Fast Cordova applications This document provides techniques for improving the performance of Cordova applications. It discusses ways to use the DOM efficiently by caching elements, minimizing reflows, and interacting with the DOM less frequently. It also covers optimizing events, network usage, CSS features, and memory management. The overall goal is to understand what makes a Cordova app fast and provide techniques to improve performance.

1. Fast Cordova applications
Ivano Malavolta
DISIM | University of L’Aquila

2. Roadmap
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I implemented many best practices and advices in this
presentation in a generic app template available here:
Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues
https://github.com/iivanoo/cordovaboilerplate

3. Introduction
You will leave this presentation with...
understanding of what makes a Cordova app fast
techniques to improve the performance of your Cordova apps
In any case, step zero to success is to be technologically ready,
for example many people tend to underestimate JavaScript, don't!
http://eloquentjavascript.net

4. Roadmap
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Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues

5. Always “cache” elements from the DOM
every time you do $(‘id’) the browser must parse the whole DOM
Don’t do this
Do this

6. Minimize DOM reflows
Reflow:
the browser process for calculating positions and geometries
for HTML DOM elements
A reflow is triggered every time the content of the DOM changes, DOM elements are resized, CSS
positioning/padding/margins are changed, etc.
• Use CSS transforms, transitions and animation
• Use fixed widths and heights (when possible)
• Avoid changing elements of the DOM
They change the appearance of the DOM, but do not
trigger a reflow op

7. Keep your DOM slim
Reflow operations are much heavier when
• the number of nodes in the DOM is large
• there are deeply nested DOM nodes
Element
Element
Element
Element
Document
HTML
Body
Element
Element
Element
Element
Element

8. Navigate the DOM with built-in methods
Avoid to continuously query the DOM if you can reach specific DOM nodes using its built-in methods
Don’t do this
Do this

9. Examples of built-in JS properties for navigation
element.parentNode — returns the node containing the element in the DOM
element.childNodes; — returns all the nested nodes of the element in the DOM
element.firstChild — retrieves the first nested node of the element in the DOM
element.lastChild — retrieves the last nested node of the element in the DOM
element.nextSibling — returns the node immediately following the element in the DOM
element.previousSibling — returns the node immediately preceding the element in the DOM

10. Avoid to interact with the DOM too often
Every time you append a node in the DOM, a reflow operation is triggered
Don’t do this
Do this

11. Prefer built-in JavaScript methods
Under the lines, all the JS frameworks end up in calling standard JavaScript methods
 when it’s possible, prefer JavaScript built-in methods to pass through a framework
Don’t do this
Don’t be tempted by the jQuery’s omni-present
$("selector"), it is much more slower than JS built-in
methods
Do this
Many frameworks contain a lot of workarounds and fallbacks for older
browsers that we are not targeting (e.g., Internet Explorer 7)

12. Examples of built-in JS methods
element.innerHTML; — returns the contents of the DOM node
element.innerHTML = " contents "; — appends contents to the DOM node
element.hasAttribute(" attribute") — tests whether the DOM node has a specific attribute
element.getAttribute(" attribute") — returns the value of a specific attribute
element.setAttribute(" name", " value ") — adds a specific attribute to the DOM node
element.removeAttribute(" attribute") —removes a specific attribute to the DOM node

13. Examples of built-in JS methods
domNode.innerHTML; — returns the contents of the DOM node
domNode.innerHTML = " contents "; — appends contents to the DOM node
domNode.parentNode.removeChild(domNode); — remove the node from the DOM
element.classList.add() — adds a specific class to the DOM
element.classList.remove() — adds a specific class to the DOM
element.classList.toggle() — adds a specific class to the DOM
... and many more

14. Try to avoid using Regular expressions
When used frequently with large inputs a Regex can be a performance killer
 when it’s possible, prefer HTML5 form validation attrib utes or String operations
Don’t do this
Do this

15. Try to avoid using Regular expressions
If the input is of a form input is knowna priori, use one of the following:
• date
• datetime
• datetime-local
• email
• month
• number
• range
• search
• tel
• time
• url
• week
date
tel
• color
http://mobiforge.com/design-development/html5-mobile-web-forms-and-input-types
number

16. Roadmap
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Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues

17. Events
Every time the user interacts with the DOM, a set of events is triggered in your JS application
We can listen to these events by means of registered eventHandlers
An eventHandler is a function automatically called by the browser, where data about the triggered
event is available as a parameter
Event handlers can be unregistered

18. Events example
name of the envet
manage the event in
the capture phase
data about the event
callback function

19. Touch events
Touch events are triggered when the user touches the display
The event can describe one or more points of contact
Touches are represented by the Touch object containing:
• position
• size and shape
• amount of pressure
• target element
Lists of touches are represented by TouchList objects

20. Touch events
Main attributes of a touch event:
TouchEvent.touches
a TouchList of Touches
TouchEvent.type
the type of touch
TouchEvent.target
touchstart
touchend
touchmove
touchenter
touchcancel
the element in the DOM
TouchEvent.changedTouches
a TouchList of all the Touches changed between this event and the previous one

21. The Touch and TouchList objects
relative to the
whole display
relative to the
viewport

22. Event default behaviour
Each element in the DOM has a default behaviour
ex.
if you tap on an <a> element, it will make the browser to point to another location
event.preventDefault();
Cancels the event if it is cancellable, without stopping further propagation of the event
Usually, this is the last instruction of an event handler

23. Capturing and bubbling
When an event is triggered in the DOM, it can be:
captured by all the elements containing the target element
 event capturing
captured first by the target and then BUBBLE up through all
the HTML elements containing the target
event bubbling

24. Event delegation
Delegation
The act of establishing event handlers at higher levels in the DOM than the items of interest
Don’t do this
Do this
WHY
Using a lot of event handlers may lead
to performance problems and they can
be sources of memory leaks
TIP
Unbind event handlers as soon as possible so that they can be
garbage collected

25. Event throttling
delay
number of milliseconds
function
the function to be executed
You get a new function, that when called repetitively,
executes the original function no more than once every
delay milliseconds.
Given a specific delay, throttling limits the execution rate of the function
• Useful when
handling events with very high frequencies and whose execution rate must be limited
ex. drag,scrolling, etc.

26. Event debouncing
delay
number of milliseconds
function
the function to be executed
You get a new function, that when called repetitively, executes
the original function just once per “bunch” of calls, effectively
coalescing multiple sequential calls into a single execution at
either the beginning or end
Given a specific delay, debouncing guarantees that the function will be executed only once
• Useful when
handling events with very high frequencies and that must be executed once
ex. toggle state, Ajax request, etc.

27. Throttling VS debouncing
Throttling
Debouncing
Ben Alman’s famous implementations available here:
http://benalman.com/projects/jquery-throttle-debounce-plugin/

28. Roadmap
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•
•
•
•
•
Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues

29. Network usage
The network is the most umpredictable and memory
consuming resource you have
• Try to prefetch data as much as possible (possibly using Web Workers)
• Bundle static data into the app
• In any case, give visual feedback to the user when accessing the network
http://fgnass.github.io/spin.js

30. Network usage
• Be robust with respect to 404 errors, especially for images
• Try to avoid synchronous network calls

31. Web Workers
Javascript is a single-threaded language
 If a tasks take a lot of time, users have to wait
Web Workers provide background processing capabilities to web applications
They typically run on separate threads
 apps can take advantage of multicore CPUs

32. Web Workers
Web Workers can be used to:
• prefetch data from the Web
• perform other ahead-of-time operations to provide a much more lively UI.
Web Workers are precious on mobile applications because they usually need to load data over a
potentially slow network

33. Usage of web workers
Any JS file can be launched as a worker
Example of Web Worker declaration:
In order to be independent from other workers, each worker script cannot access the DOM

34. Usage of web workers
The main thread and the worker can communicate via postMessage() calls and onmessage events
send message to the worker
response from the worker
receive message
respond

35. Roadmap
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•
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•
•
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Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues

36. Rule of thumb
If you can do something with CSS, do it!
Corollaries:
• Don’t use JavaScript for style-related operations
• Don’t use images for something you can do with CSS
CSS3 has many interesting features, like:
• gradients
• text manipulations & effects
• the flex box
• fonts
• visual effects
• media queries

37. Gradients
They can be used in every place you can use an image
linear  the type of gradient (also radial, or repeating-linear)
right-top  start of the gradient
left-bottom  end of the gradient
from  starting color
to  final color

38. Text manipulations
text-align
left | right
center | justify
text-transform
none | capitalize |
lowercase | uppercase
Text-decoration
none
underline
overline
line through

39. Text effects
2px  horizontal shadow
10px  vertical shadow
5px  blur distance
#FF0000  color

40. Other text properties

41. The flex box
It helps in styling elements to be arranged horizontally or vertically
box:
• a new value for the display property
• a new property box-orient

42. Flex Box main elements
display: box
opts an element and its immediate children into the flexible box model
box-orient
Values: horizontal | vertical | inherit
how should the box's children be aligned?
box-direction
Values: normal | reverse | inherit
sets the order in which the elements will be displayed

43. Flex Box main elements
box-pack
Values: start | end | center | justify
Sets the alignment of the box along the box-orient axis

44. Flex Box main elements
box-align
Values: start | end | center | baseline | stretch
Sets how the box's children are aligned in the box

45. Flex box children
By default child elements are not flexible
 their dimension is set according to their width
The box-flex property can be set to any integer, it sets how a child element occupy the box’s space

46. Fonts
Before CSS3, web designers had to use fonts that were already installed on the user's device
With CSS3, web designers can use whatever font they like
To use the font for an HTML element, refer to the font-family property
font-weight
normal
bold
100
200
…
font-style
normal
italic
oblique

47. Iconic fonts
Iconic fonts contain ICONS instead of letters and symbols
Advantages:
• icons are now part of the CSS  you do not pollute the DOM with non-semantical elements
• fonts are vector-based  you do not have to care anymore about the resolution of your icons
• icon sets conform to the same style  you do not need to think about how your icons fit together

48. Example of iconic font
http://typicons.com

49. Visual effects
Three main mechanisms:
1. Transforms (both 2D and 3D)
2. Transitions
3. Animations

50. Transforms
A transform is an effect that lets an element change shape, size, position, …
You can transform your elements using 2D or 3D transformations
http://bit.ly/IroJ7S

51. Transforms
http://bit.ly/IrpUnX

52. Transforms
http://bit.ly/IrpUnX

53. Transitions
They are used to add an effect when changing from one style to another
The effect will start when the specified CSS property changes value
Properties:
property - the name of the CSS property the transition effect is for (can be all)
duration - how many seconds (or milliseconds) the transition effect takes to complete
timing-function - linear, ease, ease-in, ease-out, ease-in-out
delay- when the transition effect will start

54. Transition example

55. Animations
An animation is an effect that lets an element gradually change from one style to another
You can change style in loop, repeating, etc.
To bind an animation to an element, you have to specify at least:
1. Name of the animation
2. Duration of the animation

56. Animations
An animation is defined in a keyframe
It splits the animation into parts, and assign a specific style to each part
The various steps within an animation are given as percentuals
0%  beginning of the animation (from)
100%  end of the animation (to)

57. Example of animation
result in
http://goo.gl/ejp4Fy

58. Animation properties
http://bit.ly/IrpUnX

59. Transition VS animation
Trigger
Transitions must be bound to a CSS property change
Animations start autonomously
States
Transitions have start and end states
Animations can have multiple states
Repeats
Transitions can be perfomed once for each activation
Animations can be looped

60. Media types
Media Queries are based on Media Types
A media type is a specification of the actual media that is being used to access the page
Examples of media types include
• screen: computer screens
• print: printed document
• braille: for Braille-based devices
• tv: for television devices

61. Media types
There are two main ways to specify a media type:
• <link> in the HTML page
• @media within the CSS file

62. Media queries
They allow you to to change style based on specific conditions
For example, they can be about
• device’s display size
• orientation of the device
• resolution of the display
• ...
http://bit.ly/I5mR1u

63. Media queries
A media query is a boolean expression
The CSS associated with the media query expression is applied only if it evaluates to true
A media query consists of
1. a media type
2. a set of media features
@media screen and orientation: portrait

64. The full media feature list
http://slidesha.re/I5oFHZ

65. Operators
AND
to combine multiple expressions
COMMA
it acts as an OR operator
NOT
to explicitly ignore a specific type of device
ONLY
to “hide” the CSS to older browsers that can read media types but cannot handle media queries
In this case the styling information will not be visible to those browsers

66. Examples
Retina Displays
iPad in landscape orientation
iPhone and Android devices

67. Roadmap
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•
•
•
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Introduction
Use the DOM efficiently
Master events
Be smart with the network
Take advantage of CSS features
Take care of memory issues

68. Take care of memory issues
Wait, why should I worry if JavaScript has a garbage
collector that automatically cleans my memory?
True, it automatically cleans up and deallocates memory, but it must be sure about what it is deleting forever
 you have to make it clear what code you are no longer using
Definition of memoryleak
A memory leak is the situation in which the available memory of your app gets gradually lost.
In JavaScript a memory leak can happen when an object is stored in memory but cannot be accessed by the
running code

69. The retaining tree
The garbage collector cleans up he portions of tree isolated from the root node
In our apps the window object is the root of the tree
https://developers.google.com/chrome-developer-tools/docs/javascript-memory-profiling

70. Rules of thumb
Use appropriate scope
Better than de-referencing, use local scopes
The key is to always have
an healthy retaining tree
Unbind event listeners
Unbind events that are no longer needed, specially if the related DOM objects are going to be
removed
Manage local cache
Be careful with storing large chunks of data that you are not going to use
http://slid.es/gruizdevilla/memory

71. http://goo.gl/UAZQvl
How to detect a memory leak
1. Open the Memory View in the Timeline tab
2. start recording
3. execute the suspicious actions
4. during the execution force a GC different times
5. If you see a sawtooth-shaped wave
 no relevant memory leaks 
You have a memory leak if one of the following do not drop down:
used memory – number of DOM nodes – number of event handlers

72. http://goo.gl/UAZQvl
Examples of pattern
No memory leak
Memory leak detected
It’s normal that during the investigation your memory grows, nothing is free!
You have to pay for what you are doing in your app

73. http://goo.gl/UAZQvl
How to detect the source of your memory leak
1. Open the Heap Profile
2. Take a heap snapshot
3. Perform suspicious actions
4. Take a heap snapshot
5. Perform suspicious actions again
6. Take a final heap snapshot
7. Select the most recent snapshot taken
8. find the drop-down that says "All objects" and switch this to "Objects allocated between snapshots 1
and 2". (You can also do the same for 2 and 3 if needed)
There, you will find the objects which have not been collected during the snapshots.
It works because when you take a heap snapshot, Chrome forces also a GC execution.

74. How to detect the source of your memory leak
Now, start from the first object and check which references it is holding
Shallow size
the size of the object
Retain size
Detached DOM tree is a subtree of the DOM that
1) has been removed from the DOM, and
2) cannot be GCed because some JS objects is still
referencing it
the size of memory that can be freed once an object is deleted
Yellow DOM nodes some JS object is referencing them
Red DOM nodes Detached DOM trees, no JS object is referencing them, but they are alive

75. http://goo.gl/UAZQvl
Example
#leaf maintains a reference to it's parent (parentNode) and recursively up to #tree,
so only when leafRef is nullified is the WHOLE tree under #tree a candidate for GC.

76. References
https://developer.mozilla.org/en-US/docs/JavaScript/Guide
https://github.com/iivanoo/cordovaboilerplate

77. Contact
Ivano Malavolta | DISIM
+ 39 380 70 21 600
iivanoo
ivano.malavolta@univaq.it
www.ivanomalavolta.com