I'm facing some difficulty in retrieving properties of "id" type object. This is how I'm accessing it:
I'm doing following to assign an object to id type object from a generic array containing different types of objects and calling method "savedata" to which I'm passing the object as well as its type:
for(id objInArray in genericArray){
NSString *objType = [objInArray valueForKey:#"type"];
[objInArray retain];
[self saveData:objInArray :objType];
}
In savedata method I'm writing following code to retrieve the properties of id object:
-(void)saveData:(id)object :(NSString *)objectType
{
self.managedObjectContext = appDelegate.managedObjectContext;
if([objectType isEqualToString:#"event"])
{
Event * newEvent = (Event *)[NSEntityDescription
insertNewObjectForEntityForName:#"Event"
inManagedObjectContext:self.managedObjectContext];
[newEvent setEletitle:[NSString stringWithFormat:#"%#", [object valueForKey:#"eletitle"]]];
[self saveAction];
}
But the object "object" containing the values fails to assign them to object newEvent.
I also tried to retrive this value in a string object like this:
NSString *eletit = [object valueForKey:#"eletitle"];
[eletit retain];
But eletit is also invalid at the end of this transaction.
Can anybody please help? This' really urgent.
Thanx in advance.
I don't have you answer unfortunately but I have few comments on your code.
Are you sure it's normal you array contain so generics object? It's strange because all your object contained in your array need to respond to "type" or "eletitle" messages, so I guess objInArray is less generic than just "id".
Second, it's not recommended to have selector like saveData::, in Objective-C it's usual and recommended to name the arguments, it's more understandable.
Related
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.
Let's say I have 2 instances of Person class. One named john and one mary. The Person class has 2 properties age and gender. Is there a way to make an iteration thorough all the instances' properties and check if the current property is equal to a given property? Something like this:
for (iterate thorough all properties of instance mary) {
//first iteration
#selector(mary.age)==#selector(john.age) //this would be YES;
//second iteration
#selector(mary.gender)==#selector(john.age) //this would be NO;
}
This question addresses how to list the selectors that an object will respond to:
List selectors for Objective-C object
Using a combination of that and the NSObject protocol's -respondsToSelector: method, you could list all the selectors for john, check if mary responds to them, and vice versa.
You can get the property name as NSStrings and use isEqualToString: method to compare them.
for (iterate thorough all properties of instance mary) {
//first iteration
NSString *marryProperty = [NSString stringWithCString:property_getName(mary.age)
encoding:NSUTF8StringEncoding];
NSString *johnProperty = [NSString stringWithCString:property_getName(john.age)
encoding:NSUTF8StringEncoding];
if([marryProperty isEqualToString:johnProperty])
NSLog(#"YES");
else
NSLog(#"NO");
}
See Printing all declared properties of an object to iterate and get the name of properties. You may need to add them to a set and then compare. If you want to check the type it may help you to read Declared Properties in the objc runtime guide.
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.
I'm working with a subclass of NSManagedObject. Actually, it inherits from a class that inherits from a class that itself inherits from NSManagedObject (that shouldn't be a problem, right?).
The problem
After I make changes to the properties of the object, the object remembers the changes for its lifetime, but the changes are never saved to the database.
How Do I Know This?
I know this because:
when I restart the app, the changes I've made are lost.
telling the context to refresh the object – AFTER I've made changes to the object and told the context to save – sets the object's values back to their original state before I made the changes.
when running the app in the simulator, I can look at the sqlite database file in the Finder, and it's modified date isn't updated when I attempt to save the context.
Nothing is being written to the database!
Context
I'm using the auto-generated delegate methods to create the store coordinator and the context. Then I'm passing the context to the view controllers in their init methods, as recommended in the docs. The store is SQLite.
I am able to successfully insert objects into the database and read them. I can even make property changes to the newly inserted object and save it successfully. I simply don't seem to be able to update object properties when the object is pulled back out of the database.
The object is fetched from the store via a relationship from another object. After making changes to its properties, I call the context's save method. However, before doing so, I call the object's isUpdated method and the context's hasChanges method, and both return false. Shouldn't they return true since I've just made changes to the object's properties but haven't saved the context?
More
If I call the object's committedChanges method before saving the context, however, passing in the names of the properties that I've changed, I get back the correct values of the properties. I'm not sure what this means. I would have thought that this means that the object's new property values have been successfully saved, but clearly they are not saved.
I know that the result objects is registered with a context. If I call
[[result managedObjectContext] refreshObject:result mergeChanges:YES];
the object reverts back to the original property values. This means that the context is there and that it is the same context from which the record was fetched. And it means that the new property values are never written tot he database.
Some Code
Here's the code where I'm poking around with all of these things. There are other places in my code where I'm making property changes, but the changes are never saved.
- (IBAction)statusControlChanged:(UISegmentedControl *)control {
WCAAssessmentResult *result = [self currentResult];
/* printing the existing property values */
if (![result.complete boolValue]) NSLog(#"result is in progress!");
else if ([result.passed boolValue]) NSLog(#"result is passed!");
else NSLog(#"result is not passed!");
/* changing the property values */
switch (control.selectedSegmentIndex) {
case 0:
NSLog(#"setting incomplete");
result.complete = [NSNumber numberWithBool:NO];
break;
case 1:
NSLog(#"setting passed");
result.passed = [NSNumber numberWithBool:YES];
result.complete = [NSNumber numberWithBool:YES];
break;
case 2:
NSLog(#"setting failed");
result.passed = [NSNumber numberWithBool:NO];
result.complete = [NSNumber numberWithBool:YES];
break;
default:
break;
}
/* this method always returns an empty dictionary */
NSLog(#"%#", [result changedValues]);
/* this method returns the values that I just set */
NSLog(#"%#", [result committedValuesForKeys:[NSArray arrayWithObjects:#"complete", #"passed", nil]]);
/* isUpdated returns false */
if (![result isUpdated]) {
NSLog(#"result is not updated?! WTF!?!?");
}
/* hasChanges returns false */
if (![[result managedObjectContext] hasChanges]) {
NSLog(#"context has no changes!? WTF!?!?");
}
/* saving the context produces no error */
NSError *error = nil;
if (![[result managedObjectContext] save:&error]) {
NSLog(#"save failed");
NSLog(#"%#",[error description]);
}
}
A Twist
If I create a new result object by inserting a new record into the context, I can set that object's properties and they are saved successfully. In the above code, I'm fetching the object as a member of a to-many association from another object. Is that a clue?
I'm tearing my hair out over this. What the hell could be going wrong here?
What's NOT The Problem
I've logged the object's class, and it is indeed the correct class
I've made sure that the managedObjectContext I'm saving is the same as the object's context
I haven't made any changes to the auto-generated setter/getter methods of my managed object classes
I've tried using the setValue:forKey: method instead of object's properties
I've used the -com.apple.CoreData.SQLDebug 1 argument to log Core Data SQL, and no SQL is logged when I update and save the object's properties
I do not really understand your statement
WCAAssessmentResult *result = [self currentResult];
Indeed, if you are accessing a to-many relationship from an object, you should get back a set, not an object. Anyway, without seeing the code it's hard to tell. The problem you are experiencing may or may not lie there.
I would rather expect in your code something like the following snippet to access objects belonging to a to-many relationship. I assume that yourObject is the object you use to access the WCAAssessmentResult objects in the to-many relationship, which I call results.
NSMutableSet *resultObjects = [yourObject mutableSetValueForKey:#"results"];
NSPredicate *predicate = ...
[resultObjects filterUsingPredicate:predicate];
for(WCAAssessmentResult *result in resultObjects){
// modify as needed the current result object
}
NSError *error = nil;
if (![managedObjectContext save:&error]) {
NSLog(#"save failed");
NSLog(#"%#",[error description]);
}
Did you verify that the managedObjectContext you are using to save the object is valid (not nil) ?
Some ideas in no particular order:
I would log the class of the result object and make sure it is the class you think it is. Some confusion with super/sub classes could result in certain values not being saved.
If you made any alterations in the setter/getter methods in any class in the hierarchy, look closely at those methods especially if you used a primativeValue method. Simply leaving out willChangeValue and didChangeValue can cause changes to be invisible to the context and sometimes to the object itself.
I would log the context you are saving as well as managedObjectContext property of the result object. Confirm they are indeed the same context.
Skip using the property accessors (the dot notation) and use setValue:forKey and see if that makes any difference. If so, you have an accessor problem. Likewise, you could try setPrimativeValue:forKey to also check for an accessor problem.
If I had to bet, I would put my money on you assigning the objects returned by a fetch to the wrong class.