Fast Cordova applications

Fast Cordova applications

Ivano Malavolta
DISIM | University of L’Aquila

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

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

Roadmap
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Introduction

Use the DOM efficiently
Master events

Be smart with the network
Take advantage of CSS features

Always “cache” elements from the DOM
every time you do $(‘id’) the browser must parse the whole DOM

Don’t do this

Do this

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

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

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

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

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

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)

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

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

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

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

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

Events example
name of the envet

manage the event in
the capture phase

data about the event
callback function

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

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

The Touch and TouchList objects
relative to the
whole display

relative to the
viewport

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

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

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

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.

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.

Throttling VS debouncing
Throttling

Debouncing

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

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

Network usage
• Be robust with respect to 404 errors, especially for images

• Try to avoid synchronous network calls

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

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

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

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

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

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

Text effects

2px  horizontal shadow
10px  vertical shadow
5px  blur distance
#FF0000  color

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

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

box-pack
Values: start | end | center | justify

Sets the alignment of the box along the box-orient axis

box-align
Values: start | end | center | baseline | stretch

Sets how the box's children are aligned in the box

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

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

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

Example of iconic font

http://typicons.com

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

3. Animations

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

Transforms

http://bit.ly/IrpUnX

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

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

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)

Example of animation

result in
http://goo.gl/ejp4Fy

Animation properties

http://bit.ly/IrpUnX

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

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

Media types
There are two main ways to specify a media type:
• <link> in the HTML page

• @media within the CSS file

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

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

The full media feature list

http://slidesha.re/I5oFHZ

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

Examples
Retina Displays

iPad in landscape orientation

iPhone and Android devices

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

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

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

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


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


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.

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


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.

References

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

Contact

Ivano Malavolta | DISIM
+ 39 380 70 21 600

iivanoo
ivano.malavolta@univaq.it
www.ivanomalavolta.com

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