest critics. The topic: git internals. We are thrilled!

Stratagems and strategies

The next topic contained another talk, not at a conference, but in the context of a “general topics” series at a local university (the Duale Hochschule in Karlsruhe). The talk introduces the concept of the 36 stratagems and of modern strategies to the audience. We talked a bit about the concept itself and found that the list of logical fallacies is somewhat similar. We even found an application of the stratagems in local history (sorry, only german source found): The Bretten’s Hundle
The talk itself is this monday, so you’ll need to hurry if you want to attend.

Psychology of deception

As often during the dev brunch, one topic led to the other, and we soon talked about morale and ethics. The concept of micro-expressions to reveal the hidden agenda of others came up, as well as the TV series “lie to me” that is inspired by the work of Paul Ekman, a professor of psychology. There even is a commercial training program to improve your skill of “spotting the liar”.

Games with morale aspects

Well, we are nerds. While crime investigation is thrilling, there is the even more enthralling topic of games with psychological and moralistic aspects. We soon exchanged our experiences with games like “Haze” or “Spec Ops: The Line”. But it doesn’t stop at shooter games, you can have similar insights by playing “Papers, Please” (a strong favorite for one of our next Schneide game nights) or “This War Of Mine”. You can even try some multiplayer games specifically designed for social insights, like “The Ship: Murder Party”.
And if you haven’t got much time but still want to learn something about yourself, little games like “60 Seconds!” are a great start.
This topic lead to some ideas for upcoming Schneide game nights in 2016.

Book review: A tour of C++

One attendee of the brunch provided a summary of the book “A Tour of C++” from Bjarne Stroustrup, that recently got updated to the language possibilities of C++ 11. In his words, the book is a rather incomplete introduction to the language, with way too many aspects described in a way too short manner. It’s more of a reading list to really grasp the concepts, so it may serve as a source of inspiration. For example, the notion of “move semantics” was explained, but to discover the consequences is up to the developer. The part about template programming was well done and every chapter has a suitable list of advices in the tradition of “Effective XYZ” at the end. So it’s not a bad book, but too short to be satisfying. It’s like a tourist’s tour around C++ 11, so the title holds its promise.

The left-pad incident

When we finished the “official” agenda, the topic of the recent left-pad incident came up and left us laughing. We really live in glorious times when the happiness of the (Javascript) world depends on a few lines of code. Not that this couldn’t happen in any other ecosystem.

Epilogue

As usual, the Dev Brunch contained a lot more chatter and talk than listed here. The number of attendees makes for an unique experience every time. We are looking forward to the next Dev Brunch at the Softwareschneiderei. And as always, we are open for guests and future regulars. Just drop us a notice and we’ll invite you over next time.