I am designing a new application by modernizing code I wrote in the past. This old code uses the class/delegate model and I am trying to transform them to use blocks as callbacks, not the delegate stuff.
What I do is to create a property like
#property (nonatomic, copy) void (^onTouch)(NSInteger index);
That would pass to the object using that class a block where code can be inserted and in this case executed on touch.
But my problem is this. When you use delegates and you have a method on the delegate protocol, Xcode will warn if you use that class and forget to implement the delegate protocols. Is that a way to do that with blocks? Or in other words: is there a way to make Xcode complain if a callback block is not defined by the caller?
I mean this would be the correct:
MyClass *obj = [[MyClass alloc] init];
obj.onTouch = ^(NSInteger *index){ //call back code to be executed };
This would be OK too
MyClass *obj = [[MyClass alloc] init];
obj.onTouch = nil;
but this would generate a message
MyClass *obj = [[MyClass alloc] init];
// no callback block defined.
Is this possible?
If you want to enforce setting a certain parameter, I would include it in the initializer.
MyClass *obj = [[MyClass alloc] initWithBlock:^(NSInteger *index) { /* code*/ }];
Then, in MyClass:
- (id)init {
// This will result in a runtime error if you use the wrong initializer.
NSAssert(NO, #"Use initWithBlock instead.");
}
- (id)initWithBlock(initWithBlock:^(NSInteger *)block) {
self = [super init];
if (self) {
self.onTouch = block;
}
return self;
}
Also note, attempting to execute a NULL block results in a crash, so make sure to do:
if (self.onTouch) { self.onTouch(); }
Wherever you run the block.
First, I strongly recommend defining types to represent your blocks - makes them a lot easier to work with, especially if you need to refactor the parameters.
You can't write code that distinguishes between "I set this property to nil" or "the runtime initialized this property to nil", at least not without some crazy runtime code to check the stack. Only option I can think of would be to use the null object pattern. Before I elaborate, bear in mind that I haven't actually tried to test this, but it should work. Define a block that means 'has no value' and set your property to point to that block on init. Then you can compare to that NullBlock at runtime to identify if someone explicitly set the property to nil (because it would be nil at that point) or gave it a real non-nil value.
Alternatively, if you don't mind manually writing your set accessors, you could have a BOOL that tracks if someone set the property explicitly. Then when you call the block just check if someone actually set the value or not.
#synthesize onTouchBlock=_onTouchBlock;
MyBlock _onTouchBlock;
BOOL _onTouchBlockWasSet;
- (void)setOnTouchBlock:(MyBlock)block {
_onTouchBlockWasSet = YES;
_onTouchBlock = block;
}
I would not recommend passing the value in the initializer because that makes it tied to the creation of that object type. If you wanted to change the block in code based on some condition, you'd be back to square one. Also, it prevents you from using storyboards which create that object.
Related
I have a NSString called fontType
and I am trying to have a custom setter for it:
- (void) setFontType:(NSString *) fType
{
if (self.fontType != fType){
[fontType release];
self.fontType = [fType retain];
//some more custom code
}
}
Is there any issue with this?
A few things that stand out for me:
do not use self. inside of custom accessors. access the variable directly
it's better use copy semantics for properties of a type that has a
mutable subtype
be careful with whatever is // some more custom code
My personal style preferences are like so:
-(void)setFontType:(NSString *)fontType_ {
if (fontType == fontType_) return; // get out quick, flatten the code
[fontType release];
fontType = [fontType_ copy];
// some more code
}
Cocoa with Love has a good article on this topic. It's worth a read.
When you do self.fontType = newFontType, you are doing [self setFontType:newFontType] (just with a different syntax), this means you are calling the method inside itself.
This is called recursion, but in this case, you don't have a base case in which the method will stop calling itself, so my guess is that this method will call itself until the app crashes. In short, recursion is not something you want here.
Just replace self.fontType = [fType retain] with fontType = [fType retain] (Assuming the var linked to the fontType property is called fontType as well).
PS. At the end of the question you asked
Is there any issue with this?
If you didn't try this, then you shouldn't even be asking that here on StackOverflow, and if you did try it, then you should have realized that this method didn't work, so that last line is pretty meaningless. Just saying.
i have some trouble writing a method in Objective-C to make an object nil. Here is some example :
#interface testA : NSObject
{
NSString *a;
}
#property (nonatomic, retain) NSString *a;
+(testA*)initWithA:(NSString *)aString;
-(void)displayA;
-(void)nillify;
#end
#implementation testA
#synthesize a;
+(testA*)initWithA:(NSString *)aString{
testA *tst=[[testA alloc] init];
tst.a=aString;
return [tst autorelease];
}
-(void)displayA{
NSLog(#"%#",self.a);
}
-(void)nillify{
self=nil;
}
- (void)dealloc {
[a release];
[super dealloc];
}
#end
int main(int argc, char **argv){
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
testA *test=[testA initWithA:#"some test"];
[test displayA];
test=nil;
//[test nillify];
NSLog(#"after setting to nil");
[test displayA];
[pool release];
return 0;
}
Apparently , when I set test object to nil and then call some method on it nothing happens , but if i call nillify instead of directly setting it to nil , displayA method works normally like test object is still there. Is there a workaround for nillify method to function properly ?
Your help is much appreciated !
You can't actually do something like this, because setting 'self' to nil only has any effect within the scope of that method (in your case, 'nilify'). You don't have any actual way to effect the values of pointers located on other parts of the stack or in random places in the heap, for example.
Basically any code that holds a reference to some object is responsible for maintaining and clearing those references itself. If you have some use case where random sections of code may need references to "live" objects of some kind, but where you'd want those object references to go away in response to some external event (maybe a user tracking system or something), you could do something with notifications, but the various modules tracking those "live" objects would still be responsible for listening for notifications and cleaning up references when they received them.
The 'nilify' thing, however, can't possibly work.
You cannot do what you're trying to do. self is just a local reference to an object that actually exists elsewhere. Setting it to nil doesn't mean anything. An object doesn't, in general, own itself, and it certainly doesn't control other objects' references to it. It's up to the owning objects to manage its lifetime.
There are a few things wrong with your code.
First, by convention, class names start with an uppercase letter. Please stick to these naming conventions as it will make it harder for other developers to work with your code (and even confuse you).
Next, your initWithName:... According to the naming conventions, a method with init in its name should be an instance method, not a class method. So either name it newWithName: or turn it into an instance method like this:
-(testA*)initWithA:(NSString *)aString{
self = [super init];
if (!self) return nil;
tst.a=aString;
return self;
}
If you keep it as class method (and name it newWithName:) you should not return a autoreleased object since according to the naming conventions method that start with init... or new... return a retained object. If you do not follow these conventions, the static analyzer will give you "false" warnings and it will become useless for you.
Now for the reason your nillify doesn't work: the self is in fact an argument to a method. Under the hood, your nillify method actually has two arguments that you do not see: the self pointer and the selector pointer. This means, self is actually a variable on the stack. And if you overwrite it, you only overwrite that stack variable but that doesn't influence your test variable which is somewhere else.
As an example, consider a method - (void)foo:(NSString *)bar;. The compiler turns it into the equivalent of the C function (void) foo(id self, SEL _cmd, NSString *bar).
I am newbie to iPhone programming. I have the following doubt which is stopping me to go ahead. Please consider the following code:
---------.h------
#interface myClass: UIViewController
{
UIImage *temp;
}
#property (nonatomic, retain) UIImage *temp;
---------.m------
#interface myClass
#synthesize temp;
-(void) dealloc
{
[temp release];
[super dealloc];
}
The above is the only program code. Thats it ... nothing else. Do I need to declare [temp release] in dealloc method even though I am not using the property accessor method in my program at all. What if I don't declare [temp release] in dealloc. Will that create memory leak as I am releasing something which I haven't retained as I am not calling property accessor method.
Also when i print retain count for temp why does it show 0 even though it is getting retained in #property.
Thanks in advance
If no value has ever been assigned to (an instance of) myClass.temp, then there won't be a leak. But you should release it in your dealloc.
#property is only a declaration that instance of myClass will have this property. You need to assign it a value before that value gets retained.
myClass *instance = [[myClass alloc] init];
// instance will now retain the value passed in
// and is therefore responsible for releasing it
instance.temp = [UIImage imageNamed:#"whatever"];
// if instance is not retained anywhere else,
// its dealloc will be called
[instance release];
On a sidenote, you should give your classes names that start with an uppercase
letter, i.e. MyClass. Not required, but makes things clearer.
You can also use self.temp = nil; in your dealloc You're sorta not supposed but it kinda works better and looks cleaner. It's a bit of an iffy subject...
What you are doing is correct. Scroll to the "dealloc" section of this Apple Doc: Declared Properties
Soon, however, these properties will be cleaned up automatically when you synthesize them (in the next Cocoa update) -- that being said, a convention I have personally began to follow so that my code works in the future is setting self.temp = nil; in dealloc instead of sending a release message (read the apple doc i posted, it explains this). The accessor method created at runtime releases the object first, so for me and quite a few other devs, this is a better/safer way of cleaning up declared properties in our dealloc.
Your code is correct.
The general rule is that, for all variables you declare in #interface, you must clean them up in -dealloc. Some variables will need to be released, others just need to be nil'd out, depending on how you've declared the #property.
In your example above, temp may never have been given a value explicitly by you, but the ObjC runtime will have initialized the value of temp to nil when an instance of your class gets allocated.
Sending a -release to a nil object is generally not a problem, so the [temp release] is fine. It's a no-op. When temp has a non-nil value in -dealloc, the [temp release] gets to do its job of freeing up the memory.
If you need temp to have a non-nil value on creation, you'll need to implement the -init method and make sure it gets some value. While your class is legitimate & functional without an -init method, you really should get in the habit including one in every custom class you design.
You'll need the default initializer at a minimum: -init. You may also want to design a more detailed initializer that could be used to give your temp ivar an a value, like -initWithImage:
Here's what you should also be including in your class:
#implementation MyClass
...
- (id) init {
self = [super init];
if (self != nil) {
// The minimal default initializer.
// temp will already have a value of nil, so you don't need necessarily
// need to do anything more, unless temp needs a real value on initialization.
}
return self;
}
- (void) dealloc {
...
}
#end
To implement a more detailed initializer, which would be known as the designated initializer, you would to something like this:
#implementation MyClass
...
- (id) initWithImage:(UIImage *)newImage {
self = [super init];
if (self != nil) {
temp = [newImage retain];
}
return self;
}
// Implement the default initializer using your more detailed initializer.
- (id) init {
// In this default initializer, every new instance comes with a temp image!
return [self initWithImage:[UIImage imageNamed:#"foobar"]];
}
- (void) dealloc {
...
}
#end
Here, the designated initializer -initWithImage: is the authoritative initializer. All other initializers, including -init, get implemented using -initWithImage:.
You get to exercise a lot of discretion over whether to implement any initializers beyond the minimal default initializer. Maybe -init is good enough for your purposes. That's fine. Sometimes more detailed initializers make using the class more convenient. Experience (and the Force) will be your guide.
Note that I didn't use the generated property accessor in either initializer method. If you aren't required by circumstances, you should generally avoid using property accessors in -init methods and -dealloc, primarily because of potential pain-in-the-ass issues with side effects of automatic key-value coding notifications.
The initializer and dealloc methods play a special role in a class. As the class designer, it is your responsibility to set and clean up instance variables in these methods. A good rule of thumb is to leave the use of synthesized property accessors for the callers of your class, and the implementation of other methods in the class.
When doing initialization of an instance, or deallocation, you can and should touch the ivars directly. They're yours. You declared them, so you can handle them directly. When implementing other methods in your class, you generally should use the property accessors.
JeremyP's link to the Cocoa Conceptual documentation on objects is a good one. You should definitely read the sections on Objects, and periodically re-read it as you gain more experience writing custom classes of your own. Eventually, it will all start making sense.
I initialized a class in my singleton called DataModel. Now, from my UIViewController, when I click a button, I have a method that is trying to access that class so that I may add an object to one of its dictionaries. My get/set method passes back the pointer to the class from my singleton, but when I am back in my UIViewController, the class passed back doesn't respond to methods. It's like it's just not there. I think it has something to do with the difference in passing pointers around classes or something. I even tried using the copy method to throw a copy back, but no luck.
UIViewController:
ApplicationSingleton *applicationSingleton = [[ApplicationSingleton alloc] init];
DataModel *dataModel = [applicationSingleton getDataModel];
[dataModel retrieveDataCategory:dataCategory];
Singleton:
ApplicationSingleton *m_instance;
DataModel *m_dataModel;
- (id) init {
NSLog(#"ApplicationSingleton.m initialized.");
self = [super init];
if(self != nil) {
if(m_instance != nil) {
return m_instance;
}
NSLog(#"Initializing the application singleton.");
m_instance = self;
m_dataModel = [[DataModel alloc] init];
}
NSLog(#"ApplicationSingleton init method returning.");
return m_instance;
}
-(DataModel *)getDataModel {
DataModel *dataModel_COPY = [m_dataModel copy];
return dataModel_COPY;
}
For the getDataModel method, I also tried this:
-(DataModel *)getDataModel {
return m_dataModel;
}
In my DataModel retrieveDataCategory method, I couldn't get anything to work. I even just tried putting a NSLog in there but it never would come onto the console.
Any ideas?
Most likely you are sending messages that get ignored, e.g. they're being sent to objects which don't exist/aren't the one you're looking for, and for some reason aren't crashing. This occurs in the case of messaging nil, or possibly other illegitimate values. Although you seem to expect that the m_ variables will be initialized to 0, this is not good form, and furthermore you are not following a very typical objc pattern for your singletons -- m_dataModel should be an ivar of m_instance, and m_instance should probably be declared static, as you probably don't want it accessed from other files directly. In addition, the most likely source of your bug is somehow the -init method, which should never be called on a singleton -- instead do something like this:
+ (ApplicationSingleton *)sharedInstance {
static ApplicationSingleton *instance = nil;
if(!instance) {
instance = [[self alloc] init]; //or whatever custom initializer you would like, furthermore some people just put the initialization code here and leave -init empty
}
return instance;
}
the code you have now leaks because you allocate an object (self) and don't release it before returning a potentially different instance (the shared one if one already exists), such that the newly allocated one is typically lost.
I have a class that contains a few instance methods which need to be called from another class. I know how to do that -
TimeFormatter *myTimeFormatter = [[TimeFormatter alloc] init];
[myTimeFormatter formatTime:time];
However, I don't want to have to alloc and init TimeFormatter every time I need to call one of its methods. (I need to call TimeFormatter's methods from various methods in another class).
I tried putting
TimeFormatter *myTimeFormatter = [[TimeFormatter alloc] init];
"by itself", or not in any blocks, but when I compile, I get an "initializer element is not constant" error.
Any input is greatly appreciated!
You can use the singleton pattern. You can read more about it here.
Specifically, you'd do something like:
static TimeFormatter* gSharedTimeFormatter = nil;
#implementation TimeFormatter
+ (TimeFormatter*)sharedTimeFormatter {
if (!gSharedTimeFormatter) {
#synchronized(self) {
if (!gSharedTimeFormatter) {
gSharedTimeFormatter = [[TimeFormatter alloc] init];
}
}
}
return gSharedTimeFormatter;
}
...
#end
Notice that we check if the variable is null, and if it is, we take a lock, and check again. This way, we incur the locking cost only on the allocation path, which happens only once in the program. This pattern is known as double-checked locking.
However, I don't want to have to alloc and init TimeFormatter every time I need to call one of its methods. (I need to call TimeFormatter's methods from various methods in another class).
I think it's worth clarifying some OOP terminology here.
The reason you need to alloc and init TimeFormatter is because your methods are instance methods. Because they're instance methods, you need an instance, and that's what alloc and init provide. Then you call your methods on (send messages to) the instance ([myTimeFormatter formatTimeString:…]).
The advantage of allowing instances is that you can keep state and settings in each instance, in instance variables, and make the latter into publicly-visible properties. Then you can deliberately have multiple instances, each having its own settings configured by whatever's using that instance.
If you don't need that functionality, you don't need to make these instance methods. You can make them class methods or even C functions, and then you don't need a TimeFormatter instance. With class methods, you send messages directly to the class ([TimeFormatter formatTimeString:…]).
And if you do want settings shared among all instances (and you don't have any state to keep), then you're right that you can just have one instance—a singleton.
The reason for that parenthesis is that shared state is bad, especially if two threads may use the time formatter concurrently. (For that matter, you could say that about settings, too. What if one thread wants seconds and the other doesn't? What if one wants 24-hour and the other wants 12-hour?) Better to have each thread use its own time formatter, so that they don't get tripped up by each other's state.
(BTW, if TimeFormatter is the actual name of your class: You are aware of NSDateFormatter, right? It does let you only format/parse the time.)
Here's a detail example of a sharedMethod. Credit goes here
#implementation SearchData
#synthesize searchDict;
#synthesize searchArray;
- (id)init {
if (self = [super init]) {
NSString *path = [[NSBundle mainBundle] bundlePath];
NSString *finalPath = [path stringByAppendingPathComponent:#"searches.plist"];
searchDict = [[NSDictionary alloc] initWithContentsOfFile:finalPath];
searchArray = [[searchDict allKeys] retain];
}
return self;
}
- (void)dealloc {
[searchDict release];
[searchArray release];
[super dealloc];
}
static SearchData *sharedSingleton = NULL;
+ (SearchData *)sharedSearchData {
#synchronized(self) {
if (sharedSingleton == NULL)
sharedSingleton = [[self alloc] init];
}
return(sharedSingleton);
}
#end
A very nice, and easy, way to setup a Singleton is to use Matt Gallager's SYNTHESIZE_SINGLETON_FOR_CLASS.
It sounds like you want to make TimeFormatter a singleton, where only one instance can be created. Objective-C doesn't make this super easy, but basically you can expose a static method that returns a pointer to TimeFormatter. This pointer will be allocated and initialized the first time in, and every time after that same pointer can be used. This question has some examples of creating a singleton in Objective-C.
You are trying to declare your variable outside the class? If to do it the way you want to do it you gotta declare it as static so
static TimeFormatter *myFormatter=...
From the name of the class though i dont see why you would wnat to keep one instance of your class... you can also do this with a singleton as described above, that is if you want to keep one instance of your class for the app as a whole.