I am new to objective C and I have a c++ background. I want to display a value in the label on the screen. I am calling the label value from the MainView.m. However, the label becomes blank after I click a button instead of printing a value. What is the problem? Here is the code.
MainView.h
#interface MainView : UIView {
int a;
}
-(int) vr;
#end
MainView.m
-(int) vr
{
return 100;
}
#end
MainViewController.h
#interface MainViewController : UIViewController {
IBOutlet UILabel *myLabel;
NSMutableString *displayString;
MainView *view1;
}
#property (nonatomic, retain) UILabel *myLabel;
#property (nonatomic, retain) NSMutableString *displayString;
(IBAction)showInfo;
(IBAction) pressButton:(id) sender;
#end
MainViewController.m
#synthesize myLabel, displayString;
-(IBAction) pressButton:(id) sender{
[displayString appendFormat:#"%i", view1.vr];
myLabel.text = displayString;}
- (void)viewDidLoad {
view1 = [[MainView alloc] init];
[super viewDidLoad];}
- (void)dealloc {
[view1 dealloc];
[super dealloc];}
I have not mentioned code that had been auto generated. This is enough to get the whole picture. I tried a lot to debug this thing. I believe that IBAction carries out direct command such that
myLabel.text = #"string";
but it does not invoke any method or class. Any subtle ideas? Thanks.
Few issues:
1
In MainView.h you declare -(id) vr;
And in MainView.m it returns int.
2
Maybe pressButton is not connected to the right event in Interface Builder (it is usually touch up inside).
Try to write to log in this method.
3
Maybe myLabel is not connected to the label in the Interface Builder.
Try to set tome hard-coded string to label's text property.
4
Do you initiate view1 in some place?
Can you post this piece of code too?
5
You can use [displayString appendFormat:#"%i", view1.vr];...
EDIT (due to changes in question):
6
The line [super viewDidLoad]; should be the first line inside viewDidLoad.
7
[view1 dealloc]; - never call dealloc directly on objects. Call release instead. The only place, where you can and should use dealloc is the line [super dealloc]; inside dealloc method.
8
When you format your question/answer in Stack Overflow, remember that each code line should start with at least 4 spaces (or tab). Try reformatting you question by adding 4 spaces in the beginning of each code line.
9
I think that displayString is not initiated. Add the next line in the viewDidLoad: displayString = [NSMutableString new];
Related
Part 1 of the code works fine. Part 2 shows minor changes in the code which causes the code to stop working (without errors/warnings) as expected.
Part 1: (Works)
#import "ClassA.h"
#import "ClassB.h"
#implementation ClassA
- (void) sendData
{
NSString *temp = [NSString stringWithFormat:#"HI!"];
ClassB *classBObject = [[ClassB alloc] init];
classBObject.dataToDisplay = temp;
self.view = classBObject.view;
}
#end
Interface of ClassB:
#import <UIKit/UIKit.h>
#interface ClassB : UIViewController
#property (weak, nonatomic) IBOutlet UILabel *textLabel;
#property NSString * dataToDisplay;
#end
Implementation of ClassB:
#import "ClassB.h"
#implementation ClassB
#synthesize dataToDisplay, textLabel;
- (void)viewDidLoad
{
[super viewDidLoad];
textLabel.text = dataToDisplay;
}
#end
Part 2:
But if I change - (void)sendData of ClassA to the following:
- (void) sendData
{
NSString *temp = [NSString stringWithFormat:#"HI!"];
ClassB *classBObject = [[ClassB alloc] init];
classBObject.textLabel.text = temp; // statement changed from Part 1.
self.view = classBObject.view;
}
and remove textLabel.text = dataToDisplay; from implementation of ClassB, the textLabel on view controller of ClassB does not get updated. Can you please suggest, why is it so?
Thanks!
Edit 1:
In the statement: classBObject.textLabel.text = temp; // statement changed from Part 1., I had missed .text while copy pasting. Please excuse me for that.
The reason that the second technique is incorrect (besides the missing .text at the end of textLabel) is that when you return from the class B initializer, the underlying UILabel corresponding to textLabel undoubtedly has not been created yet. If you look at this diagram you'll see that the view configuration is not completed at the end of the the initialization methods, but rather upon access. So you must defer any access of the user interface controls until viewDidLoad.
Update:
When I run the following code, I get "0x0" in my log, proving that the UILabel on my second view is still nil and has not been initialized yet, as I would have expected. When the viewDidLoad in my second controller sets self.textLabel.text = self.dataToDisplay in viewDidLoad, it works like a champ (as it does for you). But the UILabel IBOutlet property is just not reliable until viewDidLoad.
SecondViewController *controller = [[SecondViewController alloc] init];
NSLog(#"%p", controller.textLabel);
controller.dataToDisplay = #"from first view";
[self presentViewController:controller animated:YES completion:nil];
You are setting a UITextLabel to NSString. Try
classBObject.textLabel.text = temp;
Change this line...
classBObject.textLabel = temp; // statement changed from Part 1.
to
classBObject.textLabel.text = temp; // statement changed from Part 1.
Also, you should do
[self.view addSubView:classBObject.view]; //using navigation controller or presenting modal viewcontroller would be recommended.
instead of
self.view = classBObject.view;
after this line, update the label's text with your value.
classBObject.textLabel.text = temp; // statement changed from Part 1.
I noticed that you're using a weak reference to the UILabel in your classB interface. Any reason you're not using a strong reference? The only time you want to use weak references is to avoid retain cycles. Most likely, your UILabel isn't being retained.
Where do you initialize your UILabel?
I'm working through the IOS HelloWorld example and I have a question regarding setting the delegate for a TextField. In the example it was as easy as control-dragging from the TextField to the ViewController. But now say I wanted to create a custom class to act as my delegate as so:
#import <Foundation/Foundation.h>
#interface SweetAssDelegate : NSObject <UITextFieldDelegate>
- (BOOL)textFieldShouldReturn:(UITextField *)theTextField;
#end
#import "SweetAssDelegate.h"
#implementation SweetAssDelegate
- (BOOL)textFieldShouldReturn:(UITextField *)theTextField
{
NSLog(#"Calling Delegate");
[theTextField resignFirstResponder];
return YES;
}
#end
How can I set this class to be the delegate of the TextField? As far as I can tell there is not way to accomplish this through the GUI. I tried manually setting the delegation after window load with no success:
#import "ViewController.h"
#import "SweetAssDelegate.h"
#interface ViewController ()
#property (weak, nonatomic) IBOutlet UITextField *inputField;
#end
- (void)viewDidLoad
{
[super viewDidLoad];
SweetAssDelegate *foo = [[SweetAssDelegate alloc] init];
[self.inputField setDelegate:foo];
NSLog(#"Delegate: %#", self.inputField.delegate);
}
I actually receive some sort of memory exception when bringing up the keyboard? Any ideas? Thanks.
As a side question, where should I always use viewDidLoad to initialize any variables? I noticed that init was not being called???
Your delegate object, foo, is allowed to fall out of scope and is released at the end of viewDidLoad and by the time the keyboard comes up, it doesn't exist anymore. Make it an ivar (or property) of your view controller, or otherwise make sure that foo doesn't fall out of scope at the end of viewDidLoad.
Thus, it could be something like:
#interface ViewController ()
#property (weak, nonatomic) IBOutlet UITextField *inputField;
#property (strong, nonatomic) SweetAssDelegate *foo;
#end
- (void)viewDidLoad
{
[super viewDidLoad];
self.foo = [[SweetAssDelegate alloc] init];
[self.inputField setDelegate:self.foo];
NSLog(#"Delegate: %#", self.inputField.delegate);
}
Your textfield delegate must have the implemented to be your textfield delegate I guess.
A delegate manages the communication between objects, which means your custom delegate must allow communication between objects and must provide methods, the textfield can work with...
Another example is a tableView:
You can make a custom delegate which implements the delegates and then calls some tableview related Methods...
Here this code might be interesting for you:
#interface myCustomDelegateForTextFields <UITextFieldDelegate>
#end
#implementation myCustomDelegateForTextFields
- (BOOL)textFieldShouldReturn:(UITextField *)theTextField {
return TRUE;
}
#end
#implementation ViewController
myCustomDelegateForTextFields *txtfielddelegate = [[myCustomDelegateForTextFields alloc] init];
UITextField *whatever;
whatever.delegate = txtfielddelegate;
//your textfield now listens to the BOOL method in your custom delegate
#end
Is it that what u were looking for? :)
you can ofc pack the myCustomDelegateForTextField delegate in another class and call the class
I am passing an NSDictionary object from one view class to another as I transition from a table view to a normal view to show details:
Passing Controller:
[tweetController setTweet:tweet];
Receiving Controller.h:
#interface TweetViewController : UIViewController {
NSDictionary *tweet;
}
#property (nonatomic, retain) NSDictionary *tweet;
Receiving Controller.m:
#implementation TweetViewController
#synthesize tweet = _tweet;
I then try to use this information to set the properties of some fields in my view:
- (void)viewDidLoad
{
[super viewDidLoad];
tweetLabel.text = [_tweet objectForKey:#"text"];
}
The result is a blank label and if I inspect the value of _tweet at this stage it is nil.
I originally had a method which set the value of tweet which I called at the same location as I am now setting the value. If I inspected the value at this stage it was fine.
I presume that the automagic setter through #synthasize is working, but somewhere else the value is being lost.
Sorry this is my first objective C anything! Thanks for any help in advance.
You are using your "tweet" instance variable, whereas the "tweet" property is synthesized to the "_tweet" variable.
You are probably calling the setTweet method after viewDidLoad executes.
I usually pass this kind of thing into a custom init method.
Alternatively, you could do the set before pushing the detail VC onto the nav stack.
Are you sure that tweetLabel isn't nil?
I've made a few corrections & optimisations to your code. You don't need to declare ivars in the header file anymore, they are generated automatically by #synthesize
- (void)dealloc; is only needed if you're not using ARC.
//.h
#interface TweetViewController : UIViewController
#property (strong, nonatomic) NSDictionary *tweet;
#property (strong, nonatomic) IBOutlet UILabel *tweetLabel
#end
//.m
#implementation TweetViewController
#synthesize tweet = _tweet;
#synthesize tweetLabel = _tweetLabel;
- (void)viewDidLoad {
[super viewDidLoad];
self.tweetLabel.text = [self.tweet objectForKey:#"text"];
}
- (void)dealloc {
[_tweet release];
[_tweetLabel release];
[super dealloc];
}
#end
Note: strong is equivalent to retain
To expand on #Rayfleck's answer, since you are new to Objective-C, your custom init method could look like this:
In TweetViewController.h:
- (id)initWithTweet:(NSDictionary*)tweet;
In TweetViewController.m:
- (id)initWithTweet:(NSDictionary*)tweet
{
self = [super init];
if (self) {
_tweet = tweet;
}
return self;
}
and then in your passing controller you'd allocate and initialize like this:
TweetViewController *tvc = [[TweetViewController alloc] initWithTweet:myTweet];
I began from a tutorial example that had a UIViewController connected to a nib, but now i've decided to do it all programmatically. Consequently, I deleted the nib but without knowing how to implement my controller.
I did something like this:
EventsDetailController *myChild = [[EventsDetailController alloc] init];
[self.navigationController pushViewController:myChild animated:YES];
However, it crashes when I click the specific cell.
Do I have to initWith something? Before when I had a nib it was initWithNib
#import <UIKit/UIKit.h>
#interface EventsDetailController : UIViewController {
NSString *message;
}
#property (nonatomic, copy) NSString *message;
#end
#import "EventsDetailController.h"
#implementation EventsDetailController
#synthesize message;
-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
}
-(void)viewDidLoad{
UILabel *theMsg = [[UILabel alloc] initWithFrame:CGRectMake(0,0,200,30)];
theMsg.text = #"hello";
[theMsg release];
[super viewDidLoad];
}
-(void)viewDidUnload{
self.message = nil;
[super viewDidUnload];
}
-(void)dealloc{
[message release];
[super dealloc];
}
#end
Perhaps, I'll start from scratch, I think it is still looking for the nib I deleted.
I'm assuming that you have the app delegate's window.rootViewController set to a NavigationController instance, and that instance was created with a initWithRootViewController, as you mentioned that you're getting to the point where it crashes when you click on a cell.
Looking at the added code, the only thing I see that's strange is in the 'veiwDidLoad' method - you alloc and release 'theMsg', but don't use it. I'm assuming you've cut out some code for brevity.
I ended up starting from scratch when I went away from .nib files (about 5 minutes after starting my first iPhone app). I would run with that, adding in your functionality a little at a time.
I need to view a subview with an activity indicator.
This is my code but the subview doesn't appear:
#interface ProgressViewController : UIViewController {
IBOutlet UIActivityIndicatorView *myActivityIndicator;
}
#property (nonatomic, retain) IBOutlet UIActivityIndicatorView *myActivityIndicator;
#end
#implementation ProgressViewController
#synthesize myActivityIndicator;
- (void)viewDidLoad {
[myActivityIndicator startAnimating];
[super viewDidLoad];
}
- (void)viewWillDisappear:(BOOL)animated {
[myActivityIndicator stopAnimating];
}
#end
#import "ProgressViewController.h"
#interface MyViewController : UIViewController {
ProgressViewController *progressViewController;
}
#property (nonatomic, retain) ProgressViewController *progressViewController;
#end
#implementation MyViewController
#synthesize progressViewController
- (void)viewDidLoad
{
progressViewController = [[ProgressViewController alloc] initWithNibName:#"ProgressViewController" bundle:nil];
[self.view addSubview:progressViewController.view];
sleep(4);
[progressViewController.view removeFromSuperview];
[super viewDidLoad];
}
#end
There could be several causes, and it's still a bit unclear from the code you sent, which one it is.
First, you shouldn't use sleep(4) in your code - it messes up the application engine iOS runs to support user input, screen refresh, etc.
Your code could easily be changed to:
[self performSelector:#selector(removeMyProgressView:) withObject:progressViewController.view afterDelay:4.0];
and have removeFromSuperview in your removeMyProgressView: function.
Also, this line of code is buggy:
progressViewController = [[ProgressViewController alloc] initWithNibName:#"ProgressViewController" bundle:nil];
It should be
self.progressViewController = [[ProgressViewController alloc] initWithNibName:#"ProgressViewController" bundle:nil];
Otherwise you don't call the setter function (#sythesized property), and the object isn't retained. It could be that it is released, and therefore you don't see it.
If this none of this is right, we'll keep pounding at it :)
Good luck!
Oded.
Everything in your -viewDidLoad method happens in one runloop. This means that you add and remove the activity indicator without giving the system a chance to actually draw it. The 4 seconds of sleep don't help. Those just make the runloop take longer to finish.
call [super viewDidLoad] before anything in - (void)viewDidLoad methods