Ios on Nabeel's - iOS Developer Blog

Downgrade UI Files from XCode 8 to XCode 7

Wed, 07 Dec 2016 00:00:00 +0000

In one of my recent project I developed a module in XCode 8 as I had upgraded to it. Later I came to know that our team is currently working on XCode 7. So if we integrate XCode 8 UI files i.e .storyboard & .xib, the main project will not compile and will show following error.

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The document "Main.storyboard" requires XCode 8.0 or later.

This version does not support documents saved in XCode 8 format. Open this document with XCode 8.0 or later.

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The document "(null)" requires XCode 8.0 or later.

Although it is strongly recommended to upgrade your XCode as soon as some stable version arrives, there can come certain situations where you have to downgrade it. If you are a developer like me who is trying to downgrade his project to support XCode 7 here are two simple ways.

Using XCode 8:

If you have XCode 8 installed on your system, you can use this method to downgrade your storyboard files.

Open .storyboard/.xib file in XCode 8.0
On right side: Utility Area > File Inspecter > Interface Builder Document
Choose ‘XCode 7.x’ for ‘Opens in’s’ value. The process is shown in GIF below:

Using any text Editor

If you do not have access to XCode 8 at the moment you can use any of the text editors available to downgrade to XCode 7. So that you have no more build errors.

Just open your .storyboard/.xib file in a text editor of your choice and remove following line:

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<capability name="Document saved in the Xcode 8 format" minToolVersion="8.0"/>

After removing this line you will be able to compile your project successfully.

To understand why we removed above line you can explore the changes once a UI file is saved in Xcode 8 format. If you will open .storyboard or .xib file on XCode 8 first time. It will show you a dialog as shown below to make these files compatible with XCode 8 document format.

atomic vs nonatomic properties in Objective-C

Tue, 25 Oct 2016 00:00:00 +0000

The difference between atomic and non atomic properties remained a favourite question for interviewers. Today we will analyse these keywords in detail. These keyword define the characteristics of how these properties will behave once they will be accessed from multiple threads simultaneously.

atomic (Default)

As the word explains itself, a single thread will be able to access the property at a given time. To explain more, only one thread will be able to access getter/setter of a property. Other threads have to wait until first thread releases get/set method. Let’s say we have a property firstName which is atomic.

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- Thread A => obj.firstName=@"A"
- Thread B => obj.firstName=@"B"
- Thread C => NSLog("First Name: %@",obj.firstName);

It will ensure that the value of property remains consistent throughout the lifecycle. By default all properties are atomic.

Example atomic property:

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@property (strong,atomic) NSString *firstName;
OR
@property (strong) NSString *firstName;

Sample getter setter: This is how the methods of a property will look for an atomic property after

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@synthesize firstName = _firstName;
-(NSString *)firstName{
 @synchronized (self) {
 return _firstName;
 }
}
-(void)setFirstName:(NSString *)firstName{
 @synchronized (self) {
 if(_firstName != firstName){
 [_firstName release];
 _firstName = [firstName retain];
 }
 }
}

Note: the Objective-C 2.0 specification, mentions that locks are used internally, but it doesn’t specify exactly how. What you see above, is roughly what an atomic getter/setter would look like, but it might not be accurate.

Pros: Ensures that user gets a valid value and not some garbage

Cons: Slow, as it has code to ensure read write safety

nonatomic

For non atomic properties multiple threads can access the properties simultaneously, so there is always a risk of inconsistency between values

Example:

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@property (strong,nonatomic) NSString *firstName;

Sample getter setter: This is how the methods of a properties will look for an atomic property

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@synthesize firstName = _firstName;
-(NSString *)firstName{
 return _firstName;
}
-(void)setFirstName:(NSString *)firstName{
 if(_firstName != firstName){
 [_firstName release];
 _firstName = [firstName retain];
 }
}

Pros: Fast as compared to atomic properties, as there is no extra code to control access of multiple threads

Cons: You are not guaranteed whether or not you will receive a valid value

TODO

[] Thread safe vs Read/Write Safe
[] When is code created for properties i.e how & getter and setter code is created?
[] When to use atomic vs non-atomic some solid rules?
[] Detailed Pros & Cons
[] Multi-threading on Single CPU vs Multiple threads

synthesize vs dynamic in Objective-C

Tue, 25 Oct 2016 00:00:00 +0000

At the core of Object Orientation you should not access data directly, instead you should use accessor methods to get or set the data. In Objective-C @property directive is a simplified way in which developers can tell the compiler about the behavior required for the accessor methods, which is simple and brief.

Now the question arise, where are the accessor methods? This leads us to the key words which are our topic today i.e @synthesize and @dynamic.

@synthesize

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@synthesize propertyName = _propertyName;

@synthesize tells the compiler to take care of the accessor methods creation i.e it will generate the methods based on property description. It will also generate an instance variable to be used which you can specify as above, as a convention it starts with _(underscore)+propertyName.

@dynamic

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@dynamic propertyName;

mentioning @dynamic with a propertyName tells the compiler not to create accessor methods because user will provide the implementation dynamically in future. Apple allows its developers to provide method implementation at runtime using Dynamic Method Resolution.

Dispatch Objects in GCD

Wed, 30 Mar 2016 00:00:00 +0000

Grand Central Dispatch (GCD) is a technology by Apple to manage multiple cores of hardware at system level. As GCD is built at lower level and closer to unix system, it can manage muliple cores and developer do not have to worry about it. GCD provides a set of APIs which a developer can use to submit code blocks (dispatch objects) to it, after that GCD can do its magic.

Dispatch Object lifecycle

When compiled with Objective-C compiler dispatch objects behave like normal Objective-C objects. They are reatained and then released at the end of their lifecycle automatically in ARC.