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chap-js_commands.lyx
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#LyX 2.0 created this file. For more info see http://www.lyx.org/
\lyxformat 413
\begin_document
\begin_header
\textclass book
\use_default_options false
\maintain_unincluded_children false
\language english
\language_package default
\inputencoding auto
\fontencoding global
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\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_amsmath 1
\use_esint 1
\use_mhchem 1
\use_mathdots 1
\cite_engine basic
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\use_refstyle 0
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header
\begin_body
\begin_layout Chapter
Lift and JavaScript
\begin_inset CommandInset label
LatexCommand label
name "cha:Lift-and-Javascript"
\end_inset
\end_layout
\begin_layout Standard
In this chapter we'll be discussing some of the techniques that Lift provides
for simplifying and abstracting access to JavaScript on the client side.
Using these facilities follows Lift's model of separating code from presentatio
n by allowing you to essentially write JavaScript code in Scala.
Lift also provides a layer that allows you to use advanced JavaScript functiona
lity via either the JQuery
\begin_inset Foot
status open
\begin_layout Plain Layout
\begin_inset CommandInset href
LatexCommand href
name "http://jquery.com/"
target "http://jquery.com/"
\end_inset
\end_layout
\end_inset
or YUI
\begin_inset Foot
status open
\begin_layout Plain Layout
\begin_inset CommandInset href
LatexCommand href
target "http://developer.yahoo.com/yui/"
\end_inset
\end_layout
\end_inset
user interface libraries.
\end_layout
\begin_layout Section
JavaScript high level abstractions
\end_layout
\begin_layout Standard
You may have noticed that Lift already comes with rich client side functionality
in the form of AJAX and COMET support (chapter
\begin_inset CommandInset ref
LatexCommand ref
reference "cha:AJAX-and-COMET"
\end_inset
).
Whenever you use this support, Lift automatically generates the proper
\family typewriter
<script>
\family default
elements in the returned page so that the libraries are included.
Lift goes one step further, however, by providing a class hierarchy representin
g JavaScript expressions.
For example, with an AJAX form element in Lift the callback method must
return JavaScript code to update the client side.
Instead of just returning a raw JavaScript string to be interpreted by
the client, you return an instance of the
\family typewriter
JsCmd
\family default
\begin_inset Foot
status open
\begin_layout Plain Layout
net.liftweb.http.js.JsCmd
\end_layout
\end_inset
trait (either directly or via implicit conversion) that is transformed
into the proper JavaScript for the client.
\end_layout
\begin_layout Standard
\family typewriter
JsCmd
\family default
represents a JavaScript command that can be executed on the client.
There is an additional
\begin_inset Quotes eld
\end_inset
base
\begin_inset Quotes erd
\end_inset
trait called
\family typewriter
JsExp
\family default
that represents a JavaScript expression.The differences between them are
not usually important to the developer, since a
\family typewriter
JsExp
\family default
instance is implicitly converted to a
\family typewriter
JsCmd
\family default
.
Also note that while Lift's JavaScript classes attempt to keep things type-safe
there are some limitations; in particular, Lift can't check semantic things
like whether the variable you're trying to access from a given
\family typewriter
JsCmd
\family default
actually exists.
Besides the obvious use in techniques like AJAX and COMET, Lift also makes
it simple to attach JavaScript to regular Scala XML objects, such as form
fields.
\end_layout
\begin_layout Standard
As a simple example, let's look at how we might add a simple alert to a
form if it doesn't validate.
In this example, we'll assume we have a
\family typewriter
name
\family default
form field that shouldn't be blank.
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Simple-Form-Validation"
\end_inset
shows a possible binding from our form snippet.
Let's break this down a bit: the first thing is that in order to reference
form elements (or any elements for that matter) from JavaScript, they need
to have an
\family typewriter
id
\family default
attribute.
We add the id attribute
\begin_inset Index idx
status open
\begin_layout Plain Layout
XML ! attributes
\end_layout
\end_inset
to our text field by passing a
\family typewriter
Pair[String,String]
\family default
.
Next, we need to define our actual validation.
We do this by adding some javascript to the
\family typewriter
onclick
\family default
attribute of our submit button.
The onclick attribute evaluates whatever javascript is assigned when the
button is clicked; if the javascript evaluates to true then submission
continues.
If it evaluates to false then submission is aborted.
In our case, we use the JsIf case class to check to see if the value of
our myName field is equal to an empty string.
In this case the JE object holds an implicit conversion from a Scala string
to a Str (JavaScript string) instance.
The second argument to JsIf is the body to be executed if the condition
is true.
In our case we want to pop up an alert to the user and stop form submission.
The JsCmd trait (which Alert mixes in) provides a
\begin_inset Quotes eld
\end_inset
&
\begin_inset Quotes erd
\end_inset
operator which allows you to chain multiple commands together.
Here we follow the Alert with a JsReturn, which returns the specified value;
again, there's an implicit conversion from Boolean to JsExp, so we can
simply provide the
\begin_inset Quotes eld
\end_inset
false
\begin_inset Quotes erd
\end_inset
value.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Simple Form Validation
\begin_inset CommandInset label
LatexCommand label
name "lst:Simple-Form-Validation"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
import JsCmds._
\end_layout
\begin_layout Plain Layout
import JE._
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
var myName = ""
\end_layout
\begin_layout Plain Layout
"form" #> {
\end_layout
\begin_layout Plain Layout
"#textfield" #> SHtml.text(myName, myName = _, "id" -> "myName"),
\end_layout
\begin_layout Plain Layout
"#submit" -> SHtml.submit("Save", ..., "onclick" ->
\end_layout
\begin_layout Plain Layout
JsIf(JsEq(ValById("myName"), ""),
\end_layout
\begin_layout Plain Layout
Alert("You must provide a name") & JsReturn(false))
\end_layout
\begin_layout Plain Layout
)
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Subsection
JsCmd and JsExp overview
\end_layout
\begin_layout Standard
If you peruse the Lift API docs you'll find a large number of traits and
classes under the JsCmds and JE objects; these provide the vast majority
of the functionality you would need to write simple JavaScript code directly
in Lift.
Having said that, however, it's important to realize that the Lift classes
are intended to be used for small code fragments.
If you need to write large portions of JavaScript code for your pages,
we recommend writing that code in
\emph on
pure
\emph default
JavaScript in an external file and then including that file in your pages.
In particular, if you write your code as JavaScript functions, you can
use the
\family typewriter
JE.Call
\family default
class to execute those functions from your Lift code.
Table
\begin_inset CommandInset ref
LatexCommand ref
reference "tab:Top-Level-JsCmds"
\end_inset
gives a brief overview of the available JsCmds, while table
\begin_inset CommandInset ref
LatexCommand ref
reference "tab:Top-Level-JE-abstractions"
\end_inset
shows the JE expression abstractions.
\end_layout
\begin_layout Standard
\begin_inset Float table
placement H
wide false
sideways false
status open
\begin_layout Plain Layout
\align center
\begin_inset Tabular
<lyxtabular version="3" rows="22" columns="2">
<features islongtable="true" longtabularalignment="center">
<column alignment="left" valignment="top" width="1.25in">
<column alignment="left" valignment="bottom" width="4.5in">
<row endhead="true" endfirsthead="true">
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Command
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
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Description
\end_layout
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</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
After
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Executes the given JsCmd fragment after a given amount of time
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Alert
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Corresponds directly to the JavaScript alert function
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
CmdPair
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Executes two JsCmd fragments in order
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
FocusOnLoad
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Forces focus on the given XML element when the document loads
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Function
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines a JavaScript function with name, parameter list, and JsCmd body
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsBreak, JsContinue, JsReturn
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Corresponds directly to the JavaScript
\begin_inset Quotes eld
\end_inset
break
\begin_inset Quotes erd
\end_inset
,
\begin_inset Quotes eld
\end_inset
continue
\begin_inset Quotes erd
\end_inset
, and
\begin_inset Quotes eld
\end_inset
return
\begin_inset Quotes erd
\end_inset
keywords
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsFor, JsForIn, JsDoWhile, JsWhile
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
These define loop constructs in JavaScript with conditions and execution
bodies
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsHideId, JsShowId
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Hides or shows the HTML element with the given Id.
This is actually handled via the LiftArtifacts' hide and show methods
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsIf
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Corresponds to the JavaScript
\begin_inset Quotes eld
\end_inset
if
\begin_inset Quotes erd
\end_inset
statement, with a condition, body to execute if the condition is true,
and optional
\begin_inset Quotes eld
\end_inset
else
\begin_inset Quotes erd
\end_inset
body statement
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsTry
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines a try/catch block tha can optionally alert if an exception is caught
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
JsWith
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines a with statement to reduce object references
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
OnLoad
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines a JavaScript statement that is executed on page load
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Noop
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines an empty JavaScript statement
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
RedirectTo
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Uses window.location to redirect to a new page
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
ReplaceOptions
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Replaces options on a form Select with a new list of options.
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Run
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Executes the given string as raw javascript
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Script
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines a <script> element with proper CDATA escaping, etc to conform to
XHTML JavaScript support
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
SetElemById
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Assigns a statement to a given element by id.
Optional parameters allow you to specify properties on the element
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
SetExp
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines an assignment to an arbitrary JavaScript expression from another
JavaScript expression
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
SetHtml
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Sets the contents of a given HTML node by Id to a given NodeSeq.
This is especially useful in Ajax calls that update parts of the page
\end_layout
\end_inset
</cell>
</row>
<row>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
SetValById
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
\size small
Defines an assignment to a given element's
\begin_inset Quotes eld
\end_inset
value
\begin_inset Quotes erd
\end_inset
property
\end_layout
\end_inset
</cell>
</row>
</lyxtabular>
\end_inset