• Method – Swizzling is what

    Method-swizzling means method exchange. Its main function is to replace the implementation of one method with the implementation of another method at runtime, which is often referred to as iOS dark magic. (The so-called exchange is to change sel and IMP correspondence as shown in the following figure)

  • Method-swizzling related Api interpretation

    • class_getInstanceMethodGet instance methods through SEL
    • class_getClassMethodGet the class method through sel
    • method_getImplementationGets the implementation of a method
    • method_setImplementationSets the implementation of a method
    • method_getTypeEncodingGets the encoding type implemented by the method
    • class_addMethodAdd method implementation
    • class_replaceMethodReplace the implementation of one method with the implementation of another method
    • method_exchangeImplementationsSwap the implementations of the two methods

  • Method – Swizzling pit

    • Execute once problem

      Imagine if the method swap is performed twice and the result is that the first swap is done correctly, but the second swap will restore the result of the first swap, so the general mehod- Swizzling is written inloadMethod, butloadMethod can not help but think to continue to call, so it is possible to call many times, in view of this week’s situation can be through the singleton design principle, to achieve method transesterification once the following code:

    • A problem where a subclass of a method needs to be swapped but the parent class does

      The sample parent class containspersonInstanceMethodMethod subclass containslg_studentInstanceMethodThe method now swaps the two methods (we know that if we don’t find the implementation of the method in a subclass we go back to the parent class so the result is that the parent class’s method and the subclass’s method are swapped) as shown belowCorresponding code implementationShould contain in a subclasslg_studentInstanceMethodThe implementation of the subclass is calledpersonInstanceMethodOkay, but the superclass doesn’tlg_studentInstanceMethodThe implementation of a method that looks up the method only looks up the parent class and doesn’t look up the list of methods in the child class so it crashes when the parent class calls it Corresponding solutions:

      Before switching, check whether the current class has a corresponding method implementation. If not, the method will be replaced. If there is, the method will be swapped as followsOf course, if the method to be swapped does not exist in the current class, you can add an empty implementation and then swap the method. Either of these options can be customized according to the requirements of the project

      Solution optimization

      And of course there are incoming ones

    • The subclass doesn’t implement the parent class

      This kind of situationclass_getInstanceMethodI don’t get the value, so we just need to determineclass_getInstanceMethodIf yes, add an empty method to implement the procedure and then go through the above steps in the following code