Fast mobile web apps
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Fast mobile web apps

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  • 1. Gran Sasso Science Institute + Università degli Studi dell’Aquila Ivano Malavolta Fast mobile web apps
  • 2. Intro You will leave this presentation with... an understanding of what makes a mobile web app slow techniques to improve the performance of your mobile web 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
  • 3. Roadmap •  Use the DOM efficiently •  Master events •  Be smart with the network •  Take care of memory issues •  Take advantage of CSS3 features The best practices and advices in this presentation are implemented in a generic app template available here: https://github.com/iivanoo/cordovaboilerplate
  • 4. Always “cache” elements from the DOM every time you do $(‘id’) the browser must parse the whole DOM
  • 5. 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
  • 6. 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
  • 7. 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
  • 8. 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
  • 9. Avoid to interact with the DOM too often Every time you append a node in the DOM, a reflow operation is triggered
  • 10. Prefer built-in JavaScript methods Under the lines, all JS frameworks end up in calling standard JavaScript methods à when it’s possible, prefer JavaScript built-in methods to pass through a framework Many frameworks contain a lot of workarounds and fallbacks for older browsers that we are not targeting (e.g., Internet Explorer 7)
  • 11. 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.parentNode.removeChild(element); — remove the node from the DOM 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
  • 12. Examples of built-in JS methods 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
  • 13. Avoid using Regular expressions A complex Regex can be a performance killer à when it’s possible, prefer HTML5 form validation attributes or String operations
  • 14. Avoid using Regular expressions If the input of a form is known a priori, use HTML5 input types http://mobiforge.com/design-development/html5-mobile-web-forms-and-input-types date tel number
  • 15. Roadmap •  Use the DOM efficiently •  Master events •  Be smart with the network •  Take care of memory issues •  Take advantage of CSS3 features
  • 16. Events name of the event callback function manage the event in the capture phase data about the event
  • 17. Capturing and bubbling When an event is triggered in the DOM, it will always start from the ROOT, and then it can be: captured by all the elements containing the target element à event capturing captured first by the target and then BUBBLE up through all the HTML elements containing the target àevent bubbling http://www.kirupa.com/html5/event_capturing_bubbling_javascript.htm
  • 18. Event delegation Delegation The act of establishing event handlers at higher levels in the DOM than the items of interest
  • 19. Event throttling delay number of milliseconds function the function to be executed •  Useful when handling events with very high frequencies and whose execution rate must be limited ex. drag, scrolling, etc. Throttling limits the execution rate of the function
  • 20. Event debouncing delay number of milliseconds function the function to be executed •  Useful when handling events with very high frequencies and that must be executed once ex. toggle state, Ajax request, etc. Debouncing guarantees that the function will be executed only once
  • 21. Throttling VS debouncing Throttling Debouncing Ben Alman’s famous implementations available here: http://benalman.com/projects/jquery-throttle-debounce-plugin/
  • 22. Roadmap •  Use the DOM efficiently •  Master events •  Be smart with the network •  Take care of memory issues •  Take advantage of CSS3 features
  • 23. Network usage •  Try to prefetch data as much as possible (possibly analysing their results in 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 unpredictable and memory consuming resource you have http://fgnass.github.io/spin.js
  • 24. Roadmap •  Use the DOM efficiently •  Master events •  Be smart with the network •  Take care of memory issues •  Take advantage of CSS3 features
  • 25. Take care of memory issues True, it automatically cleans up and deallocates memory, but it must be sure about what it is deleting forever à you have to make it clear what code you are no longer using Definition of memory leak 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?
  • 26. Meet your best friend!
  • 27. The retaining tree The garbage collector cleans up the 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
  • 28. Rules of thumb Better than de-referencing 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 local scope Unbind event listeners Manage local cache http://slid.es/gruizdevilla/memory The key is to always have an healthy retaining tree
  • 29. 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 à no relevant memory leaks J 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
  • 30. Examples of pattern http://goo.gl/UAZQvl No memory leak Memory leak detected NOTHING IS FREE It’s normal that during the investigation your memory grows You have to pay for what you are doing in your app
  • 31. 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 it 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
  • 32. 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 directly 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 object is still indirectly referencing it
  • 33. 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 the WHOLE tree under #tree can be GCed.
  • 34. Roadmap •  Use the DOM efficiently •  Master events •  Be smart with the network •  Take care of memory issues •  Take advantage of CSS3 features
  • 35. Rule of thumb Corollaries: •  Don’t use JavaScript for UI-related operations •  Don’t use images for something you can do with CSS CSS3 has many interesting features, like: •  text manipulations & effects •  visual effects •  media queries •  ... If you can do something with CSS, do it!
  • 36. Text transforms text-transform none | capitalize | lowercase | uppercase
  • 37. Text effects 2px à horizontal shadow 10px à vertical shadow 5px à blur distance #FF0000 à color
  • 38. Visual effects Three main mechanisms: 1.  Transforms (both 2D and 3D) 2.  Transitions 3.  Animations
  • 39. 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
  • 40. Transforms http://bit.ly/IrpUnX
  • 41. Transforms http://bit.ly/IrpUnX
  • 42. 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 initial state final state Intermediate states automatically calculated time
  • 43. Transition example Properties: property – name of a CSS property duration – #seconds to complete the transition timing-­‐function - linear, ease, … delay-­‐ when the transition effect will start
  • 44. 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. initial state finalstate Intermediate states automatically calculated time
  • 45. Example of animation result in http://goo.gl/ejp4Fy
  • 46. 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
  • 47. 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
  • 48. Examples Retina Displays iPad in landscape orientation iPhone and Android devices
  • 49. References https://developer.mozilla.org/en-US/docs/JavaScript/Guide https://github.com/iivanoo/cordovaboilerplate
  • 50. + 39 380 70 21 600 Contact Ivano Malavolta | Gran Sasso Science Institute iivanoo ivano.malavolta@univaq.it www.ivanomalavolta.com