IOS basic principle: OC Basic interview (1)

Load, C++ constructor, initialize call sequence

Load function call timing

loadMethods in thedyldThe loading flow is called, and the function call flow is:doModInitFunctions -> libSystem_initializer -> _objc_init -> _dyld_objc_notify_register ->load_images– >.
Read more about the underlying principles of iOS: DyLD application loading.

Initialize function call timing

initializeIs called when the class is first loaded, throughlookUpImpOrForwardFunction call.
Read more about the OC Principles of Exploration: a slow search process for methods.

C++ constructor function call timing

ifC++The constructor is inobjcSource code, then it is instatic_initIs called in, we are inIOS basic principle: class loading principleAnalysis.
- The order of its function calls isdoModInitFunctions -> libSystem_initializer -> _objc_init -> static_init -> getLibobjcInitializers.
- static_initin_dyld_objc_notify_registerBefore, so inloadMethods before.
ordinaryC++Constructors are also indoModInitFunctionsInside is called, but is inlibSystem_initializerAfter, so inloadMethod.

Call order summary

objcIn the sourceC++Constructor.
loadMethods.
ordinaryC++Constructor.
initializeFunction.

What is Runtime

runtimeIs made up ofC,C++,assemblyA set of implementationsAPIforOCThe language adds object-oriented, runtime functionality.
The runtime (Runtime) is when the determination of data types is deferred from compile time to run time, for examplecategory,rweAnd so on.
Peacetime writtenOCThe code, as the program runs, will actually be converted toRuntimetheCLanguage code,RuntimeisObject-CBehind the scenes.

Third, the essence of the method, what is SEL? What is IMP? What is the relationship between the two?

The essence of a method is to send a message, which has the following flow:
1. Quick find ~ (objc_msgSend) ~ cache_tCache messages.
2. Slow to find ~ recursively their own living parent ~lookUpImpOrForward.
3. Message not found ~ dynamic method resolution ~resolveInstanceMethod.
4. The message is quickly forwarded ~forwardingTargetForSelector.
5. The message is forwarded slowly ~methodSignatureForSelector & forwardInvocation.
selIs the method number, inread_imagesAnd then it’s compiled into memory.
impThat’s our function implementation pointer, findimpIt’s the process of finding the function.
selIt’s like a table of contents for a booktittle.
impIt’s the page number of a book.
To look for specific functions is to look for specific chapters in this book
1. We first know what we want to see ~tittle(sel).
2. According to the page number of the directory ~ (imp).
3. Turn to the details.

Can I add instance variables to the compiled class? Can I add instance variables to classes created at run time?

You cannot add instance variables to a compiled class.
- Our compiled instance variables are stored in theroOnce compiled, the memory structure is completely determined and cannot be modified.
- You can add methods and properties (associated objects) to a class by classification

It can be added as long as it is not registered in memory, as shown in the following code:

Void createClassAddIvar(void) {// Create a Class with objc_allocateClassPair const char * className = "MyClass"; Class SelfClass = objc_getClass(className); if (! SelfClass){ Class superClass = [NSObject class]; SelfClass = objc_allocateClassPair(superClass, className, 0); } // Add three member variables using class_addIvar class_addIvar(SelfClass, "name", sizeof(NSString *), log2(_Alignof(NSString *)), @encode(NSString *)); class_addIvar(SelfClass, "age", sizeof(NSString *), log2(_Alignof(NSString *)), @encode(NSString *)); class_addIvar(SelfClass, "sex", sizeof(NSString *), log2(_Alignof(NSString *)), @encode(NSString *)); objc_registerClassPair(SelfClass); } // Print the member of MyClass void logIvars(void) {const char * className = "MyClass"; Class SelfClass = objc_getClass(className); unsigned int outCount; Ivar * ivars = class_copyIvarList(SelfClass, &outCount); for (unsigned int i = 0; i < outCount; i ++) { Ivar ivar = ivars[i]; NSString * key = [NSString stringWithUTF8String:ivar_getName(ivar)]; NSLog (@ "-- -- -- -- -- -- -- -- the key: % @", key); }} / / print result -- -- -- -- -- -- -- -- the key: the name -- -- -- -- -- -- -- -- the key: the age -- -- -- -- -- -- -- -- the key: sexCopy the code

[self class] and [super class]

Create the SSLPerson and SSLStudent classes:

@interface SSLPerson : NSObject

@end
@implementation SSLPerson

@end


@interface SSLStudent : SSLPerson

@end
@implementation SSLStudent

- (instancetype)init
{
    if (self = [super init]) {
        NSLog(@"%@ - %@",[self class],[super class]);
    }
    return self;
}

@end
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SSLStudentInherited fromSSLPerson.
SSLStudentIn the implementationinitMethod, print in method[self class].[super class]And what do they print out?

Results analysis

SSLStudent: [self class] SSLStudent: [self class] SSLStudent: [self class] SSLStudent: [self class]

Clang-rewrite-objc sslStudent.m -o sslStudent.cpp

Objc_msgSend (self, Sel_registerName ("class")) // [super class] compiled C++ code objc_msgSendSuper({self,class_getSuperclass(objc_getClass("SSLStudent"))},sel_registerName("class"))Copy the code

[super class]Programming intoC++After isobjc_msgSendSuperFunction call, look at its structure:

objc_msgSendSuper(struct objc_super *super, SEL op, ... )

struct objc_super {
    __unsafe_unretained _Nonnull id receiver;
    __unsafe_unretained _Nonnull Class super_class;
};
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throughclass_getSuperclassFunction get toThe parent class, as a parameter.

The objc_msgSendSuper method is the old version of the call method, assembly and source check the actual function call:

What’s really called isobjc_msgSendSuper2Its structure is similar toobjc_msgSendSuperIt’s the same:
```
objc_msgSendSuper2(struct objc_super * _Nonnull super, SEL _Nonnull op, ...)
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```

See the implementation of objc_msgSendSuper2:

You can see the callobjc_msgSendSuper2Function, is passed inThe current class, obtained by assembly source codeThe parent classCall again,CacheLookupThe delta function, I’m going to sum hereObjc_msgSend processThe same.
If I call thetaobjc_msgSendSuperFunction, which is passed in directly to get goodThe parent classAnd then jump toL_objc_msgSendSuper2_bodyIs transferred toCacheLookupFunction.

The difference between self class and super class is that [super class] starts with the parent class, and ultimately depends on the implementation of class methods.

- (Class)class {
    return object_getClass(self);
}
Class object_getClass(id obj)
{
    if (obj) return obj->getIsa();
    else return Nil;
}
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selfIt’s the hidden parameter that we pass in which isSSLStudentInstance object of theisaNature isSSLStudentClass.

5. Memory offset interview questions

Memory offset interview question 1

Create the SSLPerson class:

@interface SSLPerson : NSObject
- (void)say1;
@end

@implementation SSLPerson
- (void)say1
{
    NSLog(@"%s",__func__);
}
@end
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Add the following code to viewDidLoad, and you can guess what it will print:

- (void)viewDidLoad {
    [super viewDidLoad];
    
    SSLPerson *person = [[SSLPerson alloc] init];
    [person say1];
    
    Class cls = [SSLPerson class];
    void *ssl = &cls;
    [(__bridge id)ssl say1];
}
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View the print result:

-[SSLPerson say1]
-[SSLPerson say1]
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Results analysis:

The essence of bothobjc_msgSendThe call,Objc_msgSend processWe have analyzed inCompile the source codeChinese will pass firstisa & ISA_MASKAccess to theclass, and then search the method.
[person say1]It can be called successfully, and we all understand that without too much explanation.
[(__bridge id)kc say1]It can also be called successfully becausesslStored in theclassThe first address of,Compile the source codeThrough theClass address & ISA_MASKOf course you can get itclassAnd the following process is the same.

Here is:

Memory offset interview question 2

Modify the SSLPerson class:

@interface SSLPerson : NSObject
@property (nonatomic, copy) NSString *ssl_name;
- (void)say1;
@end

@implementation SSLPerson
- (void)say1
{
    NSLog(@"%s - %@",__func__,self.ssl_name);
}
@end
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Add code to viewDidLoad:

- (void)viewDidLoad {
    [super viewDidLoad];
    
    SSLPerson *person = [[SSLPerson alloc] init];
    person.ssl_name = @"SSL";
    [person say1];
    
    Class cls = [SSLPerson class];
    void *ssl = &cls;
    [(__bridge id)ssl say1];
}
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View the print result:

-[SSLPerson say1] - SSL
-[SSLPerson say1] - <SSLPerson: 0x6000015e48d0>
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Results analysis:

Clang-rewrite-objc sslPerson. m -o sslPerson. CPP to get sslPerson. CPP:

OBJC_IVAR_$_SSLPerson$_ssl_name = __OFFSETOFIVAR__(struct SSLPerson, _ssl_name); Ssl_name _I_SSLPerson_ssl_name(SSLPerson * self, SEL _cmd) {return self + OBJC_IVAR_$_SSLPerson$_ssl_name;  }Copy the code

You can see_ssl_nameThe value of, is calculated first its offset, and then the memory translation to get the value.

LLDB debugging verification:

Here is:

Memory offset interview question 3

Look at the following code, stack structure print:

- (void)viewDidLoad { // (id self, SEL _cmd) [super viewDidLoad]; // struct {receiver, class} class CLS = [SSLPerson class]; void *ssl = &cls; SSLPerson *person = [SSLPerson alloc]; person.ssl_name = @"SSL"; Void *sp = (void *)&self; void *end = (void *)&person; long count = (sp - end) / 0x8; for (long i = 0; i < count; i++) { void *address = sp - 0x8 * i; if (i == 1) { NSLog(@"%p : %s",address, *(char **)address); } else { NSLog(@"%p : %@",address, *(void **)address); }}}Copy the code

View the print result:

0x7ffee876e208 : <ViewController: 0x7fc7b050a940>
0x7ffee876e200 : viewDidLoad
0x7ffee876e1f8 : ViewController
0x7ffee876e1f0 : <ViewController: 0x7fc7b050a940>
0x7ffee876e1e8 : SSLPerson
0x7ffee876e1e0 : <SSLPerson: 0x7ffee876e1e8>
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Results analysis:

<ViewController: 0x7fc7b050a940>for- (void)viewDidLoadtheid selfPressure stack.
viewDidLoadfor- (void)viewDidLoadtheSEL _cmdPressure stack.
ViewControllerOf the structureclassPressure stack.
<ViewController: 0x7fc7b050a940>Of the structurereceiverPressure stack.
SSLPersonforsslPressure stack.
<SSLPerson: 0x7ffee876e1e8>forpersonPressure stack.

What would weigh on the stack:

This methodParameters can be pushed,viewDidLoadthe(id self, SEL _cmd).
forobjc_superThis structure argument is equivalent to the code below creating onessl_objc_superA temporary variable, so it can also be pressed.
```
struct objc_super ssl_objc_super;
ssl_objc_super.super_class = class;
ssl_objc_super.receiver = receiver;
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```

Method Swizzling

The basic principle of

Take a look at the Method Swizzling code:

@interface SSLRuntimeTool : NSObject /** switch method @param CLS switch object @param oriSEL Original method number @param swizzledSEL switch method number */ + (void)ssl_methodSwizzlingWithClass:(Class)cls oriSEL:(SEL)oriSEL swizzledSEL:(SEL)swizzledSEL; @end @implementation SSLRuntimeTool + (void)ssl_methodSwizzlingWithClass:(Class)cls oriSEL:(SEL)oriSEL swizzledSEL:(SEL)swizzledSEL { if (! CLS) NSLog(@" The exchange class passed cannot be empty "); Method oriMethod = class_getInstanceMethod(cls, oriSEL); Method swiMethod = class_getInstanceMethod(cls, swizzledSEL); method_exchangeImplementations(oriMethod, swiMethod); } @endCopy the code

Method Swizzling uses:

@interface SSLPerson : NSObject
- (void)personInstanceMethod;
@end

@implementation SSLPerson
- (void)personInstanceMethod {
    NSLog(@"%s",__func__);
}
@end


@interface SSLStudent : SSLPerson
@end

@implementation SSLStudent
+ (void)load{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [SSLRuntimeTool ssl_methodSwizzlingWithClass:self oriSEL:@selector(personInstanceMethod) swizzledSEL:@selector(studentInstanceMethod)];
    });
}
- (void)studentInstanceMethod {
    [self studentInstanceMethod];
    NSLog(@"%s",__func__);
}
@end
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Prevent repeated exchange, useThe singletonMore secure.

Method Swizzling principle diagram:

A pit point

The code above makes the relevant call:

- (void)viewDidLoad {
    [super viewDidLoad];
    
    SSLPerson *person = [[SSLPerson alloc] init];
    [person personInstanceMethod];
}   
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Error:

'-[SSLPerson studentInstanceMethod]: unrecognized selector sent to instance 0x600002fbc400'
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Because of method swapping, we actually call this method:

- (void)studentInstanceMethod {
    [self studentInstanceMethod];
    NSLog(@"%s",__func__);
}
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At this momentselfisSSLPersonInstance,SSLPersonThere is nothing in thestudentInstanceMethodtheSELCan’t find the correspondingSELSo it willCRASH.

Solve a problem:

@implementation SSLRuntimeTool + (void)ssl_betterMethodSwizzlingWithClass:(Class)cls oriSEL:(SEL)oriSEL swizzledSEL:(SEL)swizzledSEL{ if (! CLS) NSLog(@" The exchange class passed cannot be empty "); Method oriMethod = class_getInstanceMethod(cls, oriSEL); Method swiMethod = class_getInstanceMethod(cls, swizzledSEL); // personInstanceMethod(sel) - studentInstanceMethod(imp) // Try adding the method you want to swap - studentInstanceMethod BOOL success = class_addMethod(cls, oriSEL, method_getImplementation(swiMethod), method_getTypeEncoding(oriMethod)); If (success) {studentInstanceMethod (SEL) -personInstancemethod (imp) class_replaceMethod(CLS, swizzledSEL, method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod)); } else {// implementations has - exchangemethod_exchangeImplementations (oriMethod, swiMethod); } } @endCopy the code

class_replaceMethodFunction:
- If theClassNo specification existsSEL,class_replaceMethodThe action of theta is thetaclass_addMethodThe same;
- If theClassExists the specifiedSEL,class_replaceMethodThe action of theta is thetamethod_setImplementationThe same.

Pit point 2

If personInstanceMethod is not implemented, run the program and an infinite loop will occur:

Solve a problem:

@implementation SSLRuntimeTool + (void)ssl_bestMethodSwizzlingWithClass:(Class)cls oriSEL:(SEL)oriSEL swizzledSEL:(SEL)swizzledSEL{ if (! CLS) NSLog(@" The exchange class passed cannot be empty "); Method oriMethod = class_getInstanceMethod(cls, oriSEL); Method swiMethod = class_getInstanceMethod(cls, swizzledSEL); if (! OriMethod) {// If oriMethod is nil, After replacement, copy swizzledSEL as an empty implementation that does nothing class_addMethod(CLS, oriSEL, method_getImplementation(swiMethod), method_getTypeEncoding(swiMethod)); Method_setImplementation (swiMethod, imp_implementationWithBlock(^(id self, SEL _cmd){NSLog(@" there's an empty IMP "); })); } // try adding the method you want to swap - studentInstanceMethod BOOL success = class_addMethod(CLS, oriSEL, method_getImplementation(swiMethod), method_getTypeEncoding(oriMethod)); Override a class_replaceMethod(CLS, swizzledSEL, method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod)); } else {// implementations has - exchangemethod_exchangeImplementations (oriMethod, swiMethod); } } @endCopy the code

IOS basic principle: OC Basic interview (1)

Load, C++ constructor, initialize call sequence

Load function call timing

Initialize function call timing

C++ constructor function call timing

Call order summary

What is Runtime

Third, the essence of the method, what is SEL? What is IMP? What is the relationship between the two?

Can I add instance variables to the compiled class? Can I add instance variables to classes created at run time?

[self class] and [super class]

Results analysis

5. Memory offset interview questions

Memory offset interview question 1

Memory offset interview question 2

Memory offset interview question 3

Method Swizzling

The basic principle of

A pit point

Pit point 2

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