I have an iOS app that matches incoming text fields to standard fields used to import records. My problem is that a NSMutableDictionary that uses those fields is empty! Here is the code that saves the mapping:
-(void)mapUserFields: (id) sender { // move contents of each textField when user has finished entering it
SingletonDictionary *sd = [SingletonDictionary sharedDictionary];
UITextField *tf = (UITextField *)sender; // textfield contains the pointer to user's half of the equation
int tagValue = (int)tf.tag; // get the tag value
[sd.dictionaryOfUserIndexes setObject:tf.text forKey:[NSString stringWithFormat:#"%d", tagValue]]; // value found in textField id'd by tag
NSLog(#"\nfield.text: %# tagValue: %d nsd.count: %d\n",tf.text, tagValue, sd.dictionaryOfUserIndexes.count);
}
This is the result of the NSLog:
field.text: 1 tagValue: 38 nsd.count: 0
This is the definition of the singleton in the .h file:
#property (nonatomic, retain) NSMutableDictionary *dictionaryOfUserIndexes;
This is the code to initialize the singleton in the .m file:
//-- SingletonDictionaryOfUserIDs --
+ (id) sharedDictionary {
static dispatch_once_t dispatchOncePredicate = 0;
__strong static id _sharedObject = nil;
dispatch_once(&dispatchOncePredicate, ^{
_sharedObject = [[self alloc] init];
});
return _sharedObject;
}
-(id) init {
self = [super init];
if (self) {
dictionaryOfUserIndexes = [NSMutableDictionary new];
}
return self;
}
#end
I believe my problem is because the sd.dictionaryOfUserIndexes is not initialized, but I am not sure if that's true, and if so, how to initialize it (I tried several different variants, all of which created build errors). I looked on SO and Google, but found nothing that addresses this particular issue. Help would be greatly appreciated!
There are a few things we could improve in this code, but the only thing wrong with it is the reference to dictionaryOfUserIndexes in the init method. The code as posted wouldn't compile, unless: (a) you have a line like:
#synthesize dictionaryOfUserIndexes = dictionaryOfUserIndexes;
so that the backing variable is named without the default _ prefix, or (b) you refer to the ivar with the default prefix, as in:
_dictionaryOfUserIndexes = [NSMutableDictionary new];
The other way -- preferable in most every context except within an init method -- is to use the synthesized setter, like:
self.dictionaryOfUserIndexes = [NSMutableDictionary new];
But with that change alone (so it will compile) your code runs fine, adds a value to the dictionary and logs an incremented count.
Related
I use lazy instantiation on my properties, to have my class created and used as fast as possible. To achieve this, I write lots of 'empty' getters like this:
- (VMPlacesListFilter *)currentFilter
{
if (!_currentFilter) {
_currentFilter = [[VMPlacesListFilter alloc] init];
}
return _currentFilter;
}
They are all the same: if the instance variable is nil, call the -alloc and -init on the class of the property, then return the instance variable. Very common and straightforward.
If I don't create this getter by myself, Objective-C's automatic synthesization creates a getter for me, which does only the returning part (does not init the object if the instance variable is nil).
Is there any way to avoid writing this boilerplate code?
Nope, I'm afraid there's no good way around it, if you really want to have lazy initialization. Personally, I usually save lazy initialization for stuff that could really be time consuming or memory intensive (say, loading images or view controllers), and initialize cheap stuff (like simple data structures or model objects) in init.
- (instancetype) init {
self = [super init];
if( self ) {
_cheapThing1 = [NSMutableArray array];
_cheapThing2 = [[MyModelObject alloc] init];
}
return self;
}
- (ExpensiveThing*) expensiveThing
{
if( _expensiveThing == nil ) {
_expensiveThing = [[ExpensiveThing alloc] init];
}
return _expensiveThing;
}
Unless you're loading something from disk or the network, I wouldn't worry too much about initialization time. Of course, profile it.
I know this is an Objective-C question, but it's worth noting that Swift has lazy initialization built-in.
lazy var currentFilter = VMPlacesListFilter()
First off, I totally agree with #zpasternack that "lazy load" should not be misused. However, automatically generating setters and getters is completely doable with the power of Objective-C runtime. In fact, CoreData is doing this.
Anyway, I have come up with some stupid code implementing a class called LazyClass, in which you can declare dynamic properties like lazyArray (see below). Using dynamic method resolution, when the property is accessed for the first time, a getter that calls the corresponding class's default +alloc and -init method will be automatically added to the class. All underlying instance variables are stored in an NSMutableDictionary called myVars. Of course you can manipulate ivars through the runtime API as well, but using a dictionary should save some work.
Please note that this implementation just shows the basic idea of how it works. It lacks error checking and is not supposed to be shipped.
LazyClass.h
#interface LazyClass : NSObject
#property NSMutableDictionary *myVars;
// lazily initialized property
#property NSArray *lazyArray;
#end
LazyClass.m
#import "LazyClass.h"
#import <objc/objc-runtime.h>
#implementation LazyClass
#dynamic lazyArray;
- (instancetype)init {
self = [super init];
self.myVars = [NSMutableDictionary dictionary];
return self;
}
- (NSMutableDictionary *)getMyVars {
return self.myVars;
}
// the generated getter method
id dynamicGetterMethodIMP(id self, SEL _cmd) {
// selector name, which is also the property name
const char *selName = sel_getName(_cmd);
NSString *selNSName = [NSString stringWithCString:selName encoding:NSUTF8StringEncoding];
NSString *keyPath = [NSString stringWithFormat:#"myVars.%#", selNSName];
if (![self valueForKeyPath:keyPath]) {
// get the actual type of the property
objc_property_t property = class_getProperty([self class], selName);
const char *attr = property_getAttributes(property);
NSString *attrString = [[NSString alloc] initWithCString:attr encoding:NSUTF8StringEncoding];
NSString *typeAttr = [[attrString componentsSeparatedByString:#","] firstObject];
NSString *typeName = [typeAttr substringWithRange:NSMakeRange(3, typeAttr.length - 4)];
// the default initialization
Class typeClass = NSClassFromString(typeName);
[self setValue:[[typeClass alloc] init] forKeyPath:keyPath];
}
return [self valueForKeyPath:keyPath];
}
// the generated setter method
void dynamicSetterMethodIMP(id self, SEL _cmd, id value) {
// get the property name out of selector name
// e.g. setLazyArray: -> lazyArray
NSString *propertyName = NSStringFromSelector(_cmd);
propertyName = [propertyName stringByReplacingOccurrencesOfString:#"set" withString:#""];
propertyName = [propertyName stringByReplacingOccurrencesOfString:#":" withString:#""];
propertyName = [NSString stringWithFormat:#"%#%#", [propertyName substringToIndex:1].lowercaseString, [propertyName substringFromIndex:1]];
NSString *keyPath = [NSString stringWithFormat:#"myVars.%#", propertyName];
[self setValue:value forKeyPath:keyPath];
}
// dynamic method resolution
+ (BOOL)resolveInstanceMethod:(SEL)aSEL {
if ([NSStringFromSelector(aSEL) containsString:#"set"]) {
class_addMethod([self class], aSEL, (IMP)dynamicSetterMethodIMP, "^?");
} else {
class_addMethod([self class], aSEL, (IMP)dynamicGetterMethodIMP, "v#:");
}
return YES;
}
#end
Documentation
If it's the verboseness that bothers you, I suppose you could compress lazy initialisers that only need one-line initialization using the ternary operator:
- (VMPlacesListFilter *)currentFilter
{
return _currentFilter ? : (_currentFilter = [[VMPlacesListFilter alloc] init]);
}
DISCLAIMER: I don't do this, but it's interesting that it can be done
I am trying to create a set of objects using NSMutableSet. The object is a tag, each tag has an id and a name.
The tag class is defined like so:
#import "Tag.h"
#implementation Tag
#synthesize id, name;
+(id) populateTagObjectWithId:(NSString *)id andName:(NSString *)name
{
Tag *myTag = [[self alloc] init];
myTag.id = id;
myTag.name = name;
return myTag;
}
... remainder of code snipped out
Somewhere else in my application I use SQLite to fetch tags in the TAG table. I iterate using a while loop, for each iteration I build a tag object and then try to add it to the set. Code below:
... previous code snipped out...
NSMutableSet *thisTagSet;
while(sqlite3_step(tag_statement) == SQLITE_ROW)
{
NSString *thisTagId = [[NSString alloc] initWithUTF8String:(const char *) sqlite3_column_text(tag_statement, 0)];
NSString *thisTagName = [[NSString alloc] initWithUTF8String:(const char *) sqlite3_column_text(tag_statement, 1)];
[thisTagSet addObject:[Tag populateTagObjectWithId:thisTagId andName:thisTagName]];
... rest of code snipped out...
So, as I mentioned, as I iterate through this while loop, i get the object and its id and name ARE populating (I have confirmed this by inspecting the debugger and also using NSLog). The thisTagSet NSMutableSet however remains empty even though I'm using the addObject method. Is there something I'm doing wrong here? I have also tried separating out the two steps like so:
Tag *thisTagObject = [Tag populateTagObjectWithId:thisTagId andName:thisTagName];
[thisTagSet addObject:thisTagObject];
Again, the same result. I successfully get a thisTagObject, but nothing in the thisTagSet...
After reading your code two things pop out:
You're not initializing your NSMutableSet, and you're leaking your tags by returning a retained object in your class method.
Edit: Added leak-fix code
+(id)tagObjectWithId:(NSString *)id andName:(NSString *)name
{
Tag *myTag = [[self alloc] init];
myTag.id = id;
myTag.name = name;
return [myTag autorelease];
}
2nd edit: The code above only applies as long as ARC is disabled. Otherwise it is not needed as ARC takes care of the memory management.
Problem context is a ViewController with several button handlers and a scores.list data file of 1000 NSString objects. If I click on buttonOne, the handler code checks if the file scores.list exists in the User Documents directory. If yes, it loads the data in an NSMutableArray called score, if not it creates the NSMutableArray in memory (to be stored on disk later) like this:
- (void)readScores
{
// Setup path + filename pathUserDocDirScorelist);
...
// test for presence scores.list in documents dir
if ([fileManager fileExistsAtPath: pathUserDocDirScorelist])
{ // Read scores.plist into NSMutableArray
score = [NSMutableArray arrayWithContentsOfFile:pathUserDocDirScorelist];
} else { // Initialize empty scores array with 1000 empty NSString entries
score = [[NSMutableArray alloc] init];
int i;
for(i = 0; i < 1000; i++) {
[score addObject:#""];
}
}
// at this point there is always a valid array score with 1000 entries
}
Basically this code works; in both cases (reading data from scores.list or in-mem build-up) I can verify in the debugger (with 'po score') that an array with 1000 entries is present afterwards.
Now comes the problem that is blocking me for 2 days now:
In the handler of buttonTwo, statements like [score count] crash, but only in case the array score gets its data from disk, not if build-up in memory. In the first case is the array still valid though until the last line of handler code of buttonOne, but then 'evaporates' as soon as the array is addressed in a next handler (EXC_BAD_ACCESS).
No, it is not caused by a premature release statement, since there are none (yet) in my entire app. :)
(not concerned with memory leaks yet).
How is this possible that within one view a NSMutableArray is valid at the end of button handler 1, but invalid at the beginning of the next button handler 2 if no explicit release statement is executed in between?
Extra info:
ViewController.h:
#interface ViewController : UIViewController {
NSMutableArray *score;
...
}
#property (nonatomic, retain) NSMutableArray *score;
...
- (IBAction) buttonOne: (id) sender;
- (IBAction) buttonTwo: (id) sender;
#end
And in ViewController.m I have:
#synthesize score;
...
- (IBAction) buttonOne: (id) sender {
if (score == nil) {
[self readScores];
}
...
NSLog(#"Number of entries in score = %i", [score count]); // never crashes
}
- (IBAction) buttonTwo: (id) sender {
NSLog(#"Number of entries in score = %i", [score count]); // **crash point**
}
P.S. I tried NSZombieEnabled by starting the app with alt/cmd/R and adding 'NSZombieEnabled=YES', but that does not result in extra information in the debug console.
It's always a little dangerous to do this:
score = [[NSMutableArray alloc] init];
outside of an init method. Because, the problem is that perhaps your method readScores is executed twice. I'd guess it almost certainly is.
What happens then is the the first scores object is never released. That causes a memory leak and sooner or later you get the dreaded EXC_BAD_ACCESS.
So, the best thing is either to first check:
if (score)
{
[score release];
}
Or, alternatively, set up scores as a synthesized retained object in your .h file. Then you can replace your code as follows and let everything happen automatically:
self.scores = [NSMutableArray array];
Note that here, I didn't use:
self.scores = [NSMutableArray alloc] init];
because that would cause two retains to happen, and of then EXC_BAD_ACCESS due to over-retention.
As a relative Objective-C beginner, I'm obviously still not grasping certain memory management rules. I can't figure out how to make this not crash:
#interface MyClass { NSArray *playerArray4th; }
- (void) viewDidLoad { playerArray4th = [self getAudioPlayersForSoundFile:#"rimshot" ofType:#"aif"]; }
- (NSArray*) getAudioPlayersForSoundFile:(NSString*)soundFileName ofType:(NSString*)soundFileType {
//code instantiating objects....
NSArray *toRet = [[NSArray alloc] initWithObjects:toRetTickPlayer,toRetTickPlayerCopy,toRetTickPlayerCopy2,toRetTickPlayerCopy3, nil];
return toRet;
}
Then later, in a different function:
NSArray *currentArray = playerArray4th;
[currentArray release];
currentArray = nil;
currentArray = [self getAudioPlayersForSoundFile:fileName ofType:ofType];
And it crashes when trying to access the array again:
- (void) playSound:(NSString*)soundType {
AVAudioPlayer *currentPlayer;
if ([soundType isEqualToString:#"4th"]) {
if (playerArray4thCounter >= [playerArray4th count]) playerArray4thCounter = 0;
NSLog(#"Playing from array: %#",playerArray4th);
currentPlayer = [playerArray4th objectAtIndex:playerArray4thCounter];
playerArray4thCounter++;
}
}
Try to learn about properties and about using getters and setters. Don't take shortcuts unless you know exactly what's going on.
So define the playerArray4th property in your header file:
#property (nonatomic,retain) NSArray *playerArray4th;
And then in your .m file create getter/setter:
#synthesize playerArray4th;
Then, always use self.playerArray4th for assigning and getting the variable. The prior objects will be released when needed.
So this will not leak:
self.playerArray4th = [NSArray arrayWithObjects:#"text",#"text",nil];
self.playerArray4th = [NSArray arrayWithObjects:#"new array",#"text",nil];
because the second assignment releases the first array.
Furthermore, read about using autorelease. In short, if you alloc, copy or new, you should either release or autorelease. There's a lot to read about this here on SO which I will not repeat here now.
Don't forget to put self.playerArray4th = nil; in your dealloc method.
I've been reading about NSArrays and NSDictionaires and I think I need the later. I'm trying to populate an object from a small database table. So I can access the string values via a record id. I have to do this several times so putting it into an object makes sense.
I have the basics...
- (void)viewDidLoad {
// WORKING START
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
[dictCategories setValue:#"Utility" forKey:#"3"];
[dictCategories setValue:#"Cash" forKey:#"5"];
NSString *result;
result = [dictCategories objectForKey:#"3"];
NSLog(#"Result=%#", result);
// WORKING END
// Can't get this bit right, current error Request for member
// 'getCategories' in something not a structure or union
NSMutableDictionary *dictCategories2 = self.getCategories;
NSLog(#"Result2=%#", [dictCategories2 objectForKey:#"5"]);
[super viewDidLoad];
}
-(NSMutableDictionary*)getCategories {
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
[dictCategories setValue:#"Utility" forKey:#"3"];
[dictCategories setValue:#"Cash" forKey:#"5"];
return dictCategories;
}
you are calling the method wrong,try [self getCategories]
You're not being clear on what isn't working, but a few things that are obviously wrong (JonLOo might be spot on though) ...
Firstly. You're using the wrong methods, or at least there's a better one -- setValue:forKey: should/could be setObject:forKey: instead. This might be one of the reasons for your issue.
Secondly. You're over-allocating and not releasing properly. dictCategories2 in your viewDidLoad will vanish into the void and bring with it the allocated memory for dictCategories defined in the getCategories method. An easy standard fix for this is to change
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
in getCategories into
NSMutableDictionary *dictCategories = [NSMutableDictionary dictionary];
It will be autoreleased using the latter method by the system.
Thirdly. You want to read up on #property. Instead of getFoo, setBar, the Ob-C standard is to use #properties to (pre)define setters and getter methods. You can then override these to populate default data into your methods when appropriate. You also (probably) want to store the dictionary in your interface as an instance variable, rather than letting it be deallocated all the time. Example of a #property implementation that does this:
#interface foo {
NSMutableDictionary *ingredients;
}
#property (nonatomic, retain) NSMutableDictionary *ingredients;
#end
// ....
#implementation foo
#synthesize ingredients;
// ...
// the #synthesize command above will create getter and setter methods for us but
// we can override them, which we need to do here
- (NSMutableDictionary *)ingredients
{
if (ingredients != nil) {
// we've already got an ingredients variable so we just return it
return ingredients;
}
// we need to create ingredients
ingredients = [[NSMutableDictionary alloc] init];
[ingredients setObject:#"foo" forKey:#"bar"]
return ingredients;
}
In the viewDidLoad method (or anywhere else where you think ingredients might not have been initialized yet), you would do e.g.
NSMutableDictionary *dict = self.ingredients;
Anywhere else you can opt to use just ingredients without self, but if it's nil, your method will never be called, and you will get nil thrown at you.
This is useful in many cases, and is necessary if we want to ever read or write the ingredients variable from outside of our class. It's outside of what you're asking about, but I brought it up because you're trying to do something similar with self.getCategories.
Hope that helps.