It is a common init pattern to do self = [super init]; which should assign the self pointer
But can i forward the init like this?
- (id)initWithObject:(id)object {
return [self initWithObject:object scrollTo:nil];
}
The code works, but not sure if it is Kosher... and also how can it work without self = [super init]
moving further, is this ok?
- (id)initWithObject:(id)object {
self = [self initWithObject:object scrollTo:nil]; // NOT super
if (self) {
//...
}
return self;
}
Yes, this is fine. I have done this myself a couple of times without problems, and I found a code example in the Apple documentation (scroll down to "Multiple Initializers and the Designated Initializer").
It is absolutely legitimate only if in return operator you call designated initializer or initializer which calls the one. Make sure that one way or other the designated initializer is called.
Yes, sure you can! Note that initWithObject:scrollTo has to return a valid self object.
Related
I have general question about designated initializer. I have a some class and from there i want to call a initializer, but before i started to fill my #properties with passing data i want to make data default. For example:
-(id)initWithDefault:(NSDictionary*)defaultTemplate
{
self = [super init];
_fontColor = [defaultTemplate objectForKey:#"color"];
_fontSize = [[defaultTemplate objectForKey:#"size"] intValue];
return self;
}
-(id)initWithTemplate:(NSDictionary*)template
{
self = [self initWithDefault:myDefaultTemplate];
//now i doing something with my template
return self;
}
Is this is a way to prevent null #properties? It this a correct use of designated initializer? Of course you can assume that myDefaultTemplates is not null, and has not null object in keys.
This seems fine with me. I would use the safe way (presented below), but otherwise your code is fine.
-(id)initWithTemplate:(NSDictionary*)template
{
if(self = [self initWithDefault:myDefaultTemplate]) {
//now i doing something with my template
}
return self;
}
Your implementation is perfectly fine, given the fact that _fontColor and _fontSize variables are your local variables of properties.
adig's suggestion is just an enhancement on what you already have implemented. This check takes care of the situation, when your object does not get allocated due to any reason.
This is currently what I have for my init,
- (id)init
{
self = [super init];
if (self) {
self.url = [[NSURL alloc] init];
self.blurb = [[NSString alloc] init];
self.author = [[NSString alloc] init];
}
return self;
}
It does nothing, but I have another method called initWithObject: that will use its argument to fill up the instance variables url, blurb, and author. I don't know what I should be doing with this init. Should I throw an exception? What other options do I have?
If you want to override your standard -init method you could either return nil (if you do not want -init to be used) or do:
- (instancetype)init
{
return [self initWithObject:nil];
}
If you want to stop the use of -init completely you can tag it as an unavailable attribute or use NSAssert:
// .h
// ...
- (instancetype)init UNAVAILABLE_ATTRIBUTE;
// ...
// .m
- (instancetype)init
{
NSAssert(NO, #"This method should not be used");
return nil;
}
You can use either UNAVAILABLE_ATTRIBUTE or NSAssert(), but if you use UNAVAILABLE_ATTRIBUTE you need some kind of implementation of -init, even if it just returns nil.
You don't have to have a plain init—you can simply have your initWithObject: method. If you're doing any basic setup that will remain the same 80% of the time, or if you have some common code in all your initializers, you can override init, but you are not required to.
Also consider changing your initWithObject: name to be more specific, to something like initWithPost: (I'm assuming this is some kind of blog-entry fetcher based on your ivars) so it's more apparent what object is desired.
I think you misinterpreted what you read. I don't think you would throw an exception. You could; however, leak memory. If your initWithObject: method looks like this:
- (id)initWithObject:(id)obj {
if ((self = [self init])) {
self.url=[obj url];
self.blurb=[obj blurb];
self.author=[obj author];
}
return self;
}
And you would be perfectly fine. You could get an exception if your object was instantiated with -init and you used a variable which was assigned, assuming it was real. So in your subsequent methods be sure to check that the objects exist before using them.
If you made your object with -init rather than -initWithObject this could throw an exception:
- (void)dealloc {
[url release];
[blurb release];
[author release];
[super dealloc];
}
The rule that Apple has established for Cocoa programming is that every class must have one initializer method which is the "Designated Initializer". Every other initializer for the class must call that D.I.* The D.I. itself must call the superclass's D.I. Generally, the initializer with the greatest number of arguments (the one that most completely specifies the state of the new object) is the D.I.
In your case, with the bare init, and initWithObject:, the second would likely be the D.I. You would therefore override init to call initWithObject: with some default argument:
- (id) init {
return [self initWithObject:[Object objectWithURL:[NSURL URLWithString:#"http://www.apple.com"]
blurb:#""
author:#""]];
}
This will result in a sort of dummy object, which is correctly initialized with useless data. (Outside of ARC, be sure to watch the memory management of the default argument(s) -- you want to use an autoreleased/unowned object(s).)
*Sometimes an exception is made for initWithCoder:.
If you have any method that you don't want called and that you don't want your subclass to support, throwing an exception in a Debug build is perfectly reasonable.
What is the proper way of overloading init methods with variable param number? I'm doing it:
- (id)initWithInt:(int)a
{
return [self initWithInt:a andString:nil];
}
-(id)initWithInt:(int)a andString:(NSString*)str
{
self = [super init];
if (self) {
NSLog(#"%# %i",str,a);
}
return self;
}
It works, but the return [self initWithInt:a andString:nil]; does not seem right to me(there is no self at the moment, right?)
No, this is perfectly valid. Self is already setup, the idiom of reassigning self is for a couple of different edge cases (initialization fails, the initializer wants to return a different instance then the one you allocated, etc).
For aesthetic reasons, I decided to change this:
if ((self = [super init])) {
// init self
}
return self;
Into this:
if (!(self = [super init])) return nil;
// init self
return self;
In theory, they do the same thing. The first one is the classic way, simply works. Debugging the second one, I found that it almost worked. The "if" does it right, the init code also, but, after returning "self", the debugger get back to the "if" and returns nil!
All classes I made with the second one I'm reverting to use the "correct" way because they where initing with nil, but I really want to know why does it behaves like that! I'm afraid that this may be the result of something else wrong!
There's absolutely no difference between your two versions other than aesthetic preference, so something else must be going wrong. Perhaps you should post your whole init method?
I created a test class for this, with the following init method:
- (id)init
{
if (!(self = [super init])) return nil;
[self setText:#"foo"];
return self;
}
It initializes as expected, and I can access the text property. So as Nick pointed out, something else must be malfunctioning.
I've read in many places that you should always initialize Objective-C objects like so:
- (id) init {
if (self = [super init]) {
....
}
return self;
}
Because the super's init method may return a separate object from the current self.
Now I'm trying to do something like this, and I'm not sure if I have it right, vis-a-vis how retaining and releasing should work:
- (id) init:(int)idx {
id obj = [Cache findSelf:idx];
if (obj) {
[self release];
self = [obj retain];
} else {
self = [self doLoad];
}
return self;
}
I'm mostly curious if this is the correct way to do the retaining and releasing of self and obj. Is there a better way?
You're correct about the self = [super init] part, since some Cocoa classes actually do return a different object than the one that was allocated. However, this is the exception rather than the rule, and doing so in your own code should be exceedingly rare or not done at all. Although it may be tempting to intercept -init calls, you'd be going against the grain of established convention and what Objective-C programmers expect the code to do.
This type of -init method is generally a bad approach, since -init methods should be as straightforward as possible, and should really be concerned with initializing the object. I'd probably write a convenience method like this:
+ (id) instanceForIndex:(NSUInteger)index {
id obj = [Cache findSelf:index];
if (obj == nil) {
obj = [[self alloc] init];
// Add to cache
}
return [[object retain] autorelease];
}
Then call this method instead of -init. This will make the -init logic much cleaner.
Also, I'm not sure what your Cache class does, but it could be worth rethinking that implementation, and using a hidden static variable to store instances (for example, an NSMutableDictionary, where the key is an NSNumber created from the index). This SO question may be of use.
I agree with Quinn that you should use a convenience class method. Still, I think that your init method is mostly correct, except in your else clause you need to call the parent initializer, i.e. self = [super init].