I sometimes like to organize IB elements into NSArrays primarily to help me organize my elements. Most often, different classes of objects make it into the same array with each other. While this is a convenient way of organization, I can't seem to wrap my head around why if I have an array like this:
NSArray *array = [NSArray arrayWithObjects:((UITextField *)textField), ((UISegmentedController *)segmentedController), nil];
Why I get "Does not respond to selector" messages when I put a for loop like this:
for (UITextField *text in array) {
[text setText:#""];
}
The for loop seems to be passed objects that are not of class UITextField.
What is the point of declaring the object's class if all objects in the specified array are passed through the loop?
EDIT Just for reference, this is how I'm handling it as of now:
for (id *object in array) {
if ([object isMemberOfClass:[UITextField class]]) {
foo();
} else if ([object isMemberOfClass:[UISegmentedController class]) {
bar();
}
}
When you do
for (UITextField *text in...
the object pointers from the array are cast to UITextField* type - so if the object isn't actually a UITextField, all sorts of weird things may happen if you try to call UITextField methods.
So instead use the id type (no * needed, btw):
for (id obj in array)
Then check the type as you do and call the appropriate methods. Or, filter the array to get only objects of a certain type, then go though that type only:
for (UITextField* text in [array filteredArrayUsingPredicate:...])
Edit: here's how to build class filter predicates:
Is it possible to filter an NSArray by class?
What is the point of declaring the object's class if all objects in the specified array are passed through the loop?
The class name is just there to let the compiler know what it should expect to find. This allows it to try to figure out what methods it should expect you to call and how you might treat the object. It's the same idea as passing in an int to a method that takes float. The method will not ignore ints - it's assuming you're passing the correct type. You're just giving this construct a little more credit than it's due:
for (UITextField *text in array)
It just doesn't have that functionality. How you're handling it now is the correct way.
Are you sure you don't get an error when you run that code? The "does not respond to selector" message is a runtime error, not a compile time error. The compiler has no idea whether the objects in the array implement -setText:, but you should certainly get an error when you actually send that message to an instance of UISegmentedControl.
Another possibility is that you've got a class called UISegmentedController that does have a -setText: method. The name of the class that implements the multi-part bar-graph-looking user interface widget is UISegmentedControl. So either the code you're showing isn't real, tested code, or you've got a class that we don't know about.
Related
I have a mutable array property declared and synthesized:
#property (nonatomic, strong) NSMutableArray *arrayOfTasks;
I am using KVC collection Accessors for the same property and also I have other methods which will internally call this KVC Collection accessor method like this:
-(void)insertObject:(CSTaskAbstract *)inTask inArrayOfTasksAtIndex:(NSUInteger)index
{
[[self arrayOfTasks] insertObject:inTask
atIndex:index];
}
-(void)addObjectInArrayOfTasks:(CSTaskAbstract *)inTask
{
[self insertObject:inTask
inArrayOfTasksAtIndex:[[self arrayOfTasks] count]];
}
I had to do some modifications and add the object into the array only when a particular condition is satisfied, so to make sure that this check goes into the designated method, I included the following in the -insertObject KVC Collection accessor method:
-(void)insertObject:(CSTaskAbstract *)inTask inArrayOfTasksAtIndex:(NSUInteger)index
{
if ([inTask isOperatable])
{
[[self arrayOfTasks] insertObject:inTask
atIndex:index];
}
}
-(void)addObjectInArrayOfTasks:(CSTaskAbstract *)inTask
{
[self insertObject:inTask
inArrayOfTasksAtIndex:[[self arrayOfTasks] count]];
}
Now each time when I trigger -addObjectINArrayOfTasks method and if the -isOperatable condition returns boolean NO, the app crashes with no stack trace at all! (Stack trace is at main() of the application). All it says is "index 0 beyond bounds for empty array error".
I am not understanding the reason for this, I am not trying to access the array yet, so I am not giving a chance for framework to complain me that there is no element at index 0. Moreover, I am doing the count of array items check everywhere before accessing the objects out of array. For, if I was trying to access and element out of the bounds index, the app would crash at the same point and let me know exactly where I was trying to access the index out of bounds. That would have been a simple straightforward fix.
Now, to just cross verify, I made a small change in the code like this, which seems to work:
-(void)insertObject:(CSTaskAbstract *)inTask inArrayOfTasksAtIndex:(NSUInteger)index
{
[[self arrayOfTasks] insertObject:inTask
atIndex:index];
}
-(void)addObjectInArrayOfTasks:(CSTaskAbstract *)inTask
{
if ([inTask isOperatable])
{
[self insertObject:inTask
inArrayOfTasksAtIndex:[[self arrayOfTasks] count]];
}
}
I can go ahead with this approach which is working and does not crash, but my concerns are the following:
Adding the same check in designated method would be an added advantage in future if some other programmer would want to invoke the designated method from somewhere else.
Why would the app crash in first case when I wont insert the object into the array in KVC collection accessors based on some condition check?
Thanks for any inputs,
Raj
I think the crash you are seeing is more likely related to internal KVC behavior than your array. That might be the reason you don't see a usable stack trace. Have you enabled the exception breakpoint in Xcode?
KVC basically expects that -insertObject:in<Key>AtIndex: will insert a new object at the given index (presumably 0 in your case). Since it assumes that the object was inserted it should now be accessible by queuing the data structure (NSMutableArray) for the object at the given index. When the condition evolves to NO, you fail to insert this object, which means that an index out of bounds exception is possible when KVO tries to query using the provided index.
The second code snipped you posted avoids this error by not calling the KVC collection accessor when an insertion is not needed.
If you want to minimize the chance of someone incorrectly using those methods, expose just -addObjectInArrayOfTasks: in your public header. In addition you can document this. If you want to make it absolutely certain that -insertObject:in<Key>AtIndex: can't be accessed on int's own, you can add an NSAssert, that checks if the method was called from -addObjectInArrayOfTasks:.
I have just spotted something that I am a little puzzled about, I wonder if someone would be so kind as to clarify it for me.
NSArray *nextArray = [NSArray arrayWithObjects:#"ONE", #"TWO", #"THREE", nil];
for(id eachObject in nextArray) {
NSLog(#"COUNT: %d", [eachObject length]);
}
Why does the above not complain/warn about the fact that I am asking for the length of an id?
In Objective-C id is the general type for any kind of object regardless of class and can be used for instances of a class and for class objects themselves.
The id type is completely nonrestrictive it has no information about an object, except that it is an object. So there's no way for the compiler to know whether or not that object can respond to a method because it doesn't know what kind of object it is.
By using it in your code you're basically saying 'to whatever this is pointing to, perform this operation'.
You use id when you specifically do not want compiler type checking. You can send any message to an id type without a warning, and you can assign an id to any other type without a type cast.
This allows you to fetch an object from an array without using a cast. E.g., you're free to assume that the array contains NSStrings:
NSString* someString = [myArray objectAtIndex:1];
It also allows you to send a message to an object without a cast or a warning. In fact, the message you wish to send may not be part of any formal class or protocol:
id someObject = [myArray objectAtIndex:1];
if ([someObject respondsToSelector:#selector(setName:)])
{
[someObject setName:#"Foo"];
}
The compiler will never type check messages sent to an id. It's partly what enables Objective-C's dynamism.
If eachObject was any other type, then you would get an error if the compiler couldn't resolve the method name.
The NSArray might contain different object types, for example:
NSArray *thArray = [[NSArray alloc] initWithObjects:#"Stack",#"Overflow",[NSNumber numberWithInt:10],nil];
for(id theObject in thArray) {
NSLog(#"COUNT: %lu", [theObject length]);
}
The id can represent any object (in this case NSString or NSNumber),
therefor the compiler cannot know whether the primitive method length exists.
Simply, id means all Objective-C Class. so, length method it belongs a NSString Class. Compiler no throw warning to you.
id is determined dynamically at runtime, is unknown at compile time.
Each object in Objective-C knows what class it has and if it can handle a message. It's not the compiler who checks the class, it is the object itself at runtime.
The class of an object can be undefined at compilertime, but at runtime each object has a defined class.
I am having two arrays, Namely
NMutableArray* first;
NMutableArray* second;
Now I am copying first object to the second array like
for (int i=0;i<first.count; i++)
{
[second addObject:[first objectAtIndex:i];
}
This is ok. I don't know how to access the value of the First Array. I tried like this ,
[second addObject:[[first objectAtIndex:i]name]];
I want to get the name value which is in the first object of first array. I tried using the above line, it is showing some warning. Please help me
Assuming you started with an array like this:
NSArray *array1 = #[#{#name : #"Fred"},
#{#name : #"Bill"}];
You could create a second array that contains the value of a given property of each element of the first array as follows:
NSArray *array2 = [array1 valueForKey:#"name"];
If you then logged the second array...
NSLog(#"%#", array2);
...the resulting output would be
2012-04-18 16:26:11.226 ExampleRunner[23320:707] (
Fred,
Bill
)
EDIT
Note that this will work regardless of whether the objects in the first array are instances of NSDictionary as shown in the example above, or instances of a class or classes that have a name property or instance variable (or an _name instance variable, for that matter). For more information on how and why this works, see the documentation for the NSKeyValueCoding informal protocol:
http://developer.apple.com/library/ios/#DOCUMENTATION/Cocoa/Reference/Foundation/Protocols/NSKeyValueCoding_Protocol/Reference/Reference.html
The brackets are currently in the wrong place:
[second addObject:[[first objectAtIndex:i] name]];
Updated Answer:
Again, I think you should split stuff out into easy to parse lines of code:
for (id theObject in first)
{
// without an actual type, I still think the compiler might
// throw a warning on this next line of code;
// but maybe RJR III is correct and it won't warn.
// I didn't check.
NSString * nameOfObject = [theObject name];
if(nameOfObject)
{
[second addObject:nameOfObject];
}
}
Notice that I do some error checking in here as well (i.e. making sure the name is not nil).
Original Answer:
You're getting a warning because the compiler doesn't know what kind of custom object is being fetched from your call to "[first objectAtIndex: i]". In other words, it doesn't know what kind of object you're trying to get the "name" of.
Cast it to the right type and you'll get rid of the warning.
Or even better, split that one line of multiple things happening at once into two or three lines of code and make your code more readable in the process.
I've got a function called updateTheValue() that I have called using [self updateTheValue] for a while now. Two things happened recently; I added the method calling in the viewDidLoad() method and it spit out a warning saying my class may not respond to this. Second, I want to pass objects to updateTheValue() like strings, but mostly ints, so I declared an NSObject to pass into the method. Can an int fit into an NSObject slot, or what should I use instead?
I would have posted these seperately but they seem to be related since after updating updateTheValue() to accept an NSObject every reference to this function turns out the error that my class "May not respond to -updateTheValue"
You could make your method like this:
-(void)updateTheValue:(NSObject *)anObject
// or use -(void)updateTheValue:(id)anObject
{
if ([anObject isKindOfClass:[NSString class]]) {
// Do your string handling here
}
else if ([anObject isKindOfClass:[NSNumber class]]) {
// Do your number handling here
}
}
Use it like this:
[self updateTheValue:[NSNumber numberWithInt:42]];
I'd suggest doing two different methods though, i.e. updateTheValueWithInt: and updateTheValueWithString: making it easier to read and understand.
Make sure you make the method signature visible before using them, so that the compiler knows what this does.
If you use separate methods you can use int directly without wrapping them into NSNumber objects.
First problem:
updateTheValue() must be declared before you try to call it.
You can either move the definition of function before the calls to it, or add a prototype at the top - eg, add:
(void) updateTheValue;
near the top.
Second problem:
Use an NSNumber, eg [NSNumber numberWithInt:45];
I have a class that I use to setup objects in an array. In this class I have a custom "initWithDictionary", where I parse a JSON dictionary. However, as I am running into NSNull, this crashes my app. To get around this, I set up a class that handles exceptions, so when a string is NSNull, it's replace it with #"". or -1 for integers.
This is my NullExtensions class:
#interface NSNull (valueExtensions)
-(int)intValue;
-(NSString *)stringValue;
#end
#implementation NSNull (valueExtensions)
-(int)intValue {
return -1;
}
-(NSString*)stringValue {
return #"";
}
#end
However, in my initWithDictionary method, the following code crashes my app:
self.bookTitle = [[parsedDictionary objectForKey:#"book_title"] stringValue];
It doesn't work regardless of the object in the parsed dictionary being NSNull or containing a valid string. Only if I do the following (and the string is not null):
self.bookTitle = [parsedDictionary objectForKey:#"book_title"];
Is stringValue incorrect in this case? And if so, how do I use it properly in order to setup proper NSNull replacements?
Thx
You really really don't want to add a category to NSNull that adds such common methods. That will change the behavior of NSNull for all instances in the application, including ones created by the underlying frameworks solely for their private use.
If you need a value class that represents the notion of "value doesn't exist and therefore I'm going to return these default values instead", create a class or instance that represents exactly that.
As for why it crashes, I couldn't tell you without seeing the actual details of the crash.
And, yes, it really is THAT bad to add a category to a class that adds such a common method. All it takes is one bit of code in a plug-in or framework that does:
if ([fooMaybeNull respondsToSelector: #selector(intValue)] bar = [fooMaybeNull intValue];
Not terribly farfetched -- I have had to debug nasty crashers or misbehaviors due to exactly this kind of willy-nilly category addition.
If you are going to add methods to a class via categories, prefix your method names so as to isolate them from existing functionality. It is still fragile, but manageably so.
Instead of creating categories on NSNull, for which you would also have to add a similar category to NSString (that's why it crashes, because real strings do not respond to stringValue) - instead try creating a helper category on NSDictionary like "stringForKey" that uses the code Johan posted and returns an NSString, probably also should enforce all other types get mapped to empty strings as well.
The NSNull extensions you have written look ok to me but using a method like stringValue may be confusing since other classes like NSNumber use this.
Personally though, I think NSNull replacement in this instance is unnecessary. If you just made a quick test you can replace the NSNull where you need to. e.g.
id testObject = [parsedDictionary objectForKey:#"book_title"];
self.bookTitle = testObject==[NSNull null] ? #"" : testObject;
You are asking an NSString for its stringValue. No need to convert a string to a string.
Try this:
if (![[parsedDictionary objectForKey:#"book_title"] isKindOfClass:[NSNull class]]) {
self.bookTitle = [parsedDictionary objectForKey:#"book_title"];
} else {
self.bookTitle = #"";
}
Edit: You should not use the category on NSNull you created. You don't need it, nor should you want it. If the source for the dictionary inserts NSNull instances, go ahead and use my code above. Normally you would expect to simple have no value inserted for the key, at which time you can simple see if [parsedDictionary objectForKey:#"book_title"] returns anything.
Are you sure that the dictionary is returning [NSNull null]? By default, dictionaries return nil, not [NSNull null], when an value isn't found for a key.
However, in my initWithDictionary method, the following code crashes my app:
self.bookTitle = [[parsedDictionary objectForKey:#"book_title"] stringValue];
It doesn't work regardless of the object in the parsed dictionary being NSNull or containing a valid string.
That makes sense, since stringValue is not a valid method on NSString. It will work for NSValue and its subclasses, but not NSString.