i have defined variable in GroupView.h
#interface GroupView()
{
NSMutableArray *chatrooms;
}
#end
#implementation GroupView
Now i want to pass this variable in segue
#interface FriendsViewController ()
#end
#implementation FriendsViewController
else if ([segue.identifier isEqualToString:#"showGroupView"]) {
GroupView *groupView = (GroupView *)segue.destinationViewController;
groupView.chatrooms = [NSMutableArray arrayWithArray:chatrooms];
}
i know that chatrooms has to be property in header file to code this way but it is not
So is there any way to use this variable in segue.
Thanks for help.
chatrooms defined as an ivar like you have done is accessed using -> notation:
groupView->chatrooms = [NSMutableArray arrayWithArray:chatrooms]
This is generally discouraged, though. You should use a property instead:
#interface GroupView
#property (strong) NSMutableArray *chatrooms;
#end
Incidentally, if you're using an NSMutableArray, that indicates that you want to modify the element list of the array directly and not just replace the array wholesale. If you only ever want to replace the array with a whole new array every time, I suggest using NSArray instead.
Another point to make here is that you're attempting to cast the object held at segue.destinationViewController as a GroupView. You have either named a UIViewController subclass in a very misleading way, or you are not accessing the GroupView as a correct member of the UIViewController that is returned to you.
Normally, if you are not building the SDK or something. You don't really have a better reason not to expose it in the header file. However, you can expose the property in the extension and declare a private property in the host class(It's really not able to pass if you just declare a local variable). For example, You have a extension called GroupView+Helper. So, you can pass it into the property exposed in the extension. And then internally translate into the GroupView.
In GroupView.m:
#interface GroupView
#property (strong, nonatomic) NSMutableArray *chatrooms;
#end
In GroupView+Helper.h
#property (strong, nonatomic) NSMutableArray *internalChatrooms;
Also, you need to import the GroupView+Helper in the GroupView.
It will make your chatrooms private and internalChatrooms protected.
Related
This question already has answers here:
Objective-C declared #property attributes (nonatomic, copy, strong, weak)
(4 answers)
Closed 6 years ago.
I was trying to pass a custom object to the next view controller and I encountered this error -[ClassName copyWithZone:] unrecognized selector sent to instance
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
if ([segue.identifier isEqualToString:#"attemptDetails"])
{
ResultsVC *vc = segue.destinationViewController;
vc.selectedEntry = selectedEntry;
}
}
#property (nonatomic, retain) ClassName *selectedEntry; //Why is it retain and not copy?
I'm still very confused with property attributes and why certain types use certain attributes, like NSString uses (nonatomic, copy) and CLLocationCoordinate2D uses (nonatomic, readonly).
Could someone explain or link a reference to me how each property attribute works? Much thanks!
There are lots of descriptions for property attributes explanation,
Reference links,
Objective-C ARC: strong vs retain and weak vs assign
https://stackoverflow.com/a/4511004/4294543
#property and retain, assign, copy, nonatomic in Objective-C
Short & simple my understanding is like,
retain : It's working on the created object, and it just increase the reference count.
Here in your case you have already model class object so not need to copy in the second vc property,you just need to retain it to second vc property.
copy : The value you assigned to property can be copied & used for other purposes too(create shallow copy of object & need when object is mutable & need to release after finish with it).
nonatomic : Thread access is faster but you can't simultaneously access & change your property.
readonly : You can't directly assign the property new value.
Even i have run your case in the my project,
#import "ViewController.h"
#import "TestViewController.h"
#import "CustomClass.h"
#interface ViewController (){
CustomClass *classT;
}
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
classT = [[CustomClass alloc]init];
classT.test = YES;
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (IBAction)btn:(id)sender {
TestViewController * vc = [self.storyboard instantiateViewControllerWithIdentifier:#"TestViewController"];
vc.className = classT;
[self presentViewController:vc animated:YES completion:nil];
}
#end
#import <UIKit/UIKit.h>
#import "CustomClass.h"
#interface TestViewController : UIViewController
#property (nonatomic,retain) CustomClass *className; // Work as i said
//#property (nonatomic,copy) CustomClass *className; // Makes a copy of an object, and returns it with retain count of 1. If you copy an object, you own the copy. This applies to any method that contains the word copy where “copy” refers to the object being returned thats why here you will get crash
#end
I have read couple of good article for memory management. According to rypress
Retain Attribute : The retain attribute is the Manual Retain Release version of strong, and it has the exact same effect: claiming ownership of assigned values. You shouldn’t use this in an Automatic Reference Counted environment.
Copy Attribute : The copy attribute is an alternative to strong. Instead of taking ownership of the existing object, it creates a copy of whatever you assign to the property, then takes ownership of that. Only objects that conform to the NSCopying protocol can use this attribute.
Even I went through some good link of stackoverflow as well. Joshua Nozzi's answer gave good explanation for retain vs copy.
Retain vs. Copy - Declared properties use retain by default (so you can simply omit it altogether) and will manage the object's reference count automatically whether another object is assigned to the property or it's set to nil; Use copy to automatically send the newly-assigned object a -copy message (which will create a copy of the passed object and assign that copy to the property instead - useful (even required) in some situations where the assigned object might be modified after being set as a property of some other object (which would mean that modification/mutation would apply to the property as well).
Also found good example here.
Code :
NSMutableArray *array = [[NSMutableArray alloc] initWithObjects:#"First",#"Second", nil];
NSMutableArray *copiedArray = [array mutableCopy];
NSMutableArray *retainedArray = [array retain];
[retainedArray addObject:#"Retained Third"];
[copiedArray addObject:#"Copied Third"];
NSLog(#"array = %#",array);
NSLog(#"Retained Array = %#",retainedArray);
NSLog(#"Copied Array = %#",copiedArray);
Output :
array = (
First,
Second,
"Retained Third"
)
2013-12-19 17:15:49.380 RetainVsCopy[2876:c07] Retained Array = (
First,
Second,
"Retained Third"
)
2013-12-19 17:15:49.381 RetainVsCopy[2876:c07] Copied Array = (
First,
Second,
"Copied Third"
)
See, both array and Retained Array are having same contents. This is because both are pointing to same memory/instance/object. Where as contents of Copied Array are different. This is because copy created a separate instance.
In Objective C you will find that each class actually has a structure behind it. The properties are shortcuts which create the value in structure, a getter and a setter. For instance:
#interface MyClass
#property id myValue;
#end
Will create:
#interface MyClass {
id _myValue;
}
#property id myValue;
#end
#implementation
- (id)myValue {
return _myValue;
}
- (void)setMyValue:(id)myValue {
_myValue = myValue;
}
#end
Now these flags such as retain and copy add additional logic to the setters and getters. Using copy will actually create a setter as:
- (void)setMyValue:(id)myValue {
_myValue = [myValue copy];
}
Which means that the value must have the copy method implemented. Since your object does not it crashes.
Why to use copy is for safety. This is rarely important for something as strings but it is important for something like an array. So for instance you create a property #property NSArray *myArray; which expects an un-mutable array but the problem is that you can set a mutable array as well: myClassInstance.myArray = [[NSMutableArray alloc] init];. Now 2 modules have the access to the same mutable array. So if the first object starts modifying the array while the other one expects the array to always be the same you may find some issues. For instance MyClass instance may use it as a data source for the table view and at some point the array is mutated but the cells are not added/removed and the table view will cause a crash.
To be honest you can simply leave all of these as default and modify them only when you really need to. The case like above is highly unlikely anyway.
Simple question, I would just like to hear what other people do, and what best practice might be. To clear some things up.
When declaring an instance variable in one of my view controllers. Where is the proper place to declare it? I want the instance variable to available to all methods/function in the view controller
I've seen it done this way, in the .h file in curly braces after the #interface:
#interface ViewController : UIViewController {
NSString *test3;
}
I've seen it done this way, in the .m file in curly braces after the #interface:
#interface ViewController () {
NSString *test1;
}
And I've seen it done this way, in the .m file in curly braces after the #implementation:
#implementation ViewController {
NSString *test2;
}
What is the best practice? Is there any difference? If so what may they be, and what makes one way better than another?
All help will be appreciated,
Thanks.
Any of those will work, but current best practice is to use a property. If you want it to be accessible only inside your class:
#interface ViewController ()
#property (copy, nonatomic) NSString *test1;
#end
An access it like this:
self.test1 = #"Hello, world";
But, if you want it to be accessible to other classes as well, put it in the public interface:
#interface ViewController : UIViewController
#property (copy, nonatomic) NSString *test1;
#end
And then access it like this:
someInstanceOfViewController.test1 = #"Hello, world";
If you need to access the instance variable directly (and this applies only inside the class in most cases), for example if you are making a custom setter, the compiler will auto-synthesize an ivar that is the name of your property prefixed with an underscore:
- (void)setTest1:(NSString *)test1
{
_test1 = [test1 copy];
}
Note: the copy is because you might set test1 to an instance of NSMutableString, and you probably don’t want its contents getting mutated out from under you. When you override the setter, as above, you have to enforce this copy semantic yourself.
If I have viewController1 and viewController2 and user enters a double value into a textField in viewController1 this is passed to a variable in viewController2 and that value can be used.
This variable must then be able to be transferred to the UIView as that is what requires the value.
I'm struggling even at the first hurdle and that is to make a double a #property or what it needs to be, A lot of tutorials and examples show NSStrings being transferred but it is important that I transfer a double.
Here's what I have so far..
ViewControllerInputs.h - viewcontroller1
#interface ViewControllerInputs : UIViewController {
IBOutlet UITextField *thicknessField;
IBOutlet UITextField *capWidthField;
double *thicknessValue;
double *capWidthValue;
}
ViewControllerImage.h - viewcontroller2
#interface ViewControllerImage : UIViewController {
double thicknessValue1;
double capWidthValue1;
}
ViewControllerInputs.m - viewcontroller1
-(IBAction)createWeld {
ViewControllerImage *secondViewController = [[ViewControllerImage alloc]
initWithNibName:#"ViewControllerImage" bundle:nil];
ViewControllerImage.thicknessValue1 = thicknessValue;
//Require modal/push to viewControllerImage
}
UIView inside ViewControllerImage - viewcontroller2
This is where the variables thicknessValue1 and CapWidthValue1 will be used.
You have a few problems:
1) Get rid of the asterisk for each of the double ivars
2) Get rid of the double ivars and use properties instead.
#property (nonatomic, assign) double thicknessValue1;
#property (nonatomic, assign) double capWidthField;
As you have it, you are attempting to assign to a non-existent property.
Use
#property (nonatomic) double someDoubleVariable; for example to define the property, in the interface after defining the instance variable,
#synthesize someDoubleVariable; within the implementation to generate the getter and setter functions.
I suggest you read the documentation on Objective-C 2.0 properties — you're lucky to be programming for a platform whose parent company makes the cleanest documentation. Make good use of it.
Good luck.
I'm working on a project in objective C where i have to modificate some variables which are in a view controller from uiviews.
So i've tried somethings like this :
ViewController.h :
#property (nonatomic) bool Contact;
One of the UIViews :
ViewController * View;
View.Contact = YES;
I've also tried to make a setter method like this in the ViewController :
-(void) SetterContact:(bool)boolean;
And so to modificate from a UIView like this :
[View SetterContact:YES];
But it's looking working.
I've read that i have to init the object in which is containt the variable, but in memory management it's not really good to make some initializations from object who are already actives no ?
So if View is already init, i'm not going to call the init method from another UIView no ?
Thanks for your help !
If you want bool variable to be accessible from other viewController.Then simply wirte it as :-
#property BOOL Contact;
and make an object of ViewController in which you have declared contact variable as BOOL and access this variable using like this:-
OtherViewController *otherViewController=[[OtherViewController alloc] init];
otherViewController.Contact=YES;
As it is a instance variable it has to be accessed using class object.
use #property (nonatomic, assign, getter = isContact) BOOL contact; in your .h file.
Respect naming conventions
#property (nonatomic,retain) UIViewController *myController;
don't forget to synthesize
#synthesize myController = _myController;
If you want to implement your own setter do this: respect the naming convention
-(void)setMyController:(UIViewController*)controller;
or if by any bizarre reason you can't respect naming convention you can point the property to the method you want
#property (nonatomic,retain,setter=myBizarreSetterMethod:) UIViewController *myController;
this can help you out as well question in stackoverflow
This is what I got so far.
mainview.m
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender(id)sender {
secondView *secView = [segue destinationViewController]
secView.ext = #".com";
}
secondView.h
#interface secondView : UIViewController {
NSString *ext;
}
#proper (nonatomic, retain) NSString *ext;
#end
secondView.m
-(void)viewDidLoad {
NSString *url = [#"www.google" stringByAppendingFormat:ext]
}
And its returning a error saying ext is null... what am I doing wrong?
Did you try to turn ext into a property instead? My understanding is that the "dot" notation essentially turns your code into
[secView setExt:#".com"]
So turn SecondView.h into
#interface secondView : UIViewController
#property (nonatomic, copy) NSString *ext;
#end
And don't forget to #synthesize it in your .m file
Check the followings:
Make sure #synthesize ext; is in SecondView.m
In Storyboard, have you linked the segue correctly?
In Stodyboard, is the viewController that represetns SecondView defined as a class of SecondView?
Make sure that you are calling the SecondView via prepareForSegue:sender method (i.e. SecondView doesn't get called by pushViewController:animated somewhere else in your code).
Put a breakpoint at the line:
secView.ext = #".com";
and make sure that the ext ivar is properly set after the assignment. If it's not, you might be specifying that the accessors use a different ivar in your #synthesize directive for ext, or you might have provided a setter -setExt: that doesn't properly set the ivar.
If ext is set properly after the assignment, perhaps you've got two different instances of secondView. That used to happen a lot when people used .xib files -- they'd create one view controller in code and have another instance of the same class in their .xib file. I wouldn't expect that to be the case here since you're getting secView straight from the segue, but it's worth thinking about.
There are two views: view1 and view2.
Add view2.h file in view1.h
init object of view2 and set their variable.