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This article mainly shows you "how to use objcruntime to achieve iOS closed-loop lazy loading function", the content is easy to understand, clear, hope to help you solve your doubts, the following let Xiaobian lead you to study and learn "how to use objcruntime to achieve iOS closed-loop lazy loading function" this article.

Using objc runtime to realize lazy loading

Address: AutoPropertyCocoa

The lazy loading form is as follows

-(id) lazyloadProperty {if (_ lazyloadProperty = = nil) {_ lazyloadProperty = [XClass...];} return _ lazyloadProperty;}

It is generally easy to do this using macro definitions. But there is no consistency, and the migration is poor.

Using the dynamic nature of objc runtime to realize lazy loading can not only add and delete functions, but also avoid pollution types. The three parties make up for the vacancy that there is no closed loop to achieve lazy loading at present.

Main process:

Lazy loading of an instance or class if it is an instance object, hook and modify the type to subclass it to method swizzling the type. If you unbind the type now, you can determine whether it is a self-implemented method. If it is a self-implemented method-> 5, otherwise-> 6 call this method of method swizzling restore to delete this type

Difficulties:

Implement method swizzling on your own

Re-implement the objc1-era method class_removeMethods hooks up the runtimelock in the runtime to achieve security when modifying type data

Let's implement the two API of method swizzling again.

OBJC_EXPORT IMP _ Nullableclass_replaceMethod (Class _ Nullable cls, SEL _ Nonnull name, IMP _ Nonnull imp, const char * _ Nullable types) OBJC_AVAILABLE (10.5,2.0,9.0,1.0,2.0); OBJC_EXPORT voidmethod_exchangeImplementations (Method _ Nonnull M1, Method _ Nonnull m2) OBJC_AVAILABLE (10.5,2.0,9.0,1.0,2.0)

Either way, if the type does not implement the method but the parent class, you need to dynamically add a method. In the Objc1 era, the method of dynamic addition can be deleted in the following ways:

OBJC_EXPORT voidclass_removeMethods (Class _ Nullable, struct objc_method_list * _ Nonnull) OBJC2_UNAVAILABLE

There is no such method after runtime was rewritten after the Objc2 era, and the new runtime class structure does not seem to allow developers to delete methods, so write down the process here.

First of all, look at the structure of the class reader, class_rw_t.

Struct class_rw_t {/ / Be warned that Symbolication knows the layout of this structure. Uint32_t flags; uint32_t version; const class_ro_t * ro; method_array_t methods;// delete a method here property_array_t properties; protocol_array_t protocols; Class firstSubclass; Class nextSiblingClass; char * demangledName;#if SUPPORT_INDEXED_ISA uint32_t index;#endif}

Method_array_t inherits from list_array_tt, which is an array structure. The stored content is method_list_t.

Method_list_t inherits from entsize_list_tt, which is also an array structure.

The whole method_array_t structure is a two-dimensional array. Deleting one method_t at a time requires replacing the original object with a new method_list_t.

Then there is the issue of thread safety, where you need to get the read-write lock (pthread_rw_lock_t runtimelock) that Apple uses in the type of operation. Without this lock, any modification to runtime would be unreliable.

The ultimate approach is to hijack the system function that exposes the symbol and then block the thread

The hijacking system C function uses Facebook's fishhook, which actually works on macOS.

All that's left is to find the right function, which meets two conditions:

The function exists in the symbol table. Inside the function, there is a second function that satisfies condition 1 after lock runtimelock.

After searching for a long time, I found that only objc_allocateProtocol () is the most suitable. Calloc () is called inside objc_allocateProtocol, so the second hijacked function is calloc. In order to reduce the overhead of calloc, a little work needs to be done.

The operation of comparing thread ID to each call is obviously better than brute force blocking threads. Reduce the call stack of hijacked calloc.

The above is all the content of this article "how to use objcruntime to achieve lazy loading function of iOS closed loop". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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