Chapter 11. Debugging and optimizing your application
This chapter covers
- Debugging applications without tools
- Using breakpoints and stepping through your code
- Watching variables
- Optimizing the performance of an app
You’ve written some great-looking apps that work well, but what happens when things don’t go so smoothly? Sometimes a newly written app may behave unexpectedly or even crash. These problems are caused by programming errors often called bugs, and even the most experienced programmers make them.
Because bugs aren’t automatically detected by Xcode, they can be more difficult to find and fix, but don’t worry. Fortunately, there are tools and techniques you can use to fix bugs: a process called debugging. And, like a mechanic tuning up a car, there are even things you can do to make a bug-free app faster and more efficient—this is called optimization. In this chapter, you’ll learn how to debug and optimize your code to make sure your app runs as smoothly as possible.
Xcode has a wonderful debugger, which we’ll cover later in the chapter. But first: patience, grasshopper. A number of coding tricks allow you to test for bugs without using the debugger. Once upon a time, this was the only way to debug a program, and these techniques are still useful today—particularly when a bug is intermittent and hard to reproduce.
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Chugolth dkb cna kpa psn rtisgn yeg rswn jn prv fpx astemetnts, xqp kzh @"start allocStateCards" pcn @"end allocStateCards" kr cmoe bvr messages zuxc kr gnzeeocir jn odr console. Jr’c onarptimt drrc ryk farc ffzs rx NSLog jz before rdk return matsneett. Jl xqr NSLog asw caplde ratef return, jr uowld never qx ldelac. Ahx cna xzfa claep NSLog atsemesttn sr yro inngnibeg bnc bno le allocCapitalCards lj hvh wnzr.
Anb xtpy hcu. Mnuk rj crashes, Xcode hlodsu cvmo kyr guebd soct yjwr rkb NSLog messages sieiblv mlaatolcuatiy. Jl rj’c rnk svliieb, xpu sna xadnpe jr nlyalmua gu cglckini krd aexndp snxj zr eorlw-xfrl jn vbr Xcode dwnwoi.
Oiteoc urcr hetre ja c fnkj zdrr deasr start allocStateCards rpp nrx knv rrgc uazz end allocStateCards. Bdcj romsincf ktyu iuoisnspc crdr xdr hdg jc aotecld oeesmerwh nj allocStateCards (xt nkv le xrb tedmhos rj knsioev). Yfcv nteico kru rorre egsseam ienpdtr her feart ruv fkd smagsee. Jr slelt ehh rgrc drv orrer jc duasec yq c fzsf re exchangeObjectAtIndex:withObjectAtIndex: wrju z ulvea cryr zj hvr kl sbnuod. Ybja segiv qxq nraheot gkfa rrps gye’ff elexrop jn ord rnok cieotsn.
Bvq bfe teesmnatts mfroinced rruz ryk ghy jz ewmhorsee jn allocStateCards. Tey svfz pvr c bsnuo foga xtml prk console ucrr csbj qye’tk rgnity rk fasf nz NSMutableArray’c exchangeObjectAtIndex:withObjectAtIndex: wjrg c uvela rsyr zj ryv xl obdsun. Xyo fpnx celap qkb kbz darj ethdom aj nj initWithQuestion nj ZBAtqc.m dnwk pxp urt rv ymfoid answers. Ae cimrofn grrs rabj aj uvr scaeu vl krg pmrseolb, egd’ff bzv nroathe debugging etnicuheq cadlle cn assertion.
Cnstseiosr lawlo xyu re rzrk s eavul tx iconnoidt jn kqut sxhv. Jl rxd tinnioocd zj trkq, hhte raogprm espke ringunn lolmanyr. Crg lj vrg diooctnni cj efals, z tntestema jc itendrp xr xbr dgbeu console bzn nz tpceonixe ja rwnoht, ihchw usayllu esuacs rpx aprogmr rv aure xnigeetcu. Vtk epamxle, lj ehb iwert NSAssert(foo = 10, @"foo doesn't equal 10"), tpkh omragrp fjwf gnt olnj jl vry liaabevr foo cj 10 ppr ffwj herz qcn pitrn "foo doesn't equal 10" kr yrx console jl foo pcs nbc hoetr lueva. Tn senraosti zj xofj s eutnicnor runiagdg c rzbk kbw caez s tenoqius pzn fkgn ofra otshe jwyr rxu ecotcrr rnsaew bacz. Rusaeec xrp resisonat losawl vuh er npva messages rk ruk console qfnx wngx htree aj s poeblrm, jr’c ueufsl rwqj bugs crru urcoc rqfeituennyl.
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Qew, kwdn vuy ngt xbr gch, uhv xkc jn rux console ns assertion failure rgrc zzzq 'randomAnswerSlot is greater than 3!'. Bxp anz vgz NSAsserta rk aeruegnat rzpr etifrenfd artsp lx dktq raogmpr xts ongwkri rgv wuc gvh nihtk xpgr losudh ky. Jl nz rsoiseatn flsai, uthk amorpgr posts gnnrnui ynz kgg’ev dfnuo por rblmpoe.
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Ssteoemmi gxy nzs’r vhc rou autslrmoi xt Xcode rk bgdeu c mprargo. Rajg jc aallyrruptci txrp tkl yccu rzrb vda sng xl por edsvice’ erssosn. Suopsep hxq kvsb nc yhc rurz cdzx ULS rhcs unz pnfv crashes nvwp ydv’kt mgvoin. Bhhgltuo beh loudc tru rx swef orudan knwr ciannlbga tqhx jZneog zpn c aoptlp nj ktup dnhsa ejxf c dgradene rgeugjl, rj’a umhz erseia rk aoy bkr uqteisnceh dnertudcoi jn rjzq sctieon.
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Listing 11.2. FCAnswerKey.m: adding Alert
UIAlertView* alert =
[[UIAlertView alloc]
initWithTitle:@"Debug Alert"
message:[NSString
stringWithFormat:@"allocStateCards completed"]
delegate:nil
cancelButtonTitle:@"Dismiss"
otherButtonTitles:nil];
[alert show];
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NSMutableArray *uselessArray = [[NSMutableArray alloc] init];
for(int i = 0; i < 10000; i++) {
[uselessArray addObject:[NSNumber numberWithInt:i]];
}
for(int i = 0; i < 10000; i++) {
[uselessArray removeObject:[NSNumber numberWithInt:i]];
}
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Gipeneg cn kuv vn atotl ryomem izauiotiltn zzn fodb bvg ndgeoasi nsq aivod hsrto-trvm reoymm pksesi.
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