Download - Fast Cordova applications
DISIM | University of L’Aquila
Ivano Malavolta
Fast Cordova applications
Roadmap
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
I implemented many best practices and advices in this presentation in a generic app template available here:
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
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
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
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
Reflow: the browser process for calculating positions and geometries for HTML DOM elements
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
Document HTML Body
Element
Element
Element
Element
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
Do this
Don’t be tempted by the jQuery’s omni-present $("selector"), it is much more slower than JS built-in methods
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 • color
http://mobiforge.com/design-development/html5-mobile-web-forms-and-input-types
date tel number
Roadmap
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
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
callback function
manage the event in the capture phase
data about the event
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
the element in the DOM
TouchEvent.changedTouches
a TouchList of all the Touches changed between this event and the previous one
touchstart touchend touchmove touchenter touchcancel
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
Don’t do this
Do this
Delegation The act of establishing event handlers at higher levels in the DOM than the items of interest
TIP Unbind event handlers as soon as possible so that they can be garbage collected
WHY Using a lot of event handlers may lead to performance problems and they can be sources of memory leaks
Event throttling
delay
number of milliseconds
function
the function to be executed
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.
You get a new function, that when called repetitively, executes the original function no more than once every delay milliseconds.
Event debouncing
delay
number of milliseconds
function
the function to be executed
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.
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
Throttling VS debouncing
Throttling
Debouncing
Ben Alman’s famous implementations available here: http://benalman.com/projects/jquery-throttle-debounce-plugin/
Roadmap
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
Network usage
• 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
The network is the most umpredictable and memory consuming resource you have
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 respond receive message response from the worker
Roadmap
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
Rule of thumb
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
If you can do something with CSS, do it!
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 manipulations
text-transform none | capitalize |
lowercase | uppercase
text-align left | right
center | justify
Text-decoration none
underline overline
line through
Text effects
2px horizontal shadow
10px vertical shadow
5px blur distance
#FF0000 color
Other text properties
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
Flex Box main elements
box-pack
Values: start | end | center | justify
Sets the alignment of the box along the box-orient axis
Flex Box main elements
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-style normal
italic oblique
font-weight normal
bold 100 200
…
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
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
Transition example
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
Roadmap
• Introduction
• Use the DOM efficiently
• Master events
• Be smart with the network
• Take advantage of CSS features
• Take care of memory issues
Take care of memory issues
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
Wait, why should I worry if JavaScript has a garbage collector that automatically cleans my memory?
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
Better than de-referencing, use local scopes Unbind events that are no longer needed, specially if the related DOM objects are going to be removed Be careful with storing large chunks of data that you are not going to use
Use appropriate scope
Unbind event listeners
Manage local cache
http://slid.es/gruizdevilla/memory
The key is to always have an healthy retaining tree
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
http://goo.gl/UAZQvl
Examples of pattern http://goo.gl/UAZQvl
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)
http://goo.gl/UAZQvl
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
the size of memory that can be freed once an object is deleted
Yellow DOM nodes
Red DOM nodes Detached DOM trees, no JS object is referencing them, but they are alive
some JS object is referencing them
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
Example http://goo.gl/UAZQvl
#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