About @selector

本文详细介绍了Objective-C中的选择器概念及其使用方法。选择器作为方法调用的唯一标识符,在运行时系统中扮演着动态函数指针的角色,能够根据不同的类自动指向相应的方法实现。文章还讲解了如何获取及使用选择器,并提供了检测类是否拥有特定选择器的方法。

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A selector is the name used to select a method to execute for an object, or the unique identifier that replaces the name when the source code is compiled. A selector by itself doesn’t do anything. It simply identifies a method. The only thing that makes the selector method name different from a plain string is that the compiler makes sure that selectors are unique. What makes a selector useful is that (in conjunction with the runtime) it acts like a dynamic function pointer that, for a given name, automatically points to the implementation of a method appropriate for whichever class it’s used with. Suppose you had a selector for the method run, and classes DogAthlete, andComputerSimulation (each of which implemented a method run). The selector could be used with an instance of each of the classes to invoke its run method—even though the implementation might be different for each.

Getting a Selector

Compiled selectors are of type SEL. There are two common ways to get a selector:

  • At compile time, you use the compiler directive @selector.

    SEL aSelector = @selector(methodName);      // 没有冒号表示methodName没有参数
    注意:在声明selector的时候
    SEL aSelector = @selector(methodName:);	// 冒号表示methodName带有一个参数
    SEL aSelector = @selector(methodName:data:);	// 表示methodName带有两个参数
  • At runtime, you use the NSSelectorFromString function, where the string is the name of the method:

    SEL aSelector = NSSelectorFromString(@"methodName");

    You use a selector created from a string when you want your code to send a message whose name you may not know until runtime.

Using a Selector

You can invoke a method using a selector with performSelector: and other similar methods.

SEL aSelector = @selector(run);
[aDog performSelector:aSelector];
[anAthlete performSelector:aSelector];
[aComputerSimulation performSelector:aSelector];

(You use this technique in special situations, such as when you implement an object that uses the target-action design pattern. Normally, you simply invoke the method directly.)

如何检测一个类中有没有某个selector

-respondsToSelector:
+instancesRespondToSelector:
例子:
@interface Parent : 
- (void)onUpdate;
@end

@interface Child : Parent 
@end

Child * childObj = [[Child alloc] init];
BOOL hasSelector = [childObj respondsToSelector : @selector(onUpdate)];
CCLOG(@"child has selector = %d", hasSelector);
        
BOOL hasInstanceSelector = [Child instancesRespondToSelector : @selector(onUpdate)];
CCLOG(@"child has instance selector = %d", hasInstanceSelector);
        
BOOL hasParentInstanceSelector = [Parent instancesRespondToSelector : @selector(onUpdate)];
CCLOG(@"parent has parent instance selector = %d", hasParentInstanceSelector);
结果为
child has selector = 1
child has instance selector = 1
parent has parent instance selector = 1





                
### GNU Radio Selector Module Component Configuration In the context of GNU Radio, selectors can refer to components or modules that allow for conditional routing or selection within a flowgraph. The specific mention of moving `KismetNodeInfoContext.h` to be publicly accessible outside of the GraphEditor module does not directly relate to GNU Radio but indicates practices involving accessibility and modularity in software development environments[^1]. For GNU Radio specifically, one common way to implement selective behavior is through blocks designed for decision-making based on certain criteria. For instance, the **Selector Block** allows users to choose between different input streams dynamically. A typical use case involves configuring this block with parameters such as: - Input Stream Index: Determines which stream should pass through. - Output Stream Index: Specifies where data will go after processing by the selector. Additionally, when working with more complex configurations like those mentioned under various Django-related topics[^2], it's important to note these are unrelated frameworks; however, similar principles apply regarding modular design patterns and configuration management systems. To demonstrate how one might set up a simple selector mechanism using Python code (noting again this example uses generic syntax rather than direct GNU Radio API calls): ```python def select_stream(input_streams, condition): """Select an output from multiple inputs based on given conditions.""" selected_index = determine_selection_logic(condition) return input_streams[selected_index] # Example usage streams = ["stream_1", "stream_2"] condition = True # Some logic determining selection output = select_stream(streams, condition) print(output) ``` This function demonstrates basic functionality without delving into specifics about integrating with actual GNU Radio infrastructure. --related questions-- 1. How do you integrate custom C++ blocks including selectors into GNU Radio? 2. What other types of control elements exist besides selectors in GNU Radio? 3. Can you explain the process of creating dynamic signal paths using GNU Radio Companion? 4. In what scenarios would someone prefer using a switch over a selector block in GNU Radio?
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