Objc_msgSend summary of iOS Low-level analysis

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• IOS low-level analysis and other exploration – Cache insert, objc_msgSend

• Objc_msgSend Message dynamic resolution for iOS low-level analysis

• Objc_msgSend message forwarding for iOS low-level analysis

The previous articles have taken you step by step to analyze the general flow of the underlying calls to objc_msgSend, and let’s summarize it:

A flowchart

Detailed process

What has been done for each of them is described as follows:

_objc_msgSend{non-empty judgment; Get the isa; Get the Class; CacheLookup NORMAL, _objc_msgSend, __objc_msgSend_uncached }Copy the code

CacheLookup{
/** ** ** ** ** ** ** ** ** ** **
1,classMemory offset foundcache_t2,cache_tAnd on themaskgetmaskThe address,cache_tAnd on thebucketgetbucketAddress; 3,maskAnd on theselgetsel-impThe subscriptindex; 4,bucketAccording to theindexI'm going to shift it in memory and get the correspondingbucket5, removebuckettheselandobj_msgSendincomingselCompare, agree, enterCacheHit; 6, inconsistentbucketIf you can't find it, go to step 5MissLabelDynamicThe content corresponding to the parameter; }Copy the code

CacheHit{determine the value of the first argument:1If equal to NORMAL, enter the TailCallCachedImp function.2Enter AuthAndResignAsIMP if GETIMP;3If it is equivalent to LOOKUP, enter AuthAndResignAsIMP; }Copy the code

// Cache imp decode, get real IMP, and enter IMP
TailCallCachedImp{
    // $0 = cached imp, $1 = address of cached imp, $2 = SEL, $3 = isa
    eor	$0, $0, $3
    br	$0
}

// The imp is the real imp
AuthAndResignAsIMP{
    $0 = cache IMP, $1 = address. $2 = SEL $3 = class
    $0 = $0 ^ $3 = imp ^ class = imp
    eor	$0, $0, $3
}
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// This step has started the slow search
__objc_msgSend_uncached{
    1, MethodTableLookup// Perform a lookup of the method class table
    2, TailCallFunctionPointer// Return to the function
}
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// Method list lookup
MethodTableLookup{
    _lookUpImpOrForward/ / find the imp
}
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lookUpImpOrForward{
1, _objc_msgForward_impcache// Generate a default forward_IMP
2, checkIsKnownClass// Check whether the class has been registered
3, realizeAndInitializeIfNeeded_locked// Initialize class, superclass, metaclass, root metaclass
4,forThe body of the loop looks like this: look in the class method list with dichotomy, -> find, call log_and_fill_cache and write to the cache; -> curClass = 'curClass'; curClass = 'curClass'; curClass = 'curClass'; Keep going through this loop, write it to the cache when you find it, until curClass becomes nil (NSObject's parent class is nil), pass the default forward_IMP to IMP, jump outforCycle; }Copy the code

LGetImpMissDynamic{does just one thing: return0
}
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_cache_getImp{
    GetClassFromIsa_p16 p0, 0/ / get the isa
    CacheLookup GETIMP, _cache_getImp, LGetImpMissDynamic, LGetImpMissConstant// Cache lookup
}
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__objc_msgForward_impcache{
    b	__objc_msgForward // Jump to __objc_msgForward
    END_ENTRY __objc_msgForward_impcache
}
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__objc_msgForward{
    adrp	x17, __objc_forward_handler@PAGE
    ldr	p17, [x17, __objc_forward_handler@PAGEOFF]
    /** TailCallFunctionPointer = $0; /** TailCallFunctionPointer = $0; /** TailCallFunctionPointer = $0; So __objc_msgForward is going into __objc_forward_handler */
    TailCallFunctionPointer x17 
}
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__attribute__((noreturn, cold)) void
objc_defaultForwardHandler(id self, SEL sel)
{
    _objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
                "(no message forward handler is installed)", 
                class_isMetaClass(object_getClass(self)) ? '+' : The '-', 
                object_getClassName(self), sel_getName(sel), self);
}
/ / is _objc_forward_handler objc_defaultForwardHandler
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
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+ (BOOL)resolveInstanceMethod:(SEL)sel {
    // Add a method implementation to an object. For example, if sel is run, then the imp returned can be the IMP of sleep;
}

+ (BOOL)resolveClassMethod:(SEL)sel{ 
    // Add a method implementation to the class, ibid
}
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// Quick message forwarding
- (id)forwardingTargetForSelector:(SEL)aSelector{
    // Set the new message receiver
}
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// Slow forwarding of messages
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    // Return the method signature
}
/// The method invocation contains the method signature information,
// In the forwardInvocation method, we can handle the exception method invocation or not, and it won't crash
- (void)forwardInvocation:(NSInvocation *)anInvocation{
    // Some handling of exception methods
}
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