TweetFollow Us on Twitter

Getting Started: More on Xcode

Volume Number: 19 (2003)
Issue Number: 10
Column Tag: Programming

Getting Started

More on Xcode

by Dave Mark

Let's take the debugger for a spin. As we've done for the past few months, we'll work with the Sketch sample project. As a reminder, the Sketch files live in /Developer/Examples/AppKit/Sketch/. Launch Xcode and open the project Sketch.pbxproj.

Setting a BreakpointWorking with the Debugger

When the project window appears, click on the Project Symbols smart group (left side of the window in the Groups & Files list). When your symbols list appears, click in the search field (right side of toolbar) and search for the string init. As you learned last month, this will winnow the list of project symbols down to those containing the string init.

Figure 1 shows this list with the init in the file SKTGraphic.m selected. This is the file we're going to edit and debug. Double-click this line so an editing window appears, listing the contents of SKTGraphic.m.


Figure 1. Search the Project Symbols for files containing init.

Setting a Breakpoint

Scroll about one quarter of the way down the source file and click on line 178. As a reminder, the navigation bar is the strip immediately above the main editing pane. The line number follows the file name, which is immediately to the right of the navigation arrows (see Figure 2). Once you locate line 178, set a breakpoint by clicking in the left column, to the left of the line in the editing window. Take a look at Figure 2. Note that the breakpoint arrow appears to the left of line 178. This means that the debugger, once started, will stop execution just before it executes the if statement on line 178.


Figure 2. Line 178 is selected. Note the line number in the navigation bar and the breakpoint set on the left.

To start the debugger, click the Debug icon in the SKTGraphic.m editing window's toolbar. Depending on things like the speed of your machine, how much of the program is already compiled, etc., this may take a bit. Be patient.

Once your code is built, the Sketch app is launched. Click on the rectangle tool and drag out a rectangle. As you release the mouse button, the debugger will hit that breakpoint and bring the Debug window to the front. Figure 3 shows the Debug window when you hit the breakpoint.


Figure 3. TheDebug window, showing the program paused at a breakpoint. Note the display of Locals in the upper-right pane.

Take a look at the upper-right pane in Figure 3. Notice the display of variables. You can manipulate the column widths by dragging on the splits between the Variable, Value, and Summary column headers. I maximized the width of the Value column in Figure 3. You can click on the disclosure triangles to reveal fields within structs. For example, origin has an x and a y component. The origin line lists the values of all the origin fields. Open the triangle and each field gets its own line.

If you double-click on the x value, the value of x turns into a text editing field allowing you to modify the value of x. Just as you'd expect.

But if you double-click on the origin value (the one that lists x and y values separated by a comma), the origin formatter will appear in a text editing field. In this case, the formatting string is:

x=%x%, y=%y%%origin%, %size%

Double-click the size value column and the size formatter will appear:

width=%width%, height=%height%

Let's change the formatter. Change it to read:

w=%width%, h=%height%

When you finish the edit (hit enter), the change you made will propagate up through both the display of size and any other displays that reference size. To see this for yourself, click the Arguments triangle, then the self triangle, then the _bounds triangle to reveal origin and size within _bounds. On my display (see Figure 4), the _bounds value field is:

x=50, y=63, w=107, h=169

When we changed one size formatter, the change propagated to all fields that displayed a value of type size. Data formatters are tied to type definitions, which are global.


Figure 4. In this shot, notice that self._bounds.size uses the modified size formatter.

    If you are unfamiliar with formatters, grab your favorite C text and do a bit of digging. Just like a formatter in a printf(), Xcode formatters allow you to specify how the debugger displays your variables. There's even an API so you can write your own formatters. See the Xcode release notes for more info.

Debug Controls

Figure 5 shows the debugger controls you'll find at the top of your Debug window. Chances are, if you've ever used a debugger before, you'll recognize most of them. Terminate terminates the process being debugged, Restart terminates the process and restarts it from the beginning. Pause pauses and Continue continues execution from the current stopping point. Step Over executes the next line of code without stepping into any functions, while Step Into steps to the next line of code, even if it means stepping into a function. Step Out completes the current function and stops immediately after the function's return in the calling function.


Figure 5. The debugger controls at the top of the Debug window.

The (in my opinion) coolest icon of the bunch, the Fix scotch tape dispenser is the equivalent of selecting Fix from the Debug menu. As I mentioned in last month's column, there appears to be a glitch with this icon in the Jaguar build of Xcode which I believe is fixed in the Panther build. Once Panther is officially released, I'll definitely be working exclusively with the Panther tools, so we'll get a sense of what's what in the current release of the tools.

The Breakpoints icon brings up the Breakpoints window (see Figure 6), which lists all your current breakpoints, organized by source file. You can turn breakpoints on and off using the checkbox associated with each breakpoint.


Figure 6. The Breakpoints window.

The Console Drawer icon opens a console drawer below the Debug window, allowing you to directly enter gdb commands. This is extremely cool, especially if you come from the Unix world and learned debugging using gdb, adb, or the equivalent.

    You've got the gdb doc on your hard drive:

    /Developer/Documentation/DeveloperTools/gdb/gdb/gdb_toc.html

Once More Into the Breach

Let's take one more spin through the debugger. If Sketch is currently debugging, click the Terminate icon. Now restart Sketch by clicking the Debug icon. When Sketch starts running, do not do anything in Sketch, just use the dock to return to Xcode. Back in Xcode, select Debug - ... from the Window menu to bring the Debug window to the front. Now, click on the Pause button to stop Sketch at whatever line was executing when you clicked the Pause button.

Figure 7 shows the event trace pane which is a stack showing the call sequence with main at the bottom and the current function at the top (in this case, the trap mach_msg_trap). Basically, this is what your app looks like when it is in an idle state, waiting for an event to happen.


Figure 7. The event trace pane shows call sequence stack.

Next, click the Continue icon to get Sketch back up and running. Back in Sketch, click on the rectangle tool and drag out a rectangle. When you let go, you should pop back into the debugger and your familiar breakpoint.

Click Step Into a few times until this line is highlighted:

temp = _bounds;

Remember, since this line is highlighted it has not yet executed. Click Step Into one more time. As you can see in Figure 8, when you execute this line and assign a value to temp, the temp fields displayed in the Value column are displayed in red, showing that they have changed.

If you click Step Into one more time, you'll change bounds. To see this change, you'll need to open the self triangle. self._bounds will turn red and temp will return to black.


Figure 8. When we change the value in temp, the temp fields turn red.

For our final trick, click on the Console Drawer icon. Scroll to the bottom of the window till you see this prompt:

(gdb)

At the prompt, type the command continue and hit return. This is exactly as if you had clicked the Continue icon in the toolbar. You are now back in Sketch and you can drag out some more shapes, as you like.

Till Next Month...

Whelp, I'm out of space again. <sigh>. There's so much more I want to talk about. Next month, we're going to take the debugger through its paces and we'll also play around with a feature called code completion, one of my very favorite parts of XcodeYou know, there are way more things I want to write about than there is space in the magazine. Note to self - see about adding more pages to mag. Ah, well. I've got a cool column idea for next month. Not sure I can pull it off. We'll See see. Look for you then...


Dave Mark is a long-time Mac developer and MacTech contributor. Author of more than a dozen books on various Mac-development topics, Dave is all about Xcode these days. Last month's column focused on the editor interface as well as fix-and-continue. This month's installment will take the debugger through a few of its paces, and explore Xcode's code completion feature.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »
How to upgrade your character in Spellsp...
So you’ve mastered the basics of Spellspire. By which I mean you’ve realised it’s all about spelling things in a spire. What next? Well you’re going to need to figure out how to toughen up your character. It’s all well and good being able to spell... | Read more »
5 slither.io mash-ups we'd love to...
If there's one thing that slither.io has proved, it's that the addictive gameplay of Agar.io can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
Egz – The Origin of the Universe (Games...
Egz – The Origin of the Universe 1.0.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.2 (iTunes) Description: ►►► Special offer until 2nd may : get the game at 2.99€ instead of 3.99€ ! ◄◄◄ Egz is a mesmerizing mix... | Read more »

Price Scanner via MacPrices.net

Price drops on clearance 12-inch Retina MacBo...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
15-inch Retina MacBook Pros available for $20...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Target offers Apple Watch Sport for $50 off M...
Target has Apple Watch Sports on sale for $50 off MSRP for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699.99 including free shipping. Their price is $300 off MSRP, and it’s the lowest price available for this model from any reseller (and... Read more
Apple Beats Microsoft at Own Game; Amazon Pri...
First quarter seasonality combined with an overall disinterested customer base led to an annual decline of 14.7% in worldwide tablet shipments during the first quarter of 2016 (1Q16). Worldwide... Read more
Tablets Had Worst Quarter Since 2012, says St...
The global tablet market began 2016 just as 2015 left off, down. Tablet shipments fell 10% to 46.5 million units during the Q1 2016, according to the new “Preliminary Global Tablet Shipments and... Read more
Clearance 13-inch MacBook Airs, Apple refurbi...
Apple recently dropped prices on certified refurbished 2015 13″ MacBook Airs with 4GB of RAM with models now available starting at $759. An Apple one-year warranty is included with each MacBook, and... Read more
Clearance 12-inch Retina MacBooks, Apple refu...
Apple has dropped prices on Certified Refurbished 2015 12″ Retina MacBooks with models now available starting at $929. Apple will include a standard one-year warranty with each MacBook, and shipping... Read more
Aleratec Releases Mac Software Upgrade for 1...
California based Aleratec Inc., designer, developer and manufacturer of Portable Device Management (PDM) charge/sync products for mobile devices and professional-grade duplicators for hard disk... Read more

Jobs Board

Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.