I've got a UIViewController which has three views: Category -> Sub-Category -> Detail View. The content of the Sub-Category and Detail View depends on which row is clicked in the Category view. This is done with a property called 'categoryClicked' and is declared in the sub-category.m file. It's value is given in the category.m file by the following lines of code.
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath{
Sub-category *sub = [[Sub-category alloc] initWithNibName:#"Sub-category" bundle:nil];
sub.categoryClicked = [_categoryArray objectAtIndex:indexPath.row];
[self.navigationController pushViewController:rubrieken animated:YES];
}
This works perfectly fine until I want to use the value of categoryClicked in the Detail View. In both the category.m and the DetailView.m sub-category.m is imported. For testing purposes I putted a label on the Detail View and in the DetailView.m file I've got the following code:
Sub-category *sub = [[Sub-category alloc] initWithNibName:#"Sub-category" bundle:nil];
label.text = sub.categoryClicked;
I'm convinced this code should do the job but in fact I get an empty label. Is there anybody able to tell me what I'm doing wrong.
Edit
categoryClicked is a property declared in Sub-category.h and synthesized in Sub-category.m.
Wanted to post some more code but there is no more relevant code.
This line...
Sub-category *sub = [[Sub-category alloc] initWithNibName:#"Sub-category" bundle:nil];
...creates a new Sub-category. Since it's new, it doesn't know anything about what information has been given to some other Sub-category object. You need to get a reference to the existing object if you want to access its data.
When you alloc an object, you're allocating memory space for it. Then when you use something with init (like initWithNibName) you're initializing it. So when you allocate memory space for sub and initialize it, you have one object. But then you alloc and init again, which creates an entirely new (and completely unrelated) object. Make sure that you remove anything that could destroy your old object.
Also, the * symbol means that sub is a pointer (it points to a memory location). Whenever you use the assignment operator (=) you're telling it to point to a new thing. What you're doing is telling the label.text pointer to point at what sub is pointing at. But if you change what sub is pointing at and point label.text at the same thing, neither one is pointing at the value you want.
Hope this is clear enough, if it's not trying posting some more code and maybe someone can suggest exact changes.
-EDIT-
If you want to have a reference to an object you can only get it a few ways. What's important to know is that you can't really "create" a reference to an existing object. You have to have some connection to the object.
Declare the object inside of the file you want the reference in with something like Category c = [[Category alloc] init]; Remember this creates a new object, it won't create a reference to an existing object. However, creating the object inside of another object means that one "owns" the other and can do whatever it wants with it (obviously including accessing properties and calling methods).
Use a "chain" of objects to get a reference to your object. So if your file owns a file that owns the object you want you can use topfile.otherfile.objectyouwant. The most obvious example of this is getting a reference to an object owned by a subview.
That's about as basic as it gets; just remember that there aren't any "global" objects that you can just call by name. If your problem isn't solved by this, either look at some sample code and try to figure out how references work, or post another question that's more closely related to your problem
Related
My NSMutableArray lOfSegments, declared as IVAR, get populated correctly. During the debug it shows 4 object in the array.
for (int x=0; [arrayOfSegmentsTcIn count]>x; x++) {
NSDictionary *segmentDic=[[NSDictionary alloc] initWithObjectsAndKeys: [arrayOfSegmentsNumbers objectAtIndex:x],#"segment",[arrayOfSegmentsTcIn objectAtIndex:x],#"tc_in",[arrayOfSegmentsTcOut objectAtIndex:x],#"tc_out", nil];
[lOfSegments addObject:segmentDic];
[myDMXML.segments addObject:segmentDic];
}
[self.xmlTCLable setStringValue:[myDMXML startTimeCode]];
[self.xmlDurationLable setStringValue:[myDMXML duration]];
[self xmlValidationCheck];
NSLog(#"arrayController:%#",[lOfSegments valueForKey:#"segment"]);
[self.tableViewOutlet reloadData];
NSLog list the array correctly but when reloadData is executed the code jumps to
- (NSInteger)numberOfRowsInTableView:(NSTableView *)tableView {
return [lOfSegments count];
}
The array is null.
The Object is initialised in viewDidLoad as
lOfSegments = [[NSMutableArray alloc]init];
Please Help!
First, I recommend making your code more clear here by using self.lOfSegments rather than directly accessing an ivar. (The fact that the ivar lacks a leading _ is very suspicious as well, and raises the question of whether this is even the variable you think it is.)
On the assumption that this is the variable you think it is, and that you have overridden the standard behavior to make the ivar match the property or created explicit ivars (neither of which you should do), there are several common causes for this kind of problem:
The most likely cause is that you called your initialization code prior to viewDidLoad and then viewDidLoad blew away the array. Many things can run prior to viewDidLoad, and viewDidLoad can run more than once (at least this used to be true; I'd have to study whether the view-loading changes in iOS 6 made it guaranteed to be run once.)
You have some other way reset lOfSegments between the time your initialization code ran and the time reloadData ran. If you would reliably use self. then you could override setLOfSegments: so you could log this. Or you could mark this property readonly so you could prevent it. Thats one of many reasons that you should use properties, not ivars.
The setting code failed to run before reloadData. Ensure that the log statement actually printed prior to getting to reloadData and is on the same queue (the queue identifier will be part of the NSLog output in brackets). I don't think this is likely given your description, but it is a common problem.
There are two instances of this object. Make sure that the object that ran your initialization code is the same object that ran reloadData. This is a more common mistake then you may think. Log self and make sure the memory address is the same in both cases.
looks like you have variable with same name lOfSegments in method viewDidLoad. So in viewDidLoad you use stack variable, in numberOfRowsInTableView - instance variable.
Or maybe tableViewOutlete.dataSource pointing on different viewController
I would like to subclass UILabel in such a way that the user of the class cannot set the text directly through label.text = #"foo". Instead I'd like to set the text from inside the subclass depending on some values.
What I tried:
BalanceLabel.h:
#interface BalanceLabel : UILabel
#property(nonatomic,copy, readonly) NSString *text;
#end
However, I get a warning telling me I'm restricting text access (like I wanted to) but I don't get any compile time errors if I try to set the text directly using an object of my subclass.
You can't do this. As a trivial example as to why not, just think of how the following code should behave:
UILabel *label = [[BalanceLabel alloc] init];
label.text = #"string";
That code creates a BalanceLabel, but stores it in a variable of type UILabel, which means that the subsequent setting of the .text property can't know that you tried to make the property readonly in BalanceLabel.
Unfortunately there's not much you can do about this. You could override the setter to throw an exception, which will let users know what they did wrong, but of course will also crash the app.
You should be putting the logic into controller that managed the view instead of view directly.
I assume you have some view that gets updated with new values and you want to update the BalanceLabel based on these new values.
Your controller is a delegate for your view so it receives new values, from either user or other modules of your app that populated new values (like loaded from file, downloaded from network and so on).
Your controller then figures out which bits of view needs update and sets new values - in your case calculate balance, I assume
I am trying to create a mutable array in objetive c to hold references to objects. The objects in the array are regularly updated through user interaction and i want the array to automatically reflect changes made to the objects as they occur. Does anyone know if there is a way to do this? Perhaps store pointers to the objects instead of the objects themselves in the array? Any help would be much appreciated
Thanks in advance
Edit: I should mention that the objects are not exactly being updated in the strict sense of the word. They are being reinitialized. For ex if i had a controller:
MyController = [MyController alloc] initWith.....]]
the above call is made again with different init parameters.
The array always stores the pointers.... It holds a strong reference to it or sends it a retain message (if using non ARC).
So
[myMutableArray addObject: anObject];
adds the pointer to it.
If you now change anObject's properties and access it later through the array, it will
give you the pointer to just that object with the changes to its properties.
Edit:
No, if you alloc/init, you are creating a new object instance and allocate new memory for it on the heap (ie, it's another pointer to a new memory address).
What exactly are you trying to accomplish? There sure is a way, if you provide a little more detail.
If you alloc/init the object with the same class, why not just create a method to change the object's properties:
Instead of
myObject = [[MyClass alloc] initWithParameter1: one parameter2: two];
You could create a method that changes these properties:
[myObject updateParameter1: anotherOne parameterTwo: anotherTwo];
And, of course, the advantage of a mutable array is, that you can change its contents, so like #Eli Gregory pointed out, you can replace an object with another one (or rather the pointers to it).
Because you want to point to a newly allocated and initialized object, you can't 'update' the pointer, what you can do is 'replace' the pointer with a new one at a certain index.
A method you could use to do this is:
- (void)replaceObjectAtIndex:(NSUInteger)index withObject:(id)anObject
so it would look something like:
NewViewController *new = [[NewViewController alloc] init..];
[myArray replaceObjectAtIndex:x withObject:new];
I have a UIViewController that I load every time I click a button:
NearMe * temp = [[NearMe alloc] initWithNibName: #"NearMe" bundle:nil];
I parse some XML with locations and then assign the values to an NSMutableArray, which populates a UITableView.
I parse the XML on the viewDidLoad method, but I don't need to parse it every time, since the value is not going to change. I only want to parse it if the array is nil, so I put the following check in:
if (allLocations == nil) ....
So that the XML is only parsed if necessary, but every time I press the back button (in the UINavigationController) it erases all the objects in the array.... I don't have any idea why this is happening...
Since your app is creating a new NearMe instance each time, the allLocations instance variable will always be nil in viewDidLoad (which, by the way, is only called if the _view instance variable is nil).
If you don't want to recreate the array each time, your app will need to cache it somewhere else. A couple of possibilities would be to store the array in the object that creates the NearMe instance, or to store it in a static variable.
I've been attempting this for two days, and constantly running into dead ends.
I've been through Aaron Hillegass's Cocoa Programming for MAC OS X, and done all the relevant exercises dealing with NSTableview and mutable arrays, and I have been attempting to modify them to suit my needs.
However none of them seem to be using an array with objects as a data source, it seems to use the tableview as the datasource.
I'm trying to implement Jonas Jongejan's "reworking" of my code here, with a Cocoa front end to display the results.
Any pointers or suggestions I know this should be simple, but I'm lost in the wilderness here.
I can populate the table by setting the array
It's pretty simple really, once you get to understand it (of course!). You can't use an NSArray directly as a table source. You need to either create a custom object that implements NSTableViewDataSource or implement that protocol in some existing class - usually a controller. If you use Xcode to create a standard document based application, the document controller class - (it will be called MyDocument) is a good class to use.
You need to implement at least these two methods:
– numberOfRowsInTableView:
– tableView:objectValueForTableColumn:row:
If you have a mutable array whose values you'd like to use in a table view with one column, something like the following should do as a start:
– numberOfRowsInTableView: (NSTableView*) aTableView
{
return [myMutableArray count];
}
– tableView: (NSTableView*) aTableView objectValueForTableColumn: (NSTableColumn *)aTableColum row: (NSInteger)rowIndex
{
return [myMutableArray objectAtIndex: rowIndex];
}
It has just occurred to me that you could add the above two methods as a category to NSArray replacing myMutableArray with self and then you can use an array as a data source.
Anyway, with a mutable array, it is important that any time you change it, you need to let the table view know it has been changed, so you need to send the table view -reloadData.
If your table view has more than one column and you want to populate it with properties of objects in your array, there's a trick you can do to make it easier for yourself. Let's say the objects in your array are instances of a class called Person with two methods defined:
-(NSString*) givenName;
-(NSString*) familyName;
and you want your table view to have a column for each of those, you can set the identifier property of each column to the name of the property in Person that that column displays and use something like the following:
– tableView: (NSTableView*) aTableView objectValueForTableColumn: (NSTableColumn *)aTableColum row: (NSInteger)rowIndex
{
Person* item = [myMutableArray objectAtIndex: rowIndex];
return [item valueForKey: [tableColumn identifier]];
}
If you replace valueForKey: with valueForKeyPath: and your Person class also has the following methods:
-(Person*) mother;
-(Person*) father;
-(NSString*) fullName; // concatenation of given name and family name
you can add table columns with identifiers like: father.fullName or mother.familyName and the values will be automatically populated.
You could go the datasource route and do all of the heavy lifting yourself, or you could let bindings do all the heavy lifting for you. Add an NSArrayController to the nib file that has the table view in it. Make sure that the File's Owner of the nib is set to the same class that has the mutable array in it. Bind the contentArray of the array controller to File's Owner.myMutableArray. For each column bind Value to the array controller arrangedObjects and add the appropriate key path. This will allow you to get things like user sorting for free if you ever need it.
On the iPhone (I know you're talking about Mac, but maybe this could help) you have to use delegation for loading a tableView. It asks for a cell and you use your array to fill-in the data where needed.
I'm not sure if this works for the Mac, but it'd be worth looking into.
Maybe set dataSource to self and use those delegate methods to access your array based on the row and column #
Apple has a whole guide for Table View Programming so I suggest you start with the Using a Table Data Source section of the that guide.