I just started to learn iOS programming and I have a problem with inheritance. There are 2 files.
First file
Header
#import <UIKit/UIKit.h>
#interface ViewController : UIViewController {
int x;
}
#end
Implementation:
#import "ViewController.h"
#import "NewClass.h"
#implementation ViewController
#pragma mark - View lifecycle
- (void)viewDidLoad
{
[super viewDidLoad];
x = 999;
NewClass *myClass = [[[NewClass alloc] init] autorelease];
}
#end
Second file
Header:
#import "ViewController.h"
#interface NewClass : ViewController
#end
Implementation:
#import "NewClass.h"
#implementation NewClass
-(id)init {
self = [super init];
if (self != nil) {
NSLog(#"%i",x);
}
return self;
}
#end
In ViewController I set x to 999, and in NewClass I want to get it, but when I call NSLog(#"%i",x); it gives me 0.
Where did I make a mistake?
You have a timing problem.
The init method gets called first (at all levels of the inheritance hierarchy, so in both ViewController and NewClass). This is when you print out your value of x, when it is still zero.
The viewDidLoad method only gets called much later, generally at a point after a view controller's view has been added to a superview. It's functionality that's specific to the UIViewController class.
To make your example work, put an init method in your ViewController class that looks like the one in your NewClass class, and set x there.
Also, you don't need to create a NewClass instance within ViewController. When you create a NewClass object, it is automatically a ViewController as well. In the same way that a dog is an animal automatically, and so is a cat. When you create a dog, you don't want to create an animal as well!
As sidyll says, you should probably do a bit more reading about how inheritance works, I'm afraid!
You need to review you OOP concepts. Object-Oriented Programming with Objective-C is a must.
Your class NewClass indeed inherits the x variable, but not it's value. When you create an instance of it, you're creating a shiny new instance whose values have nothing to do with the parent class.
Another point of view to help you is that x was set in a object of ViewController class. The NewClass inherits from ViewController class, not from an arbitrary instance (object, where you set x).
That's because -viewDidLoad is not called until well after -init returns. Your superclass should do configuration like that in its -init method.
Related
How can I access the value from an inputField located in a second viewController?
The class name of the second view controller is SettingsViewController and the outlet name for the inputField is setRateInput.
I tried this but it didn't work…
double taxRateFromInput = [[self.settings.setRateInput text]doubleValue];
when I NSLog it comes out as The value is: (null)
Any idea what I'm doing wrong?
Here is the implementation file for the main viewController:
#import "SettingsViewController.h"
#interface ViewController ()
#property (strong, nonatomic) SettingsViewController * settings;
#end
#implementation ViewController
// lazy instantiation
-( SettingsViewController *) settings
{
if (_settings == nil) {
_settings = [[SettingsViewController alloc]init];
}
return _settings;
}
- (IBAction)calculatePrice:(id)sender {
double taxRateFromInput = [[self.settings.setRateInput text]doubleValue];
#end
In theory, you could create a global. Create a new class, call it something like taxRate (.h and .m)
In taxRate.h, add the following code:
#import <Foundation/Foundation.h>
#class MyTaxRate;
#interface TaxRate : NSObject {
}
#property (nonatomic, retain) double * taxRateFromInput;
+(TaxRate*)getInstance;
#end
Then, in your controller, put a "#import taxRate.h" in there. In your .m file, add the following:
#import "TaxRate.h"
#implementation TaxRate
#synthesize taxRateFromInput;
static TaxRate *instance =nil;
+(TaxRate *)getInstance
{
#synchronized(self)
{
if(instance==nil)
{
instance= [TaxRate new];
}
}
return instance;
}
#end
Note: This is extremely similar in structure to what I'm purposing.
if you have the reference from the object view controller you can just access by the property from your attribute.
You instantiated a new SettingsViewController, but you didn't do anything to instantiate its textfield setRateInput. You can do it when you instantiate it:
_settings = [[SettingsViewController alloc]init];
_settings.setRateInput = [UITextField alloc] initWithFrame:someFrame]];
or, as a beter solution, instantiate the text field in -init of SettingsViewController
- init {
if (self = [super init] {
self.setRateInput = [UITextField alloc] initWithFrame:someFrame]];
}
return self;
}
If you use nib files, this would be a lot easier.
Note: setRateInput is a bad name for a property. Consider rateTextField instead.
Edit I forgot to add that you have to add the text field as a subview to its parent view.
So it will be like,
_settings = [[SettingsViewController alloc]init];
_settings.setRateInput = [[UITextField alloc] initWithFrame:someFrame] autorelease];
[_settings.view addSubView:_settings.setRateInput];
In this case, the setRateInput is retained by its super view. You're not using ARC, so you can call autorelease on your text field.
The better solution: Use - (void) loadView; inside SettingsViewController. Loading the view is the responsibility of the correspondent view controller.
- (void) loadView {
self.setRateInput = [[UITextField alloc] initWithFrame:someFrame] autorelease];
[self.view addSubView:_settings.setRateInput];
}
Edit: xib files and storyboards can help you out. Give these tutorials a try.
You are on the right track, also well done with your lazy instantiation (as
a demonstration that you grasped the concept, I mean).
But note, that outlets don't get connected until viewDidLoad is called. So if you
just alloc/init your viewController (lazily), the outlet to your textfield is pointing to nil.
The outlet doesnt get connected until your controller's view property is accessed, ie the view is displayed.
What you could do is give the settings viewController a handle to your calculating viewController and let it set a public property on the calculating viewController that represents the rate.
This is a common pattern - delegation - where one viewController (settingsViewcontroller) calls a method on its delegate (calculating viewController).
You wouldn't need the settingsViewcontroller property in your calculating viewController then, but just instantiate a new settings viewController every time you want it to be brought up, giving it a reference to your calculating viewController.
Another possibility - maybe even better - is to define a model object that does calculation and takes care of the rate it needs to calculate. Then you could give your settingsViewcontroller a reference to that model object (probably instantiated in your
other viewController), so that it can change the rate on it.
PS: also re think how you instantiate viewControllers generally. The designated initialiser is -initWithNibName:bundle: - so usually, you wouldn't just alloc/ -init them.
If you use storyboards (you probably should!), use storyboard's -instantiateViewControllerWithIdentifier: or use the above mentioned designated initialiser.
This is an objective-c problem. I have created a subclass person of NSObject with parameters 'height' and 'weight', with property and synthesize in a file called Person.h that contains both interface and implementation.
I want to import Person.h into my viewcontroller.m and create person objects and alter them using 2 IBActions.
-(IBAction)alterperson_1{
person *bob = [person alloc]init];
bob.height = 72;
bob.weight = 200;
}
-(IBAction)alterperson_2{
bob.height = 80;
bob.weight = 250;
}
This arrangement does not work because the method alterperson_2 can't find Bob because it is a local variable to alterperson_1. My question is how and where in viewcontroller.m do I allocate Bob as a person so that his attributes can be altered by both IBActions.
I have tried allocing in viewdidload as well as in the initwith nibname methods. It did not work. I have also tried in the implementation{ } of viewcontroller.m but that doesn't work either because Bob's allocation is not a compile time constant.
Thanks!
Update With Code
So, I have the Person.h file importing properly now (thanks Robotnik), and am able to create instances of Person throughout ViewController.m -- however, my created instance *bob does not seem to retain values for its properties (see comments by NSLog statements in the code). I think this is an initialization issue, but I have no idea where to initialize. Currently, I get a warning when initializing in viewDidLoad. How do I get bob.weight to print 200 when my IBAction is called, instead of the 0 I currently get? Thanks.
// Person.h
#import <Foundation/Foundation.h>
#interface Person : NSObject{
int weight;
int height;
}
#property int weight, height;
#end
end Person.h
//Person.m
#import "Person.h"
#implementation Person
#synthesize weight, height;
#end
end Person.m
//ViewController.h
#import <UIKit/UIKit.h>
#import "person.h"
#interface ViewController : UIViewController{
}
#property Person *bob;
-(IBAction)persontest:(id)sender;
#end
end ViewController.h
//ViewController.m
#import "ViewController.h"
#implementation ViewController
#synthesize bob;
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
Person *bob = [[Person alloc]init]; // this causes a local declaration warning, if I remove this code, however, it still doesn't work
bob.weight = 100;
NSLog(#"viewDidLoad bob's weight, %i", bob.weight); // this will print 100, but only because I made the local initialization. The value is lost once the viewDidLoad Method ends.
}
-(IBAction)persontest:(id)sender{
bob.weight = bob.weight + 100;
NSLog(#"IBAction bob's weight %i", bob.weight); // this prints 0, probably because value is nil. How can I make it print 200?
}
- (void)viewDidUnload
{
[super viewDidUnload];
// Release any retained subviews of the main view.
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}
#end
end ViewController.m
You'll need to declare Bob in your ViewController.h if you want him to be accessible across multiple methods. You can then initialise him in viewDidLoad
#interface ViewController
{
Person *bob;
}
-(IBAction)alterperson_1;
-(IBAction)alterperson_2;
#end
You mentioned you wanted to instantiate multiple people. In that case you may want to keep multiple Person objects in an NSMutableArray or similar. This should still be declared in the ViewController.h in order to be accessible in multiple methods. You can then use the method addObject: to add people to the array. Here is an example of an array, storing strings.
NSMutableArray *stringArray = [[NSMutableArray alloc] init];
[stringArray addObject:#"Dog"];
[stringArray addObject:#"Cat"];
You seem to be confusing declaration of a variable with allocation of a variable. You're also not using properties properly.
Declaration of a variable is done like this:
Person *bob;
That's it. If you put this line in your #interface section, or in {braces} at the top of your implementation section, then this becomes an instance variable, any time you use bob in the rest of your class it will know that you are talking to a person. So, in your viewDidLoad, you can now do this:
bob = [[Person alloc] init];
And it knows you are referring to the instance variable. In your current code, you have Person * in front of this line, which, as the compiler is telling you, is declaring a local variable with the name bob, and therefore hiding your instance variable, so you are not changing your instance variable at all, so there is no value in it later.
Looking at the nature of this question, and your comments, I would strongly advise reading some objective-c introductory texts before proceeding much further. You can ask questions here but SO is not really the place to learn a language - most answerers will assume that you know the language basics.
I am trying to call a method thats in my ViewController from a NSObject Class thats doing some parsing.
I initally call a connection class I have made wich downloads some data from my server, I then pass this data over to a parser class I have made, now what I am trying to do is pass this data back to the viewcontroller and reload the tableview thats in this view (thats on a navigation stack)
anyway this is causing some errors and I think it might be the way I am trying to call this method thats doing it. here is how I call it.
MyViewController *myViewController = [[MyViewController alloc] init];
[myViewController initFilterArray:filteredArray];
Now I think this is causing an issue because I am allocating a new viewcontroller object? is that right.. not to sure of the terminoligy.. but yea..
the result of which is that reloaddata is only calling
numberOfSectionsInTableView
tableView:numberOfRowsInSection
then thats it.
any help would be appreciated.
UPDATE:
so I am trying to set up a protocol/delegate to see if that fixes my problem.
so in my class.h this is what I am doing
#protocol PassParsedData <NSObject>
#required
- (void) sendMyArray:(NSArray *)modelArray;
#end
//..
id <PassParsedData> delegate;
//..
#property (strong) id delegate;
then in class.m
//..method
[[self delegate]sendMyArray:filteredArray];
//..
so thats my class, then over in my view controller where I want to call this sendMyArray I do this
viewcontroller.h
#import "class.h" //delegates & protocols
//..
interface VehicleSearchViewController : UITableViewController <PassParsedData> {
//..
then i call it like this
viewcontroller.m
//..
- (void)sendArray:(NSArray *)array
{
ICMfgFilterArray = array;
[self.tableView reloadData];
}
One way of doing this would be the recommended approach of delegates and protocols.
Your NSObject declares a protocol. The ViewController actually implements the protocol and sets itself as the delegate. Then the NSObject calls the method (not knowing who implements it). It is a loosely-coupled way to communicate between objects.
I actually recently wrote a blog post on a basic introduction to protocols and delegates if you're interested...
UPDATE
Based on your update above in question.
Don't forget to set your ViewController to be the delegate.
- (void)viewDidLoad {
// State that you will take care of messages from graphView (provided you have the protocol implementation!)
self.yourClass.delegate = self;
}
And the method in your ViewController should match the protocol signature. So in ViewController.m
- (void) sendMyArray:(NSArray *)modelArray {
ICMfgFilterArray = array;
[self.tableView reloadData];
}
I'm fairly new to programming in Objective-C. While I have been able to find my way, there now is an issue I cannot solve, which is either caused by a mistake I made or because I have a fundamental misunderstanding about classes.
Essentially, I want one class to change a variable (or object) in another class. Here is the code I have:
// LocationManager.h
#import <Foundation/Foundation.h>
#import <CoreLocation/CoreLocation.h>
#interface LocationManager : NSObject <CLLocationManagerDelegate> {
CLLocationManager *locationManager;
CLLocation *locationByCustomLocation;
}
#property (nonatomic, retain) CLLocation *locationByCustomLocation;
#end
Of course, there's a corresponding implementation file: LocationManager.m. It synthesizes the locationByCustomLocation variable.
The point is that from another class, I'd like to manipulate the locationByCustomLocation variable.
// viewCustomLocation.h
#import <UIKit/UIKit.h>
#interface viewCustomLocation : UIViewController <UITableViewDataSource, UITableViewDelegate> {
UITableView *tblLocation;
UITableViewCell *cell;
}
--
//viewCustomLocation.m
#import "viewCustomLocation.h"
#import "LocationManager.h"
#class LocationManager;
#implementation viewCustomLocation
#synthesize tblLocation;
#synthesize cell;
// some view related selectors here, but it boils down to this one:
- (void)dismissView:(id)sender
{
[self dismissModalViewControllerAnimated:YES];
LocationManager *locationManager = [[LocationManager alloc] init];
// I made sure with NSLog that the customLoc variable contains the expected data
CLLocation *customLoc = [[CLLocation alloc] initWithLatitude:place.coordinate.latitude longitude:place.coordinate.longitude];
[locationManager setLocationByCustomLocation:customLoc];
}
Now, if I use NSLog in LocationManager.m to see what's in the LocationByCustomLocation variable, I would expect the same data as in customLoc. Instead, the variable still seems empty.
I think the problem is that I created a copy of the LocationManager class, thus filling the LocationByCustomLocation variable in the copied class, rather than the original one, which is what I want. I can't figure out how to talk to the original LocationManager class.
I know of a few ways to work around this issue, but I would like to know how to achieve it this way to improve my fundamental understanding of the language.
Thanks for reading!
That's because you are allocating a new instance of LocationManager. You can either connect the two controllers between them, like declaring properties and setting them accordingly.
For example, if you instantiate controller B from controller A, you should implement a property for controller B, like firstController, so :
B *controller = [[B alloc] init];
controller.firstController = A;
and then from inside B controller, you control what happens in controller A
An alternate way is to instantiate and control everything from the ApplicationDelegate. It's a more powerful pattern.
At the moment, the majority of my code is in the same viewcontroller, and i'd like to move some of it over to other areas. Such as moving the animations all over to somewhere else. But then how do i reference things which are in another class? And how do i reference back from that class to items in my viewcontroller class? Not going this has always disuaded me from doing it.
there is a couple of ways you can achieve that.
one way is the cocoa delegate #protocol way, the second way could be creating references to each object in the other class.
for the first way you can do something like this:
#class Class2;
#interface Class1 : NSObject {
Class2 *cls2Pointer;
}
#property Class2 *cls2Pointer;
#end
#class Class1;
#interface Class2 : NSObject {
Class1 *cls1Pointer;
}
#property Class1 *cls1Pointer;
#end
int main(){
Class1 cls1Obj = [[Class1 alloc] init];
Class2 cls2Obj = [[Class2 alloc] init];
[cls1Obj setCls2Pointer:cls2Obj];
[cls2Obj setCls1Pointer:cls1Obj];
}
the second way, is to declare a protocol in one/both of the classes to be able to pass arguments and call different methods on other objects:
#protocol Class1Delegate
- (void)class1:(Class1)obj MethodWithArg:(id)arg;
#end
#interface Class1 : NSObject {
id <Class1Delegate> delegate;
}
#end
#interface Class2 : NSObject <Class1Delegate>{
}
#end
#implementation Class2
- (void)class1:(Class1)obj MethodWithArg:(id)arg {
//do stuff when called from the 1st class
}
#end
You might like to look into this here - to create static classes in objective c and then reference them in a separate file by classname - as in the view controller quoted in the linked example.
Otherwise you can just create a new class within a separate .m file and then code it such that the calling method in another class will first create an instance of this new class and then invoke the necessary method on this instance.
Hope this helps.
Basically what you do is that you create one or more classes, move the code over to these classes and then create instances of these classes in your viewcontroller.
so if you had a method in your view controller
-(void)foo;
you would create a new class say C and move the method there.
then in your view controller you would create an instance variable of that class e.g.
C* myC;
then alloc/init and then call the foo method. This is not object oriented in the sense that foo is not really related to C in any way so method foo could have just been a static method not relating to the instance and as such called just like any other method but as [C foo] instead of [self foo] from the view controller.
the other more OOP method would be to move functionality that belongs to together into a separate class like animation in your example.