IOS interview preparation - Objective-C

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Swift And oc difference

Swift and Objective-C Share a set of runtime environment ,Swift The type can be bridged to Objective-C（ I'll call it OC）, vice versa .

Swift advantage
Swift Easy to read , Simplification of grammar and file structure .
Swift Easier to maintain , The structure of the document is clearer after separation .
Swift More secure , It's a type safe language .
Swift Less code , Simple grammar , Can save a lot of redundant code
Swift Faster , Higher performance .

Swift shortcoming
No one used oc many （ It's not obvious now ）
Because the cost of language conversion Many companies continue to use oc

KVC

KVC By NSKeyValueCoding A mechanism for enabling informal agreements , Object uses this protocol to provide indirect access to its properties .
ios13 prohibit kvc Access private properties .

kvc Set the value
Setting a value is to call a function ：setValue:forKey, The internal process is as follows ：
First, the corresponding setter Method ,setkey、_setKey、setIsKey, without ,
Check accessInstanceVariablesDirectly Method returns YES, If NO Go directly to setValue:forUndefinedKey: Method . If YES,
Search member variables in turn _key,_isKey,key,isKey . If found, assign him a value , Otherwise, call setValue:forUndefinedKey:
setValue:forUndefinedKey: Exception thrown by default

If not crash You can get one NSObject The classification of , rewrite setValue:forUndefinedKey: Make an empty implementation .

kvc Value
Value is called valueForKey: , The internal process is as follows ：

● First getter Method ,getKey、key、isKey、_key If it is found, it will directly call . If it is BOOL perhaps Int Equivalent type , It will be packaged as a NSNumber object .

● Check class method + (BOOL)accessInstanceVariablesDirectly, If you return YES( Default behavior ), So it's the same as the previous setting , Will press _key,_isKey,key,isKey Search the member variable name in the order of .
● If you haven't found it yet , Call the object directly valueForUndefinedKey: Method , This method throws an exception by default .

KVO

iOS In what way to implement the KVO？（KVO What is the essence of ？）
answer . When an object uses KVO monitor ,iOS The system will modify this object isa The pointer , To point to a new passage Runtime Subclass created dynamically , Subclasses have their own set Method realization ,set Method implementation will call in sequence willChangeValueForKey Method 、 The original setter Method realization 、didChangeValueForKey Method , and didChangeValueForKey The listener will be called inside the method observeValueForKeyPath:ofObject:change:context: Monitoring methods .

How to trigger manually KVO?
answer . When the value of the monitored property is modified , Will trigger automatically KVO. If you want to trigger it manually KVO, We need to call willChangeValueForKey and didChangeValueForKey Method can be triggered manually without changing the attribute value KVO, And these two methods are indispensable , The trigger code is still between them .

other

iOS： Understand OC Classes and metaclasses in :
https://www.jianshu.com/p/51b4019a8df2

il、Nil、NULL、NSNull The difference between
nil： A null pointer to an object , Yes objective c id Object null value .
Nil： Null pointer to a class , Indicates that the class is null .
NULL： Point to other types （ Such as ： Basic types 、C type ） The null pointer of , Used to assign null values to non object pointers .
NSNull： In a collection object , Objects representing null values .

try in return
stay try Inside return,finally Will be in return After performing , So in this case finally There will be problems in processing the return value , So don't put return discharge try Inside .

Self increasing problem
There are two ways ,count The value of will not change . principle ：count This value 0 Put it in storage , then count Self adding 1 become 1, Finally, assign the temporary value to count,count The final value is 0

int count = 0;
count = (count++);
count = count++;

The following value will be increased by one

count = ++count;