TweetFollow Us on Twitter

An Introduction to Handlers

Volume Number: 21 (2005)
Issue Number: 2
Column Tag: Programming

AppleScript Essentials

by Benjamin S. Waldie

An Introduction to Handlers

In a previous article, you may remember that we discussed methods of writing AppleScripts to watch folders for incoming items to process. In each of the methods we discussed, we made use of handlers. This month, we are going to explore handlers in more depth. Since handlers are a fairly complex subject, the full scope of handlers will not be covered in this month's article. Rather, we will cover the basics of handlers. In future articles, we will discuss handlers in more detail.

What is a Handler?

A handler is a group of one or more AppleScript statements that are associated with a single command. As we will discuss, some handlers are user defined, and some are application, or system defined.

Once the command associated with a handler is invoked, the statements within that handler will execute. Handlers allow you to group your code into logical "chunks," which may be triggered, or called, over and over again within a script. By carefully constructing your handlers, you can make scripts very modular, giving you the ability to break the scripts apart and reuse the handlers again in future scripts. Many AppleScript developers store their handlers in code libraries. These libraries can then be loaded, and handlers within them can be triggered from other scripts.

Anatomy of a Handler

Let's take a look at a basic handler. The following handler may be used to display a basic dialog message.

on displayDialog()
   display dialog "This is code within a handler."
end displayDialog

Now, let's break handlers down into different parts, and examine how they are constructed.

Handler Definition

A handler definition refers to the handler itself. The following would be considered the handler definition from the previous example.

on displayDialog()
   display dialog "This is code within a handler."
end displayDialog

A handler definition always begins with the word on or to, which indicates to AppleScript that it is the beginning of a handler. In the example above, I chose to begin my handler with the word on. However, the method you choose to use is entirely at your discretion. You should use whichever word looks more acceptable to you.

The next part of the handler definition is the name of the handler, followed by any parameters to be passed to the handler by the script. For example:

displayDialog()

In many cases, you may need to pass information to a handler to be processed. To allow for this, handlers support input parameters. Handlers also have the ability to return data back to the script. For example, a handler may return a true/false value indicating whether or not it was successfully processed.

There are a couple different methods that you can use to specify input parameters, and we will discuss them shortly. In the previous example, the empty parentheses indicate that my handler does not require any parameters.

Following the first line of a handler definition are any AppleScript statements that should be executed by the handler. You may write as few or as many statements as you need, keeping in mind that they will not execute until the handler is called.

The final line of a handler definition always begins with the word end, followed by the name of the handler.

end displayDialog

To save time when writing a handler, you may write the word end, and when you compile your code, the handler name will automatically be inserted for you.

Handler Call

To call a handler from within a script, you must specify the name of the handler, followed by values for all of its parameters. The following is the call for the previous handler example:

displayDialog()

We'll look at calling handlers that use parameters shortly, as we explore parameters.

If you need to call a handler from within an application tell block, you must specify of me after the handler call, to indicate that the handler is to be addressed within the script, and not within the application. For example:

tell application "Finder"
   displayDialog() of me
end tell

Parameters

As previously mentioned, the first line of a handler contains the handler name, followed by any parameters to be passed into the handler. A parameter is a variable that is named in the handler definition, and is assigned a value when the handler is called. When calling a handler, all parameters are required, and must be specified. Handlers do not allow optional parameters.

There are two types of parameters that may follow a handler name - labeled parameters and positional parameters.

Labeled Parameters

Labeled parameters are parameters that are associated with labels in the handler definition. When the handler is called, these labels are used to determine which parameters are which. Therefore, when working with labeled parameters, you may pass the parameters through your handler call in any order you wish, so long as they correspond with the correct labels. The exception to this rule is that, in some cases, you may choose to assign a direct parameter, which is required to fall immediately after the handler name.

There are two ways that you can assign labeled parameters in a handler. The first is to assign the parameters using one of the following predefined labels: about, above, against, apart from, around, aside from, at, below, beneath, beside, between, by, for, from, instead of, into, on, onto, out of, over, since, through, thru, under. The label of may also be used, though only to define a direct parameter. If you add a direct parameter, using the label of, then you are required to have at least one or more additional parameters. The following is an example of a handler with labeled parameters:

to displayDialog of theText above theButtons aside from theIconNumber
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

In the example above, the parameter theText is a direct parameter, as indicated by the label of. The parameter theButtons is associated with the label above, and the parameter theIconNumber is associated with the label aside from. The handler above would be called using the following line of code:

displayDialog of "Hello" above {"OK"} aside from 1

Again, since labeled parameters are associated with their labels, then you can rearrange the non-direct parameters, if desired. For example, the following handler call would function identically as the previous one:

displayDialog of "Hello" aside from 1 above {"OK"}

Another way to assign labeled parameters is to use custom labels. This is done by creating label definitions in the format labelName:parameterName, separating them by commas, and preceding them with the label given. Custom labeled parameters must follow any predefined label parameters. An example of a handler with custom labeled parameters is the following:

to displayDialog of theText given someButtons:theButtons, someIconNumber:theIconNumber
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

In the example above, the parameter theText is a direct parameter, preceded by a predefined label. The remaining parameters are custom labeled parameters. The parameter theButtons is associated with the custom label someButtons, and the parameter theIconNumber is associated with the custom label someIconNumber. This handler can be called using the following line of code:

displayDialog of "Hello" given someButtons:{"OK"}, someIconNumber:1

Again, since this handler makes use of labeled parameters, the non-direct parameters may be rearranged, if desired:

displayDialog of "Hello" given someIconNumber:1, someButtons:{"OK"}

Positional Parameters

Another type of parameter that you can use when defining a handler is a positional parameter. Positional parameters are separated by commas, and do not contain any labels. Because they do not contain labels, they are identified by their position in the handler definition. Therefore, they must be listed in the same position when the handler is called. The following is an example of a handler that makes use of positional parameters:

on displayDialog(theText, theButtons, theIconNumber)
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

The handler above may be called using the following line of code:

displayDialog("Hello", {"OK"}, 1)

Return Value

In some cases, a handler may return a value to the script that called it. This value may be placed into a variable for later usage. By default, a handler will return the result of the last AppleScript statement that executes within the handler, assuming that this statement produces a result. If desired, you may configure your handler to return a different value. For example, the following code returns the name of the button clicked in the dialog.

set theChoice to displayDialog()

on displayDialog()
   display dialog "Would you like to continue processing?" buttons {"Yes", "No"}
   return button returned of result
end displayDialog

Next, I could add additional code at the root level of my script to check the button that was clicked, now stored in a variable called theChoice, and take the appropriate course of action.

Within a handler, you may return a value at any time to cease further execution of that handler, or you may return no value to cease execution without returning a specific value. For example, the following handler will stop processing, returning no value, if the user clicks the "No" button.

displayDialog()

on displayDialog()
   display dialog "Would you like to continue processing?" buttons {"Yes", "No"}
   set theChoice to button returned of result
   if theChoice = "No" then return
   display dialog "Continuing..."
end displayDialog

Types of Handlers

There are two types of handlers in AppleScript - subroutine handlers and command handlers.

Subroutine Handlers

Subroutine handlers are groups of statements, which are defined by the developer, and called throughout a script, or from another script. Subroutines can be extremely useful if you need to perform the same exact task over and over throughout your script. For example, let's say that you need to display an informational dialog to the user 10 times throughout your script to let the user know what is occurring.

display dialog "Beginning next task..." buttons {"*"} 
   default button "*" with icon 1 giving up after 3

To display the above dialog 10 times, I could write out all of this code 10 times - or, I could write a handler 1 time.

on displayNextTaskMessage()
   display dialog "Beginning next task..." buttons {"*"} default 
      button "*" with icon 1 giving up after 3
end displayNextTaskMessage

Once written, this handler can be triggered from anywhere within my script. Rather than writing the entire lengthy line of display dialog code each time I need to display the dialog to the user, I can call my handler instead. The following code would call the displayNextTaskMessage handler.

displayNextTaskMessage()

In the example above, you may be wondering where the handler name "displayNewTaskMessage" came from. This is something that I defined myself when writing the handler. Remember, since subroutine handlers are defined by a developer, their names are user definable. So, I could have named this handler anything I wanted, as long as it was not the name of another existing handler in my script.

Command Handlers

A command handler is a group of statements that is triggered by a specific application or system related event.

Every AppleScript application contains an implied run handler. Any AppleScript statements at the top level of the script, excluding global variables, properties, other handlers, and script objects, fall within that run handler. If you prefer to make the run handler visible in your code, you may wrap these statements within an on run handler call. When the script is run, both methods will behave identically. For example, each of the examples below will perform the exact same function:

Example 1:

display dialog "Hello!"

Example 2:

on run
   display dialog "Hello!"
end run

As you can see, any code within the run handler of a script will be executed whenever the script is run. However, in some cases, you may want to execute code when other types of actions occur within your script.

The open handler may be used to initiate specific code when items are dragged and dropped onto a script in the Finder. For example:

on open theDroppedItems
   -- Process the items
end open

By adding an open handler into a script, and then saving that script as an application, the script will automatically accept dropped files and folders. It will also receive a new icon, indicating that it is now a drop script.

In the previous example code, the parameter theDroppedItems will contain a list of paths to any items dropped onto the script. If you want your script to only process folders, then you would need to add custom code within the open handler to determine which dropped items were folders, and process only these items.

Two other types of command handlers that may be added to an AppleScript application are the idle handler and the quit handler.

An idle handler is particularly useful when creating stay open AppleScript applications. In a stay open AppleScript application, by default, AppleScript will send the script an idle command every 30 seconds. At that time, any code within the idle handler will execute. For example, if I save the following code as a stay opened AppleScript application, and trigger it, the script will beep every 30 seconds:

on idle
   beep
end idle

Though the default time period between idle messages is 30 seconds, I can change this behavior if desired, by returning an integer value indicating how many seconds of a delay should occur before the next idle. In the following example, the script would beep every 10 seconds.

on idle
   beep
   return 10
end idle

The quit handler may be used in order to execute code when the script quits, whether manually quit by the user or not.

on quit
   display dialog "Task complete."
end quit

Example Handlers

Now that we have covered the basics of handlers, let's take a look at some example handlers, which may be useful to you as you write scripts in the future. Each of these handlers has been written generically, so that you can use it in virtually any script in the future.

The following handler will make a new folder in a specified output folder, using a specified name:

on makeNewFolder(theNewFolderName, theOutputFolder)
   tell application "Finder" to make new folder at folder theOutputFolder with properties 
      {name:theNewFolderName}
end makeNewFolder

The following handler will delete a folder of a file:

on moveItemToTrash(theItemPath)
   tell application "Finder" to delete item theItemPath
end moveItemToTrash

The following handler will mount an afp volume:

on mountVolume(theVolumeName, theServerIPAddress, theUserName, thePassword)
   mount volume "afp://" & theUserName & ":" & thePassword & "@" & theServerIPAddress 
      & "/" & theVolumeName as string
end mountVolume

In Closing

Again, handlers are a fairly complex aspect of AppleScript development for many users. Today, I use them regularly, and I try to make them as modular as possible. My theory is that if I write code to perform a specific task, such as opening a document in QuarkXPress, then I don't want to ever write that code again. Rather, I store the handler for later usage in future scripts, and the next time I need to perform the task, which could be a year down the line, I simply add the handler into my new script.

I encourage you to begin using handlers more in your scripting, as it will benefit you greatly. We'll discuss other aspects of handlers in more detail in future articles. In the meantime, for additional information about handlers, you may want to check out an AppleScript book, or browse the AppleScript Language Guide at <http://developer.apple.com/documentation/AppleScript/>.

Until next time, keep scripting!


Benjamin Waldie is president of Automated Workflows, LLC, a firm specializing in AppleScript and workflow automation consulting. In addition to his role as a consultant, Benjamin is an evangelist of AppleScript, and can frequently be seen presenting at Macintosh User Groups, Seybold Seminars, and MacWorld. For additional information about Benjamin, please visit http://www.automatedworkflows.com, or email Benjamin at applescriptguru@mac.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Unite 3 3.1 - Turn websites into full-fe...
Unite 3 lets you turn websites into apps on your Mac and change the way you use your computer forever Creating your app It all starts with the new Unite creation tool. Simply enter a name for your... Read more
App Tamer 2.5.2 - Efficiently manage you...
App Tamer tames your processor-monopolizing apps and keeps them from chewing up excessive CPU time and battery life. Powered by a unique AutoStop feature, App Tamer stops each application when you... Read more
Sid Meier's Civilization IV 1.74 -...
Note: Sid Meier's Civilization IV is no longer under development and the developer provides no support for it. With Civilization IV, history as you know it, is history. Rule throughout time and... Read more
Day One 4.15.3 - Maintain a daily journa...
Day One is an easy, great-looking way to use a journal / diary / text-logging application. Day One is well designed and extremely focused to encourage you to write more through quick Menu Bar entry,... Read more
Backblaze 7.0.1.450 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $6 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Visual Studio Code 1.47.0 - Cross-platfo...
Visual Studio Code provides developers with a new choice of developer tool that combines the simplicity and streamlined experience of a code editor with the best of what developers need for their... Read more
Apple Pages 10.1 - Apple's word pro...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices, and... Read more
Shimo 5.0.2 - VPN client for everyone.
Shimo is the most versatile VPN client for OS X and it enables really everybody to master secure network. It supports more protocols than any other VPN application out there! CiscoVPN, AnyConnect,... Read more
Yasu 6.0.1 - System maintenance app.
Note: Yasu is not being sold anymore and is listed only for people who had purchased it in the past. Yasu was created with system administrators who service large groups of workstations in mind, Yasu... Read more
Keynote 10.1 - Apple's presentation...
Easily create gorgeous presentations with the all-new Keynote, featuring powerful yet easy-to-use tools and dazzling effects that will make you a very hard act to follow. The Theme Chooser lets you... Read more

Latest Forum Discussions

See All

Puzzle & Dragons welcomes the cast o...
Puzzle & Dragons has a history of wild crossovers, and its latest is no exception. The gates of hell are now open, and the cast of Devil May Cry have made their grand entrance into the world Puzzle & Dragons. The collaboration is set to... | Read more »
Meteorfall: Krumit's Tale is launch...
Meteorfall: Krumit's Tale is getting an iOS & Android beta test this Thursday, and you can sign up now to get involved. [Read more] | Read more »
Marvel Duel, NetEase's promising ca...
Marvel Duel, NetEase's collectable card battler, has now opened for pre-registration in Thailand, Indonesia, Malaysia, and the Philippines. [Read more] | Read more »
PUBG Mobile teams up with Yamaha for a l...
PUBG Mobile has had various collaborations with various companies since it first burst onto the scene. The latest sees the popular battle royale title teaming up with vehicle and marine products company Yamaha, meaning players can take one of... | Read more »
Marvel Super War launches its third seas...
Marvel Super War, NetEase's popular MOBA, has kicked off its third season today, introducing its Zone Invasion beta and an all-new playable superhero. [Read more] | Read more »
Brave Dungeon, Unlock Games' idle-R...
Brave Dungeon, Unlock Games' idle-RPG and auto chess title, has been downloaded over 1 million times since it launched last week. To commemorate reaching this milestone the developers have decided to host a series of in-game events. [Read more] | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Steam Link Spotlight - Disco Elysium
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was Signs of the Sojourner Read about how it plays using Steam Link over here. | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Pokemon Go's July Community Day wil...
Pokemon Go developers have announced the details concerning the upcoming Gastly Community Day. This particular event was selected by the players of the game after the Gas Pokemon came in second place after a poll that decided which Pokemon would... | Read more »

Price Scanner via MacPrices.net

Apple has 2020 13″ 4-Core MacBook Airs availa...
Apple has a full line of Certified Refurbished 2020 13″ 4-Core MacBook Airs available for $200 off the cost of new models. Each MacBook features a new outer case, comes with a standard Apple one-year... Read more
Apple restocks clearance 2019 13″ MacBook Pro...
Apple has restocked Certified Refurbished 2019 13″ 1.4GHz 4-Core Touch Bar MacBook Pros starting at $979 and up to $440 off original MSRP. Apple’s one-year warranty is included, shipping is free, and... Read more
US Cellular offers $300 off any new Cellular...
US Cellular is offering a $300 discount on any new Cellular Apple iPad, iPad Air, iPad Pro, or iPad mini with a new line of service. According to US Cellular, “Promotional pricing requires purchase... Read more
Sale! 16″ MacBook Pros for $400 off Apple’s M...
Amazon has 16″ MacBook Pros on sale for $400 off Apple’s MSRP today with prices starting at $1999. Shipping is free: – 2019 16″ 2.6GHz 6-Core MacBook Pro Space Gray: $1999.99 $400 off MSRP – 2019 16... Read more
Lowest price ever: Apple AirPods Pro for $214...
Amazon-owned Woot has new Apple AirPods Pro on sale for today only (Monday, July 13th) for $214.99 including free shipping for Prime members. Their price is $35 off Apple’s MSRP, and it’s the lowest... Read more
Woot is selling refurbished Apple iPhones tod...
Amazon-owned Woot is selling refurbished unlocked Apple iPhones today for as low as $99. Shipping is free for prime members. The following models are available, and note that many will sell out... Read more
Take $50 off the 64GB Apple iPhone SE at Simp...
Use coupon code SUMMER2020 at Simple Mobile through July 31, 2020, and take 20% off the 64GB Apple iPhone SE + service plan. This is in addition to their standard $50 discount on the iPhone SE.... Read more
Weekend Sale: Woot offers clearance MacBook,...
Amazon-owned Woot is blowing out a wide range of MacBooks, MacBook Airs, and MacBook Pros starting at only $389. All models are open-box returns, and according to Woot, “these MacBooks are in New-... Read more
Apple’s 2020 Mac minis on sale for up to $100...
B&H Photo has Apple’s 2020 Mac minis on sale today for $40-$100 off MSRP, each including free expedited delivery for many US addresses. These are the same Mac minis sold by Apple in their retail... Read more
Apple restocks refurbished 10.5″ iPad Airs fo...
Apple has restocked 10.5″ WiFI iPad Airs models for up to $100 off MSRP, Certified Refurbished. Each iPad comes with Apple’s standard one-year warranty and includes a new outer case. Shipping is free... Read more

Jobs Board

Perioperative RN - ( *Apple* Hill Surgical C...
Perioperative RN - ( Apple Hill Surgical Center) Tracking Code 62018 Job Description Monday - Friday - Full Time Days Possible Saturdays General Summary: Under the Read more
Surgical Coord/Scheduler-MG - *Apple* Hill...
Surgical Coord/Scheduler-MG - Apple Hill Medical Center - (full-time) - Days Tracking Code 62537 Job Description General Summary: Under general supervision, provides Read more
Office Assistant - *Apple* Hill Medical Cent...
Office Assistant - Apple Hill Medical Center- (part-time) - Days Tracking Code 62649 Job Description General Summary: Under general supervision, performs diversified Read more
Blue *Apple* Cafe Student Worker - Fall - P...
…to enhance your work experience. Student positions are available at the Blue Apple Cafe. Employee meal discount during working hours is provided. Duties include food Read more
Geek Squad Advanced Repair *Apple* Professi...
**769390BR** **Job Title:** Geek Squad Advanced Repair Apple Professional **Job Category:** Store Associates **Store Number or Department:** 000580-Reading-Store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.