Debugging And Debugging An Objective-C Runtime System
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Recently, some readers have reported an encounter with the Objective-c runtime system.
Objc_msgSend Function
In Objective-c posts, implementation of methods is positive only at runtime. The compiler converts the message expression,
[recipient's message]
deliver results to a major messaging service by calling objc_msgSend . This function takes a recipient and a name as specified in the message – i. H. exact method selector – as are your two legal settings:
objc_msgSend (getter, selector)
All arguments passed in the message are also passed to objc_msgSend :
objc_msgSend (getter, selector, arg1, arg2, ...)
- First found (procedure implementation method) referenced by the person picker. Since the same method can be implemented differently in different grades, the exact procedure in which it is found depends on the college grade of the recipient.
- Then it usually calls the procedure and passes it the one that receives the product (one to reference its data) and any arguments that were method-safe. They
- finally pass the return value of the action as their own return valuee value.
The basic idea is the kitchen appliances that the compiler generates for almost every class and every object. Each class break order contains the following two main elements:
- Superclass tip.
- Cool post desk. It contains table entries, which in turn associate method selectors with the corresponding address classes of the found methods. The for selector, the
setOrigin ::method, is considered to be associated with an address that is related to (the procedure that implements)setOrigin ::, the to selector,display method is associated with the displayURL and beyond.
When a new object is created, a callback is assigned to it and the dog instance variables are initialized. First, somewhere between the variables of an object, there is a pointer that simply refers to its class structure. This note, isa , is called and passes the object’s input to its class, and through the class, to any classes from which it inherits.
When a new object is formed, memory is allocated to it, and its instance variables are usually initialized They become. The first variable of an object is considered a pointer to its class system. This pointer, isa , gives any object access to its class and even through the class to all some of the classes that it inherits. A
When a commercial message is sent to an object, the entire messaging function follows the object’s isa clause for the class structure from which it looks for the technology picker in the delivery table. If the situation cannot find a selector there, objc_msgSend follows the pointer to, and the superclass continues to try to find that selector in its dispatch table. Consecutive unnecessary reasons to climb objc_msgSend every class in our hierarchy until it reaches the entire NSObject class. Once it finds a common selector, the function calls the accessed method in the table and passes that situation to the structure of the receiving data object.
This is undoubtedly how method implementations can be chosen at runtime – or, more importantly in an object-oriented programming language, whyThe codes are dynamically linked to messages.
To speed up the messaging process, replay stores selectors between the system and validates the methods used. There is a separate cache for each individual class, which can contain items such as selectors for inherited methods, as well as methods defined in a category. Before looking up the allocation tables, this mail routine first checks the destination object’s school cache (assuming that the method that most have used one day is likely to be used again). When the solution selector is in the cache, the Messaging Solitary is slightly slower than a function call. After the program has been running long enough to warm up its caches, almost all sent item messages find some kind of cached structure. Caches are dynamically expanded to accommodate new voicemail messages while the program is running.
Use Hidden Arguments
When objc_msgSend finds a procedure that implements the method, it calls the human procedure and passesim all the exact arguments of the incoming message. Likewise, it passes two or three hidden arguments to the procedure:
- Receiver object
- Its method selector
These arguments provide each method with explicit information about the two sections of the message expression being called. They must be “hidden” because they are not declared in the source code that defines the operation. They are included in the implementation even though the code is compiled.
Although these reasons are not explicitly declared, the source code can still refer to them (just as it can refer to getting instance variables from this object). The method matches a receiving object of the form AND self , its own selector, as does _cmd . In the following example, _cmd refers to the selector for getting the dirty and self method to a specific object that has fancy <. get / code> message.
self is the more useful of two or three arguments. In fact, the temporary variables of the receiver object are made available through the definition of the method.
Resolution Didynamic Method
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There are complexities for which you might need to implement a dynamic method.
Dynamic Loading
An Objective-C program can also load new classes and categories at runtime. The new code is usually integrated into a refined program that is identical to the overflowing classes and categories at the beginning.
Obviously, dynamic loading can be used for many different purposes. For example, many types of modules are loaded dynamically in System Preferences.
Forwarding Messages
Sending an implication to an object that does not process this message is a serious new error. However, before reporting the error, some part of the executive gives the object a second chance to process the general message.
If you send a message to be sent to an object that will not accept this message, the runtime sends an important message forwardInvocation: to the object with NSInvocation incappromises the original message and your arguments passed through it.
The forwardInvocation: method can act as a centralized distribution for unrecognized messages and send these products to different recipients. This could be a forwarding position where all messages are sent to the same destination. He can translate one feeling into another, or simply “swallow” several messages so that there is no reaction and even an error message. forwardInvocation: can also concatenate multiple messages into a last individual response. forwardInvocation: what actually happens is up to the developer. The ability to connect points in a transmission chain opens up possibilities for program development.
Declared Property
When the entire compiler matches the declaration property, out generates descriptive metadata associated with the enclosing class, category, and protocol. You can access this metadata using functions that help you find a property by name in your own class or protocol, recognizing The character of the property is in the form of a large @encode and a line, a list with a. copy the property attributes as a sequence of C strings. A list of declared properties is available for each field and protocol.
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