I created a UIImageView in IB and connected it to an IBOutlet:
#property (nonatomic, retain) IBOutlet UIImageView *alertImage;
However, the UIImageView doesn't react to my code at all. I tried setting the outlet and then changing a property to test:
alertImage = [[UIImageView alloc] init];
alertImage.hidden = YES;
But the image didn't disappear. What am I doing wrong?
The problem is that you are re-initializing alertImage, so it's not connected to the "original" one defined in IB anymore.
Simply just use:
alertImage.hidden = YES;
Drop that line: alertImage = [[UIImageView alloc]init];
Write only alertImage.hidden = YES;
You declared your IBOutlet as a property, so why don't you use it as a property?
Try self.alertImage.hidden = YES;
Related
This question already has answers here:
Should IBOutlets be strong or weak under ARC?
(11 answers)
Closed 7 years ago.
When not using interface builder I always keep strong references to UI elements:
#interface myViewController : UIViewController
#property (nonatomic, strong) UILabel *folderLabel;
And then add them like this:
[self.view addSubview self.folderLabel];
where the initialiser is thus:
-(UILabel *)folderLabel{
if(!_folderLabel) {
_folderLabel = [[UILabel alloc] init];
_folderLabel.text = #"foo";
}
return _folderLabel
}
I have been told that this is bad for some reason and they should always be weak..
#property (nonatomic, weak) UILabel *folderLabel;
-(UILabel *)folderLabel{
if(!_folderLabel) {
UIlabel *folderLabel = [[UILabel alloc] init];
folderLabel.text = #"foo";
[self.view addSubview:folderLabel];
_folderLabel = folderLabel;
}
return _folderLabel
}
Is the strong reference a bad thing here?
When you add the subview to self.view, it gets retained. One pattern that works is to alloc the subview and store it in a local variable, add it to self.view then assign it to the ivar.
For some reason the image is not showing with this code:
YellowClass.h
#property (strong, nonatomic) IBOutlet UIImageView *myImage;
BlueClass.m
YellowClass *yellowClass = [[YellowClass alloc] init];
yellowClass.myImage.image = [UIImage imageNamed:#"Img.png"];
Two things. The instance of YellowClass that you create with alloc init is probably not the one you have on screen (though I can't be sure with that small snippet of code). Secondly, the view of yellowClass has not yet been loaded at the time you access its IBOutlet (myImage), so that outlet will be nil.
I am trying to create a UIImageView but I have to make it programmatically and I have to be able to declare it with an instance variable (in the .h file or something of the sort). Here is the code for creating it; however, this does not allow me to use it in other methods.
UIImageView *airImage = [[UIImageView alloc]
initWithFrame:CGRectMake(29, 7, 82, 96)];
[myScrollView addSubview:airImage];
I have looked on other people asking similar questions however none of them will allow me to create an instance variable. BTW that code is in my viewDidLoad. Thanks in advance!
In your .h use:
UIImageView *airImage;
In your viewDidLoad:
airImage=[[UIImageView alloc] initWithFrame:CGRectMake(29, 7, 82, 96)];
[myScrollView addSubview:airImage];
Or you can declare it as a property:
#property (nonatomic, strong) UIImageView *airImage;
and use to access it:
self.airImage = [[UIImageView alloc] initWithFrame:CGRectMake(29, 7, 82, 96)];
[myScrollView addSubview:self.airImage];
To be more specific, instance variables should be created in a specific place in an interface (can be both in your .h and .m files, but use .h as it is more common).
If you want to declare it in your .h file, then you will want your code to look like this:
#interface ClassName : UIViewController {
UIImageView *_airImage; //many developers use _ to represent ivars
}
#end
To set the value of the variable, then you can use
_airImage = [[UIImageView alloc]init...];
Property's are another option. Instead, you can declare this like so:
#interface ClassName : UIViewController
#property (strong, nonatomic) UIImageView *airImage;
#end
To set this value, simply use,
self.airImage = [[UIImageView alloc]init...];
Hope this helped clear some things up. Use this question to help understand the difference and when to use ivars vs properties: What is the difference between ivars and properties in Objective-C
This tutorial shows how you can use both ivars and properties together, and just help you understand them both better: http://www.icodeblog.com/2011/07/13/coding-conventions-ivars/
in your .h
#property (nonatomic, strong) UIImageView *airImage; // public
in your .m (viewDidLoad or wherever you want to init your ImageView)
self.airImage = [[UIImageView alloc] initWithFrame:CGRectMake(29, 7, 82, 96)];
[myScrollView addSubview:self.airImage];
I am working on the UIPopover and in one of the example I found that the Popover object is
created but then the object is assigned to the property of the Viewcontroller.
UIPopoverController* aPopover = [[UIPopoverController alloc] initWithContentViewController:content];
self.popoverController = aPopover;
What is the merit in such assignment and any reason for not assigning object to the property directly?
There is no "merit" in it. Saying
self.popoverController =
[[UIPopoverController alloc] initWithContentViewController:content];
would be absolutely equivalent.
On the other hand there is nothing wrong with using a temporary variable (aPopover) as shown in your example. It's just a name (a pointer); there is no significant waste of space or time. Moreover, saying self.popoverController repeatedly (either to set or to get its value) is to be avoided, because this is a method call - you are passing through the setter method or getter method (which may be synthesized, may have side effects, and does in fact take some extra time). Thus, when there is much configuration to be done (for example), it is best to do it as shown in your example:
UIPopoverController* aPopover =
[[UIPopoverController alloc] initWithContentViewController:content];
// lots of configuration here, using the local automatic variable aPopover...
// ...and then, only when it is all done, call the setter:
self.popoverController = aPopover;
The only reason for that is that you probably read this in a tutorial somewhere. And the author did it for readability for beginners. You could absolutely use:
self.popoverController = [[UIPopoverController alloc] initWithContentViewController:content];
All depending on how familiar you are with programming in general and how readable you want your code to be.
I agree with the others that, in this case, the assigning of the popover controller to a local variable before later assigning it to a class property is largely a stylistic matter. But this is only the case because you are keeping a strong reference to that popover controller. There are other situations where you have weak properties, in which this local variable pattern is critical.
For example, let's assume that we have a bunch of controls that we're going to add to our view controller's view programmatically:
#property (nonatomic, strong) UIView *containerView;
#property (nonatomic, strong) UILabel *usernameLabel;
#property (nonatomic, strong) UILabel *emailLabel;
// and more labels
When you want to add these to your view controller's view, you could do something like:
- (void)addSubviewsAtPoint:(CGPoint)location
{
self.containerView = [[UIView alloc] initWithFrame:[self frameForContainer:location]];
[self.view addSubview:self.containerView];
self.usernameLabel = [[UILabel alloc] initWithFrame:[self frameForUsernameLabel]];
[self.containerView addSubview:self.usernameLabel];
self.usernameLabel.text = self.username;
self.emailLabel = [[UILabel alloc] initWithFrame:[self frameForEmailLabel]];
[self.containerView addSubview:self.emailLabel];
self.emailLabel.text = self.email;
// etc.
}
But it also means that when you remove the subviews, you not only have to remove the container view from your view hierarchy, but you also have to remember to nil all of the properties for all of those subviews:
- (void)removeSubviews
{
[self.containerView removeFromSuperview];
self.containerView = nil;
self.emailLabel = nil;
self.usernameLabel = nil;
// etc.
}
This introduces a maintenance issue, that every time you add a new control via addSubviewsAtPoint, that you also have to remember to add it to removeSubviews, too, or else you might be hanging on to the control well after you've removed it from the screen.
To simplify your life, you might make all of these properties weak (with the intuition being that it's the view that owns these subviews, not the view controller):
#property (nonatomic, weak) UIView *containerView;
#property (nonatomic, weak) UILabel *usernameLabel;
#property (nonatomic, weak) UILabel *emailLabel;
// etc.
But now, using ARC, your addSubviewsAtPoint no longer works, because when you assign an object to a weak property, if there are no other strong references, it will be immediately become nil:
self.containerView = [[UIView alloc] initWithFrame:[self frameForContainer:location]];
[self.view addSubview:self.containerView]; // FAIL!!! self.containerView will be nil!
So, instead, we employ that local variable pattern of your question to ensure that the controls are not prematurely deallocated while we're adding them to our view:
- (void)addSubviewsAtPoint:(CGPoint)location
{
UIView *containerView = [[UIView alloc] initWithFrame:[self frameForContainer:location]];
[self.view addSubview:containerView];
self.containerView = containerView;
UILabel *usernameLabel = [[UILabel alloc] initWithFrame:[self frameForUsernameLabel]];
[containerView addSubview:usernameLabel];
usernameLabel.text = self.username;
self.usernameLabel = usernameLabel;
UILabel *emailLabel = [[UILabel alloc] initWithFrame:[self frameForEmailLabel]];
[containerView addSubview:emailLabel];
emailLabel.text = self.email;
self.emailLabel = emailLabel;
// etc.
}
And, as a result, because we're using weak properties, our removal of those subviews is now much simpler, as we don't have to nil all of those properties when we remove the containerView from our view controller's view:
- (void)removeSubviews
{
[self.containerView removeFromSuperview];
// because all of those `containerView` subviews were `weak`,
// we don't have to manually `nil` them
}
I've seen some solutions, none of which have worked for my issue.
I have a UIButton created by simply drag/dropping into the UIViewController in Storyboard editor.
The UIButton has an outlet linked to the .h file for the UIViewController associated to that view. It is also Synthesized in the .m file.
#property (strong, nonatomic) IBOutlet UIButton *answerButton;
I want to change the location of the button during run time like this;
CGRect btFrame = answerButton.frame;
btFrame.origin.x = xOne;
btFrame.origin.y = yOne;
answerButton.frame = btFrame;
However whenever I try this, the button refuses to move.
All other editing functions (like setTitle etc) are functional, but for some reason the frame won't move how I want it to.
Simply uncheck "Use Autolayout" in the file inspector..
.h file
#import <UIKit/UIKit.h>
#interface ViewController : UIViewController{
IBOutlet UIButton *theButton;
}
#property(nonatomic, strong) IBOutlet UIButton *theButton;
-(IBAction)moveTheButton:(id)sender;
#end
.m file
-(IBAction)moveTheButton:(id)sender{
CGRect btFrame = theButton.frame;
btFrame.origin.x = 90;
btFrame.origin.y = 150;
theButton.frame = btFrame;
}
This code moves the button from one point to another.
Replace your code by the below, which includes code to remove auto resizing mask.
CGRect btFrame = answerButton.frame;
btFrame.origin.x = xOne;
btFrame.origin.y = yOne;
answerButton.autoresizingMask = UIViewAutoresizingNone;
answerButton.frame = btFrame;