I've linked a button to the following method:
- (IBAction)searchButton
{
NSString *searchText = _searchField.text;
NSLog(#"lol");
[_search testSearch:searchText];
}
The last line calls the method testSearch within an object named search, defined as follows:
#property (strong, nonatomic) Search *search;
Within Search, testSearch is defined as follows:
-(void)testSearch:(NSString *)testString
{
NSLog(#"HELLO");
}
My final output, when I click search, is only "lol" (each time I click the button). It does NOT print "HELLO", as testSearch should be doing. I have included testSearch in Search.h, so it should be accessible...why isn't this method being called?
You should start by initializing your _search ivar to an instance of Search in your designated initializer (or in viewDidLoad or some other "user gonna use this" method).
- init {
if ((self = [super init])) {
_search = [[Search alloc] init];
}
return self;
}
You should generally avoid lazy initialization in getter methods for a variety of reasons:
It adds unnecessary code; use #property and the default synthesized implementations. Leads to simpler code and less of it.
a getter that does lazy initialization yields a getter that causes mutation. That is inconsistent it is quite odd to see a KVO change notification when calling a getter (unless, of course, you don't fire the KVO notification... at which point, you have non-observeable mutation).
a getter that causes mutation is inherently not thread safe unless you add the code, tricky code, to make it so.
lazy initialization is generally a premature optimization. Unless you have an identifiable memory or CPU performance issue caused by initializing a resource "too soon", then adding the complexity of lazy initialization is wasted effort.
lazy initialization can lead to weird ordering dependencies and other complexities. Far better to have a known entry point for initializing a subsystem than to rely on subsystem X being initialized prior to Y, both by side effect.
The search object you are sending the message to has not been instantiated so you are sending a message to nil. In Obj-C this does not crash the program, instead it does nothing. Its a best practice in Objective-C programming to perform lazy instantiation in the getter method of the iVar. Additionally, you would have to couple this best practice with not accessing your iVars directly and use the setters and getters for whichever iVar you are trying to access. Below is an example of lazy instantiation in the getter method for your search iVar:
-(Search *)search
{
if(!_search){
_search = [[Search alloc]init];
}
return _search;
}
Here is your method call while NOT accessing the iVar directly:
[search testSearch:searchText];
Related
What does self mean in Objective-C? When and where should I use it?
Is it similar to this in Java?
self refers to the instance of the current class that you are working in, and yes, it is analagous to this in Java.
You use it if you want to perform an operation on the current instance of that class. For example, if you are writing an instance method on a class, and you want to call a method on that same instance to do something or retrieve some data, you would use self:
int value = [self returnSomeInteger];
This is also often used for accessor methods on an instance (i.e. setters and getters) especially with setter methods, if they implement extra functionality rather than just setting the value of an instance variable, so that you do not have to repeat that code over and over when you want to set the value of that variable, for example:
[self setSomeVariable:newValue];
One of the most common uses of self is during initialization of a class. Sample code might look like:
- (id)init
{
self = [super init];
if(self!=nil) {
//Do stuff, such as initializing instance variables
}
return self;
}
This invokes the superclass's (via super) initializer, which is how chained initialization occurs up the class hierarchy. The returned value is then set to self, however, because the superclass's initializer could return a different object than the superclass.
self is an implied argument to all Obj-C methods that contains a pointer to the current object in instance methods, and a pointer to the current class in class methods.
Another implied argument is _cmd, which is the selector that was sent to the method.
Please be aware that you only get self and _cmd in Obj-C methods. If you declare a C(++) method, for instance as a callback from some C library, you won't get self or cmd.
For more information, see the Using Hidden Arguments section of the Objective-C Runtime Programming guide.
Yes, it's exactly the same as "this" in Java - it points to the "current" object.
Two important notes:
The class itself, e.g. UIView (I'm NOT talking about a UIView object) is itself an object, and there is a self associated with it. So for example, you can reference self in a class method like this:
// This works
+(void) showYourself { [self performSelector: #selector(makeTheMostOfYourself)]; }
// Class method!
+(void) makeTheMostOfYourself { }
Note that the compiler does NOT raise any warnings or errors, even if the self you mean to reference is an object and not a class. It is VERY easy to cause crashes this way, for example:
// This will crash!
+(void) showYourself { [self performSelector: #selector(makeTheMostOfYourself)]; }
// Object method!
-(void) makeTheMostOfYourself { }
// This will crash too!
-(void) showYourself2 { [self performSelector: #selector(makeTheMostOfYourself2)]; }
// Class method!
+(void) makeTheMostOfYourself2 { }
Sadly, this makes class methods a bit harder to use, which is unfortunate because they are a valuable tool for encapsulation through information hiding. Just be careful.
Wow, that many half-correct answers and misleading hints. This let me answer the Q even there is a accepted answer for years:
First of all: It is really hard to compare a concept of messaging/calling in the context of an early binding, static typing language as Java with a late binding, dynamically typing languages as Objective-C. At one point this will break. I would say: No, this is not similiar, since the typing and dispatching concepts of both language are fundamental different so nothing can be similar to the other one. However, …
Then we should differ between the "two sides" of self.
A. Using self
When you use it in a message, it is simply an object reference as any other:
[self doSomething];
[anotherObject doSomething];
Technically both lines works identically (accept of having a different receiver, of course). This especially means, that the first line does not lead to an execution of a method inside the class of self, because self does not necessarily refer to "that class". As every message inside Objective-C (single exception: messages to super)this can lead to the execution of a method in a subclass:
#interface A : NSObject
- (void)doSomething;
- (void)doAnotherThing;
#end
#implementation
- (void)doSomething
{
[self doAntoherThing];
}
- (void)doAnotherThing
{
NSLog( #"A" );
}
#interface B : A
- (void)doSomething; // Not necessary, simply as a marker
#end
#implementation B
- (void)doAnotherThing
{
NSLog( #"B" );
}
In a code like this
B *b = [B new;]
[b doSomething];
The line
[self doAnotherThing];
in class A will lead to the execution of -doAnotherThing (B), because messages to self are late bound as every other message. The result on the console will b "B", not "A". Using self as a receiver you should not think of a single special rule. There is completely none.
(And the above example is a very good example for using self in class methods, because the same situation can occur on class methods. Using the class itself breaks polymorphism, what is one of the worst idea at all in OOP. DO use self in class methods, too.)
B. Getting self
What is self pointing to? It points to the object to whom the message is sent that caused the execution of the current method.
Having …
…[someObject doSomething]… // some object is a reference to an instance object
… as a message, a method is called, in the most simple case …
- (void)doSomething
{ … }
In such a case, self can point to an instance of the class, the method belongs to. And it can point to an instance of a subclass, the method belongs to, too. You don't know. (And this information is preserved using self to send a message as explained above.)
If the message is sent to a class object, self points to the class object, that was the receiver of the message. This is completely analogous. Therefore it is possible that self points to a subclass object:
#interface A : NSObject
+ (void)doSomething;
+ (void)doAnotherThing;
#end
#implementation
+ (void)doSomething
{
[self doAntoherThing];
}
+ (void)doAnotherThing
{
NSLog( #"A" );
}
#interface B : A
- (void)doSomething; // Not necessary, simply as a marker
#end
#implementation B
+ (void)doAnotherThing
{
NSLog( #"B" );
}
Having this classes
…[A doSomething]…
self inside -doSomething (A) points to the class object of B. Therefore [self doAnotherThing] of B(!) is executed. This is clearly different from
+ (void)doSomething
{
[A doAntoherThing];
}
The latter version causes relevant harm to the principles of OOP.
As a side note it is possible that self inside a class method of a root class points to an instance object of the root class or any subclass. You have to keep this in mind, when writing categories on NSObject.
self is an object pointer to the current instances dispatch table. It is an implicit first argument to every member function of an object, and is assigned when that function is called.
In functions like init, you need to be careful that when you call the super class init you reassign self to be the return value as the super class init may redefine what self points to.
super is similar to self except it points to the superclass dispatch table.
I have a property that sometimes needs to be a strong reference, and other times needs to be a weak reference. I thought I'd cracked it with the following custom getter/setter:
- (PropertyData *)property
{
return _strongProperty? _strongProperty : _weakProperty;
}
- (void)setProperty:(PropertyData *)newProperty
{
_strongProperty = newProperty;
_weakProperty = nil;
}
- (void)weaken
{
if (_strongProperty != nil) {
_weakProperty = _strongProperty;
_strongProperty = nil;
}
}
Previously there was an ivar called property and an #synthesize property. I removed both and replaced with:
PropertyData __weak *_weakProperty;
PropertyData *_strongProperty;
The class using this class calls weaken when it wants to convert to a weak reference to prevent a retain cycle.
This all works fine on iOS, but running the same code on OS X doesn't work at all. In the debugger, setProperty: has no effect on either of the new ivars right from the outset, and it crashes before weaken is ever called.
I'm confused about whether you need a #synthesize a=b statement when writing custom getter/setters (deployment target OS X 10.10). Not obvious how to do that in this case because there are two backing ivars.
Follow the conventions, or you confuse anyone including yourself.
Start your instance variables with an underscore. Don't use synthesise.
Apart from that, you have two problems. One, calling weaken twice sets the property to nil. Second, there's a good chance that your property has only one reference count and goes away when you call weaken.
I am working on a subclass of SKNode called UtilityNode
#implementation UtilityNode
- (id)initWithName:(NSString *)rootName {
self = [super init];
if(self) {
[self setName:rootName]; // ?
}
return self;
}
#end
I am setting up a designated initialiser for the new node initWithName: where I am trying to initialise the name of the superclass SKNode when creating the new subclass. I was under the impression that I could just write _name = rootName; but _name was flagged as undeclared. I have got it working (as you can see above) by using [self setName:rootName]; Can anyone shine some light on this, am I doing this correctly?
This is correct. The _name instance variable was probably declared as #private (the default for auto-synthesizing properties) and therefore it is inaccessible by subclasses.
In well-designed class hierarchies and specifically framework classes for which there is no source code available you will find most if not all instance variables inaccessible by subclasses because it hides the implementation detail and makes future changes to the base class possible. Imagine if the base class needed to verify the name property whenever it changes - if subclasses could assign directly to the ivar they would bypass the checks added to the ivar setter method.
PS: I find dot notation friendlier on the eyes:
self.name = rootName;
UPDATE regarding the "don't send messages to self in init/dealloc" rule:
This can easily be avoided by redesigning the class to not take non-essential parameters in its init method. A cleaner version of this subclass would be:
UtilityNode* un = [UtilityNode node];
un.name = #"some name";
If the node absolutely requires the parameter to be set, warn the user via an NSAssert in the method where the name is required to be valid.
I am making a simple app and this piece of code has been giving me issues.
Here is my property.
In ConverisonCalculator.h
#property (strong, nonatomic)NSString *startingUnit;
In Viewcontroller.m I am using this code and everytime I NSLog it I am getting (null)
_calculator.startingUnit = #"FPS";
Also here is my lazy instantiation of the object.
- (ConversionCalculator *)calculator{
if (!_calculator) _calculator = [[ConversionCalculator alloc]init];
return _calculator; }
I hope this is enough for you to answer my question. I am not override the default setter either.
Here is my logging.
NSLog(#"%#", [_calculator startingUnit]);
_calculator.startingUnit = #"FPS";
This is not using your property. This is direct access to the instance variable, so your lazy loading code is never called.
If you define properties, always access them through the property:
self.calculator.startingUnit = #"FPS";
Otherwise, you might as well be using instance variables. The only exception is inside the accessor methods themselves, or in init or dealloc methods (in some cases).
I get the above message in XCode 4.6. I've done a pretty thorough search and but nothing seems to match the exact circumstances surrounding my issue. Admittedly, I'm relatively new to iOS dev, and memory-management has never been my strong suit, but this just has me completely miffed.
I have an instance variable theLink which is defined in the class Game as follows:
#interface Game : NSObject
// Class objects
#property(nonatomic,retain) NSMutableArray *queryItems;
#property(nonatomic,retain) NSMutableArray *theArray;
#property(nonatomic,retain) NSString *theLink;
#property(nonatomic,retain) NSString *thePath;
theLink is set in the makeGame method which is called in the method initialiseGame in my view controller:
- (void) initialiseGame
{
bool gameCreated = FALSE;
while (!gameCreated)
{
gameCreated = [theGame makeGame:#"ptl"];
}
[loadingIndicator stopAnimating];
[loading setText:#"Tap to Start"];
[self performSelector:#selector(setLabels) withObject:nil afterDelay:0.0];
}
(Note: the performSelector afterDelay is used to allow the view to update before continuing. Bit of a hack but I couldn't work out a better way!)
The app then loads the game, and when the user taps the screen to start, the next method which is called from the view controller is:
- (void) setupLink
{
...
for(int i=0; i<[theGame.theLink length]; i++) {
...
}
}
It is on this reference to theGame.theLink where I'm am getting the crash.
What has me most confused is that if I call theGame.theLink from inside the initialiseGame method, it is displays correctly, and also calling any other variable from the Game class (such as thePath or theArray works perfectly, so theGame object has not been deallocated in it's entirety, only the variable theLink.
It seems to me that the variable is being deallocated somewhere as the view controller is being updated. I haven't released the variable, and can't work out why only this variable is being deallocated. As I said at the start, memory-management is not my strength!
Any help/ideas would be hugely appreciated. Let me know if you require any more details.
Thanks heaps,
Andrew
EDIT: Setting of theLink within makeGame
- (bool) makeGame:(NSString*)gameType
{
...
[self getLink];
}
- (void) getLink
{
...
if (... && ((arc4random() % 10) > 8))
{
theLink = #"Animals";
}
}
There are many different ways theLink may be set, depending on random numbers and other factors. This is the most basic form which simply sets it to a static string. It doesn't matter how theLink is set or what it is set to, the program always crashes at the same point.
If theLink is being set to the parameter being passed to it ,#"ptl" or some similar temporary string, it will give you a problem, because it is just a pointer pointing at the current location that is holding #"ptl". After the makeGame method is completed, your system will assume that it is all done with #"ptl" and just free it up.
When you make an #"stringwhatever" in your code, it is supposed to be the equivalent of making an NSObject that is an immutable literal instance of #"stringwhataver". It should, in theory handle all the reference counting in a nice way, but when you are doing your own memory management, there are so many ways to lose count of your references.
There's a pretty simple rule to follow. If you've declared properties, access them via the property. To do otherwise (as you are doing above, with theLink = ...) bypasses all of the memory management built into the property accessors.
self.theLink = ...
Would have solved this problem under MRC. Switching to ARC has "solved" your problem without you understanding the root cause.