Events & Callbacks(ESaaS §6.5)

© 2013 Armando Fox & David Patterson, all rights reserved

Events

• What: occurrences that affect the user interface– User interacts with a page element– Previously-set timer expires– Data received from server in response to AJAX

call (later)• Events are usually (not always) associated

with a DOM element• Bind a handler (function) for a particular

event type on one or more elements2

Manipulating the DOM Using Events: Overview

1. Identify element(s) you will want to do stuff to, make sure they’re conveniently selectable using $()

2. Similarly, identify element(s) on which interactions will trigger stuff to happen

3. Write handler function(s) that cause the desired stuff to happen

4. In a setup function, bind the handlers (third way to call $() )

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2.5. Create failing test for handler using TDD

http://pastebin.com/RSVXzgLR

Handlers on Buttons & Links

• A good summary of recognized handlers:http://www.chipchapin.com/WebTools/JavaScript/example1-04.html

• What about links & buttons that are clickable without JavaScript?– handler runs first– if handler returns false, no other action taken– otherwise, other actions follow handler– example use: client-side form validation

Summary: jQuery & DOM

• Select elements with $() (or wrapto give them secret jQuery powers)

• Inspect them…

text() or html()is(:checked), is(:selected), etc.

attr('src')• Add/remove CSS classes, hide/show• Create setup function that binds handler(s) on

element(s)– common ones: onclick, onsubmit, onchange– Pass func to $() (alias for document.ready())

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Browser will complain, but only when form’s Submit button clicked

The form will be submitted, but without inputs being checked

Nothing will happen when submit button is clicked (form won’t be submitted)

Browser will complain about malformed HTML when page is loaded (server should respect browser version and not send JavaScript)

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<form onSubmit='checkInputs()'> <input type="text"/> <input type="submit"/></form>If this form is loaded in a non-JS-aware browser:

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AJAX: Asynchronous JavaScript and XML

(ESaaS §6.6)

© 2013 Armando Fox & David Patterson, all rights reserved

AJAX ==Asynchronous Javascript And XML

• JSAPI call XmlHttpRequest (a/k/a xhr) contacts server asynchronously (in background) and without redrawing page – Normal HTTP request, w/special header:X-Requested-With: XmlHttpRequest

• Controller action receives request via route• What should it render in response?

render :layout => falserender :partial => 'movies/show'

render :json => @movies (calls to_json)render :xml => @movies (calls to_xml)render :text => @movie.title render :nothing => true

The Basic AJAX Cycle, As Seen From Browser JSAPI

r = new XmlHttpRequest;r.open(method,URL,async);

method GET,POST,HEAD,PUT,DELETE...async: true means script should not block (important!)

r.send("request content");r.abort();

• Callbacks during XHR processingr.onReadyStateChange=function(XmlHttpRequest r) { ... }

– function inspects r.readyState uninitialized,open, sent,receiving,loaded

• r.status contains HTTP status of response• r.responseText contains response content string

The jQuery Way

$.ajax({type: 'GET', url: URL, timeout: milliseconds, success: function, error: function

// many other options possible });

Rails Cookery:AJAX with Rails+jQuery

• javascript_include_tag 'application'• your code in app/assets/javascripts/*.js• Define handler function that...

– optionally inspects element state, attributes, …– calls $.ajax()

• Define controller action & route to receive request, and determine what will be returned

• Define callback function to receive server reply– Unpack JSON objects & modify DOM?– Replace existing HTML element (e.g. <div>) in place?– Add/change/remove CSS classes/properties?

A Simple Example

Server side:

class MoviesController < ApplicationController def show @movie = Movie.find(params[:id]) render :partial=>'movie', :object=>@movie if request.xhr? # else render default (show.html.haml) endend

Client side:14

http://pastebin.com/zZPKvmVW

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In general, the server must rely on explicit hints (like headers) to detect XHRThe response to an AJAX request can be any content type (not just HMTL)If the server fails to respond to an XHR request, the browser’s UI will freeze

AJAX requests can be handled with their own separate controller actions

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Which is FALSE concerning AJAX/XHR vs. non-AJAX interactions?

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Intro to Jasmine: TDD for JavaScript and AJAX

(ESaaS §6.7)

© 2013 Armando Fox & David Patterson, all rights reserved

Jasmine: Testing JavaScript

• Idea: run your tests in browser– Install jasmine gem & jasmine-jquery add-on– rails generate jasmine:install

• Jasmine “server” (rake jasmine) on localhost:8888 serves Jasmine test “page”– Reload page == rerun tests!

• How to construct tests?• How do tests “see” the page on which they

would normally operate?

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Jasmine is Like RSpec

• Sets of related examplesdescribe "things" do it "..." do...endend describe("things", function() { it("...", function() { ... } );});

• Setup/teardownbefore(:each) do...end beforeEach(function() {...})

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Expectations

expect {...}.to == "foo" expect(expr).toEqual("foo") .toBeTruthy(),.toBeFalsy() .toBeHidden() .toHaveClass() .toContainText()

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Expectations: Jasmine-jQuery

toBeSelected(), toBeChecked()toBeDisabled(), toBeVisible(), toBeHidden()toHaveClass("foo"), toHaveId("foo") toHaveAttr("href", "http://saasbook.info")

• Putting it together example:describe('Clicking Hide button', function() { it('hides Movie div', function() { $('a#hide').trigger('click'); expect($('div#movie')).toBeHidden(); });});

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(a) & (c)

(b) & (c)

(a), (b) & (c)

(a) & (b)☐

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Which are always true of Jasmine’s it() method:(a) it can take a named function as its 2nd arg(b) it can take an anonymous function as its 2nd arg(c) it executes asynchronously

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More Jasmine:Spies & Fixtures

(ESaaS §6.7)

© 2013 Armando Fox & David Patterson, all rights reserved

Stubs (Spies)

• Strategy: create “spy” method that replaces real method that is a property of an object

spyOn(MoviePopup,'new').andReturn(value) .andCallThrough() .andCallFake(func)

– Why no quotes around MoviePopup?

• Examine calls on spy:expect(MoviePopup.new.mostRecentCall.args). toContain("Gravity")

expect($.ajax.mostRecentCall.args[0]['url']). toEqual("/movies/1")

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HTML Fixtures

• Goal: provide enough HTML for JS code to do its thing in a familiar environment<table id="movies"> <tbody> <tr class="adult"> <td>Casablanca</td> <td>PG</td> <td><a href="/movies/1">More about Casablanca</a></td> </tr> </tbody></table>

• loadFixtures('movie_row.html') loads this into div#jasmine-fixtures (automatically cleaned out)

• Fallacies & Pitfalls for §6.7 shows how to auto-generate these fixtures from real views

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Stubbing Redux

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Putting it all together example

• Example

• Jasmine tests• (why no call to MoviePopup.setup?)

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http://pastebin.com/9xRk1CW1

http://pastebin.com/QNAuLLAt

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$.ajax doesn't actually return the server reply content, so andReturn won't workajaxArgs is in the wrong scope, so its value is not "visible" to andReturnWe needed to be able to spy on (monitor) the AJAX call

We could have done it either way; just a matter of choice

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In the example, why did we use andCallFake() to pass ajaxArgs in stubbing the AJAX call, rather than just andReturn(ajaxArgs) ?

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Single-Page Apps and JSON APIs

(ESaaS §6.8)

© 2013 Armando Fox & David Patterson, all rights reserved

Single-Page Apps

• Load page once; no further page reloads, since everything else done via AJAX

• Uses same MVC flows we already know!– but rendered content from server is handed to

your JS code, not to browser• So, what should controller method render?

– Certainly not a full HTML page– HTML snippet? incorporate into page– “Raw” text? hard to send structured data– XML or JSON!

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A Simple Example

Server side:

class MoviesController < ApplicationController def show @movie = Movie.find(params[:id]) render :json => @movie if request.xhr? # else render default (show.html.haml) endend

Client side:

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http://pastebin.com/QNAuLLAt

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In an association such as Movie has-many Reviews, the owned objects must be returned in 1 or more separate JSON objects

JSON objects can only be consumed by a JavaScript-capable clientNone of the above are true

A JSON object’s properties must exactly match the corresponding ActiveRecord model

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Which, if any, of the following statements are TRUE regarding JSON objects in Rails apps?

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And In Conclusion:JavaScript Wrapup

(ESaaS §6.9-6.11)

© 2012 Armando Fox & David PattersonLicensed under

Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License

Emerging JS Use Cases

• Server-side frameworks: Node.js– JS library for creating server-side apps– Uses event-driven programming in JS– Potentially much more memory-efficient per user

compared to traditional server-side frameworks– LinkedIn – Node.js 20x faster than Rails

• Client-side frameworks: Backbone.js– Express views & models entirely in JS– Server just returns JSON

• Client-side frameworks: Yahoo Mojito• CoffeeScript

http://pastebin.com/JJ9rEBDz

JavaScript & Performance

• The browser is increasingly the “client-side OS” that matters

• CSS has made layout increasingly sophisticated

• JavaScript + AJAX has made UI increasingly rich & desktop-like

• What’s the cost of all this?

Where Does the Time Go?

• CSS selectors + JavaScript interpreter = 41% of total client rendering time– Esp. selectors that require walking

DOM tree, e.g. div > li

• Browsers compete on speed of JavaScript interpreter => selector/parser performance increasingly important

• Use GPU for rendering

parserDOM CSS rules

rendering

21%

23%

12%

29%

9%

JavaScript interpreter

layout engine

HTML parser

selector engine

rendering

JavaScript interpreter

Courtesy Leo Meyerovich, UC Berkeley

Pitfall: “Adding JavaScript will make my app more responsive”

• Risk: too-wide API to server, with needlessly expensive database calls

• Risk: at mercy of JS interpreter on a wide variety of devices

• Risk: testing/debugging is more complicated, since must handle client-side errors

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Slow database queries associated with AJAX actionsNetwork delays due to sending too much data to browser in AJAX responsesCan’t tell without further information

Slow JavaScript interpreter (for example, users with very old browsers/computers)

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Some of the AJAX interactions in Ben Bitdiddle’s AJAX-intensive SaaS app “feel sluggish”. The likeliest cause is:

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