In the case where no custom initialization is being performed, the "new" method can often be used in place of the alloc-init messages:. Also, some classes implement class method initializers. Some class method initializers take parameters:. The alloc message allocates enough memory to hold all the instance variables for an object, sets all the instance variables to zero values, and turns the memory into an instance of the class; at no point during the initialization is the memory an instance of the superclass.
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The init message performs the set-up of the instance upon creation. The init method is often written as follows:. In the above example, notice the id return type. This type stands for "pointer to any object" in Objective-C See the Dynamic typing section. The initializer pattern is used to assure that the object is properly initialized by its superclass before the init method performs its initialization. It performs the following actions:. A non-valid object pointer has the value nil ; conditional statements like "if" treat nil like a null pointer, so the initialization code will not be executed if [super init] returned nil.
If there is an error in initialization the init method should perform any necessary cleanup, including sending a "release" message to self, and return nil to indicate that initialization failed. Any checking for such errors must only be performed after having called the superclass initialization to ensure that destroying the object will be done correctly. If a class has more than one initialization method, only one of them the "designated initializer" needs to follow this pattern; others should call the designated initializer instead of the superclass initializer.
Objective-C was extended at NeXT to introduce the concept of multiple inheritance of specification, but not implementation, through the introduction of protocols. Objective-C makes use of ad hoc protocols called informal protocols and compiler-enforced protocols called formal protocols. An informal protocol is a list of methods that a class can opt to implement. It is specified in the documentation, since it has no presence in the language.
Informal protocols are implemented as a category see below on NSObject and often include optional methods, which, if implemented, can change the behavior of a class. For example, a text field class might have a delegate that implements an informal protocol with an optional method for performing auto-completion of user-typed text.
The text field discovers whether the delegate implements that method via reflection and, if so, calls the delegate's method to support the auto-complete feature. A formal protocol is similar to an interface in Java, C , and Ada It is a list of methods that any class can declare itself to implement. Versions of Objective-C before 2. A class must be declared to implement that protocol to be said to conform to it.
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This is detectable at runtime. Formal protocols cannot provide any implementations; they simply assure callers that classes that conform to the protocol will provide implementations.
By stating in the class definition that the protocol is implemented,. Objective-C, like Smalltalk, can use dynamic typing: This can allow for increased flexibility, as it allows an object to "capture" a message and send the message to a different object that can respond to the message appropriately, or likewise send the message on to another object. This behavior is known as message forwarding or delegation see below. Alternatively, an error handler can be used in case the message cannot be forwarded. If an object does not forward a message, respond to it, or handle an error, then the system will generate a runtime exception.
Static typing information may also optionally be added to variables. This information is then checked at compile time. In the following four statements, increasingly specific type information is provided. The statements are equivalent at runtime, but the extra information allows the compiler to warn the programmer if the passed argument does not match the type specified. In the above statement, foo may be an instance of any class that conforms to the NSCopying protocol.
Objective-C permits the sending of a message to an object that may not respond. Rather than responding or simply dropping the message, an object can forward the message to an object that can respond. Forwarding can be used to simplify implementation of certain design patterns , such as the observer pattern or the proxy pattern. An object wishing to implement forwarding needs only to override the forwarding method with a new method to define the forwarding behavior. The action method performv:: The - void forwardInvocation: When compiled using gcc , the compiler reports:.
The compiler is reporting the point made earlier, that Forwarder does not respond to hello messages. In this circumstance, it is safe to ignore the warning since forwarding was implemented. Running the program produces this output:. During the design of Objective-C, one of the main concerns was the maintainability of large code bases. Experience from the structured programming world had shown that one of the main ways to improve code was to break it down into smaller pieces. Objective-C borrowed and extended the concept of categories from Smalltalk implementations to help with this process.
Furthermore, the methods within a category are added to a class at run-time. Thus, categories permit the programmer to add methods to an existing class - an open class - without the need to recompile that class or even have access to its source code. For example, if a system does not contain a spell checker in its String implementation, it could be added without modifying the String source code.
Methods within categories become indistinguishable from the methods in a class when the program is run. A category has full access to all of the instance variables within the class, including private variables. If a category declares a method with the same method signature as an existing method in a class, the category's method is adopted. Thus categories can not only add methods to a class, but also replace existing methods. This feature can be used to fix bugs in other classes by rewriting their methods, or to cause a global change to a class's behavior within a program.
If two categories have methods with the same name but different method signatures, it is undefined which category's method is adopted. Other languages have attempted to add this feature in a variety of ways. TOM took the Objective-C system a step further and allowed for the addition of variables also.
Other languages have used prototype-based solutions instead, the most notable being Self. The C and Visual Basic. NET languages implement superficially similar functionality in the form of extension methods , but these lack access to the private variables of the class.
Logtalk implements a concept of categories as first-class entities that subsumes Objective-C categories functionality Logtalk categories can also be used as fine-grained units of composition when defining e. This example builds up an Integer class, by defining first a basic class with only accessor methods implemented, and adding two categories, Arithmetic and Display , which extend the basic class. While categories can access the base class's private data members, it is often good practice to access these private data members through the accessor methods, which helps keep categories more independent from the base class.
Implementing such accessors is one typical usage of categories. Another is to use categories to add methods to the base class. However, it is not regarded as good practice to use categories for subclass overriding, also known as monkey patching. Informal protocols are implemented as a category on the base NSObject class. The program will still run. This means that it is possible to mix-and-match added categories if needed; if a category does not need to have some ability, it can simply not be compile in.
Objective-C permits a class to wholly replace another class within a program. The replacing class is said to "pose as" the target class. Class posing was declared deprecated with Mac OS X v Similar functionality can be achieved by using method swizzling in categories, that swaps one method's implementation with another's that have the same signature. For the versions still supporting posing, all messages sent to the target class are instead received by the posing class.
There are several restrictions:. Posing, similarly with categories, allows global augmentation of existing classes. Posing permits two features absent from categories:. In the C language, the include pre-compile directive always causes a file's contents to be inserted into the source at that point.
Objective-C has the import directive, equivalent except that each file is included only once per compilation unit, obviating the need for include guards. Objective-C's features often allow for flexible, and often easy, solutions to programming issues. As nothing is done to unify the semantics behind the various language features, certain restrictions apply:. Mac OS X v When run in backwards-compatible mode, the runtime turned reference counting operations such as "retain" and "release" into no-ops.
All objects were subject to garbage collection when garbage collection was enabled. The garbage collector does not exist on the iOS implementation of Objective-C 2.
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Garbage collection was deprecated in Mac OS X v Properties are, in a sense, public instance variables; that is, declaring an instance variable as a property provides external classes with access possibly limited, e. A property may be declared as "readonly", and may be provided with storage semantics such as assign , copy or retain.
By default, properties are considered atomic , which results in a lock preventing multiple threads from accessing them at the same time. A property can be declared as nonatomic , which removes this lock. Properties are implemented by way of the synthesize keyword, which generates getter and setter, if not read-only methods according to the property declaration. Alternatively, the getter and setter methods must be implemented explicitly, or the dynamic keyword can be used to indicate that accessor methods will be provided by other means.
When compiled using clang 3. Properties can be accessed using the traditional message passing syntax, dot notation, or, in Key-Value Coding, by name via the "valueForKey: In order to use dot notation to invoke property accessors within an instance method, the "self" keyword should be used:. A class or protocol's properties may be dynamically introspected.
Under the modern runtime, an extra layer of indirection is added to instance variable access, allowing the dynamic linker to adjust instance layout at runtime. This feature allows for two important improvements to Objective-C code:. Fast enumeration generates more efficient code than standard enumeration because method calls to enumerate over objects are replaced by pointer arithmetic using the NSFastEnumeration protocol. A class extension has the same syntax as a category declaration with no category name, and the methods and properties declared in it are added directly to the main class.
It is mostly used as an alternative to a category to add methods to a class without advertising them in the public headers, with the advantage that for class extensions the compiler checks that all the privately declared methods are actually implemented.
Since fast enumeration does not generate exactly the same binaries as standard enumeration, its use will cause an application to crash on Mac OS X version Blocks are only supported in Mac OS X Automatic Reference Counting ARC is a compile-time feature that eliminates the need for programmers to manually manage retain counts using retain and release. Xcode can also attempt to automatically upgrade a project to ARC. Using this format saves the programmer from having to use the longer initWithString or similar methods when doing certain operations. However, different from string literals , which compile to constants in the executable, these literals compile to code equivalent to the above method calls.
In particular, under manually reference-counted memory management, these objects are autoreleased, which requires added care when e.
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In code, subscripting is represented using brackets [ ]. After the purchase of NeXT by Apple, attempts were made to make the language more acceptable to programmers more familiar with Java than Smalltalk. One of these attempts was introducing what was dubbed "Modern Syntax" for Objective-C at the time  as opposed to the current, "classic" syntax. There was no change in behaviour, this was merely an alternative syntax. Instead of writing a method invocation like. It also includes a runtime library called ObjectPak, which is based on Cox's original ICPak library which in turn derives from the Smalltalk class library , and is quite radically different from the OpenStep FoundationKit.
Objective-C today is often used in tandem with a fixed library of standard objects often known as a "kit" or "framework" , such as Cocoa , GNUstep or ObjFW. These libraries often come with the operating system: Objective-C allows for the declaration of new root classes that do not inherit any existing functionality. Originally, Objective-C-based programming environments typically offered an Object class as the base class from which almost all other classes inherited.
With the introduction of OpenStep, NeXT created a new base class named NSObject, which offered additional features over Object an emphasis on using object references and reference counting instead of raw pointers, for example. Almost all classes in Cocoa inherit from NSObject. Not only did the renaming serve to differentiate the new default behavior of classes within the OpenStep API, but it allowed code that used Object—the original base class used on NeXTSTEP and, more or less, other Objective-C class libraries —to co-exist in the same runtime with code that used NSObject with some limitations.
The introduction of the two letter prefix also became a simplistic form of namespaces, which Objective-C lacks.
Using a prefix to create an informal packaging identifier became an informal coding standard in the Objective-C community, and continues to this day. More recently, package managers have started appearing, such as CocoaPods , which aims to be both a package manager and a repository of packages. A lot of open-source Objective-C code that was written in the last few years can now be installed using CocoaPods.
Objective-C implementations use a thin runtime system written in C [ citation needed ] , which adds little to the size of the application. In contrast, most object-oriented systems at the time that it was created used large virtual machine runtimes. Programs written in Objective-C tend to be not much larger than the size of their code and that of the libraries which generally do not need to be included in the software distribution , in contrast to Smalltalk systems where a large amount of memory was used just to open a window.
Since the programmer has such freedom to delegate, forward calls, build selectors on the fly and pass them to the runtime system, the Objective-C compiler cannot assume it is safe to remove unused methods or to inline calls. Swift Development with Cocoa. Oracle Utilities Pocket Reference. Learn FileMaker Pro Xcode 6 Start to Finish. Magento 2 Development Cookbook.
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