基础

一、堆栈

1、堆栈的区别

堆和栈都是按序排列的数据结构，只能在一端对数据进行插入和删除

heap&stack.png

栈：
先进后出（FILO）；
由编译器自动分配和释放；
空间较小，运行速度较快；
栈空间多线程不共享；

堆：
先进先出（FIFO）；
一般需要手动分配和释放；
空间较大，运行速度较慢；
堆内存多线程共享；

iOS基础

一、Proprety

@proprety的本质是什么？

@property = ivar + getter + setter
属性有两大概念：实例变量（ivar）、存取方法（access method ＝ getter + setter）
属性定义后，编译器会自动添加实例变量并编写访问这些变量所需的方法，此过程叫做“自动合成”(autosynthesis)

默认的属性特性是什么？

声明属性时，如果是基础数据类型，默认取值是atomic，readwrite、assign；如果是对象类型，默认取值是atomic，readwrite，strong

atomic是线程安全的吗？

不是，atomic通过锁机制可以保证setter和getter的完整性，是读写安全的，但不是线程安全的。

线程安全：多个线程访问同一个对象时，如果不用考虑这些线程在运行时环境下的调度和交替执行，也不需要进行额外的同步，或者在调用方进行任何其他操作，调用这个对象的行为都可以获得正确的结果，那么这个对象就是线程安全的

锁：当多个线程同时操作同一块资源或者说同一个对象的时候，可能会造成各种意想不到的情况(比如数据错乱、资源争夺、崩溃等)，而锁就是为了能够保证同一时刻只有一个线程在操作这个数据应运而生的

assign修饰对象数据类型会怎样？

编译通过，实际使用刚初始化就被释放，warning：

Assigning retained object to unsafe property; object will be released after assignment

调用崩溃，EXC_BAD_ACCESS

weak修饰基本数据类型会怎样？

编译报错：

Property with 'weak' attribute must be of object type

什么场景使用weak？

1、ARC中避免循环引用，如delegate
2、已经强引用的对象，如IBOutlet

weak实现原理？

runtime 对注册的类，会进行布局，对于 weak 对象会放入一个 Hash 表中。用 weak 指向的对象内存地址作为 key，当此对象的引用计数为0的时候会 dealloc，假如 weak 指向的对象内存地址是a，那么就会以a为键，在这个 weak 表中搜索，找到所有以a为键的 weak 对象，从而设置为 nil

Hash表，是根据关键码值(Key value)而直接进行访问的数据结构。它通过把关键码值映射到表中一个位置来访问记录，以加快查找的速度

Hash table.png

什么场景使用copy？

1、NSString、NSArray、NSDictionary 等等经常使用copy保证其封装性
2、block 使用 copy 是从 MRC 遗留下来的，在 MRC 中,方法内部的 block 是在栈区的,使用 copy 可以把它放到堆区；在ARC中，编译器自动对 block 进行了 copy 操作

注意：NSString、NSArray、NSDictionary 的可变版本不能使用copy，否则有崩溃风险

二、KVC

KVC原理

Apple注释已经写的很清楚：

/* Given a value and a key that identifies an attribute, set the value of the attribute. Given an object and a key that identifies a to-one relationship, relate the object to the receiver, unrelating the previously related object if there was one. Given a collection object and a key that identifies a to-many relationship, relate the objects contained in the collection to the receiver, unrelating previously related objects if there were any.

The default implementation of this method does the following:
    1. Searches the class of the receiver for an accessor method whose name matches the pattern -set<Key>:. If such a method is found the type of its parameter is checked. If the parameter type is not an object pointer type but the value is nil -setNilValueForKey: is invoked. The default implementation of -setNilValueForKey: raises an NSInvalidArgumentException, but you can override it in your application. Otherwise, if the type of the method's parameter is an object pointer type the method is simply invoked with the value as the argument. If the type of the method's parameter is some other type the inverse of the NSNumber/NSValue conversion done by -valueForKey: is performed before the method is invoked.
    2. Otherwise (no accessor method is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key>, _is<Key>, <key>, or is<Key>, in that order. If such an instance variable is found and its type is an object pointer type the value is retained and the result is set in the instance variable, after the instance variable's old value is first released. If the instance variable's type is some other type its value is set after the same sort of conversion from NSNumber or NSValue as in step 1.
    3. Otherwise (no accessor method or instance variable is found), invokes -setValue:forUndefinedKey:. The default implementation of -setValue:forUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Compatibility notes:
    - For backward binary compatibility with -takeValue:forKey:'s behavior, a method whose name matches the pattern -_set<Key>: is also recognized in step 1. KVC accessor methods whose names start with underscores were deprecated as of Mac OS 10.3 though.
    - For backward binary compatibility, -unableToSetNilForKey: will be invoked instead of -setNilValueForKey: in step 1, if the implementation of -unableToSetNilForKey: in the receiver's class is not NSObject's.
    - The behavior described in step 2 is different from -takeValue:forKey:'s, in which the instance variable search order is <key>, _<key>.
    - For backward binary compatibility with -takeValue:forKey:'s behavior, -handleTakeValue:forUnboundKey: will be invoked instead of -setValue:forUndefinedKey: in step 3, if the implementation of -handleTakeValue:forUnboundKey: in the receiver's class is not NSObject's.
*/
- (void)setValue:(nullable id)value forKey:(NSString *)key;

1、先搜索对应的setter
2、再搜索成员变量
3、调用-setValue:forUndefinedKey:，抛出异常

/* Given a key that identifies an attribute or to-one relationship, return the attribute value or the related object. Given a key that identifies a to-many relationship, return an immutable array or an immutable set that contains all of the related objects.
    
The default implementation of this method does the following:
    1. Searches the class of the receiver for an accessor method whose name matches the pattern -get<Key>, -<key>, or -is<Key>, in that order. If such a method is found it is invoked. If the type of the method's result is an object pointer type the result is simply returned. If the type of the result is one of the scalar types supported by NSNumber conversion is done and an NSNumber is returned. Otherwise, conversion is done and an NSValue is returned (new in Mac OS 10.5: results of arbitrary type are converted to NSValues, not just NSPoint, NRange, NSRect, and NSSize).
    2 (introduced in Mac OS 10.7). Otherwise (no simple accessor method is found), searches the class of the receiver for methods whose names match the patterns -countOf<Key> and -indexIn<Key>OfObject: and -objectIn<Key>AtIndex: (corresponding to the primitive methods defined by the NSOrderedSet class) and also -<key>AtIndexes: (corresponding to -[NSOrderedSet objectsAtIndexes:]). If a count method and an indexOf method and at least one of the other two possible methods are found, a collection proxy object that responds to all NSOrderedSet methods is returned. Each NSOrderedSet message sent to the collection proxy object will result in some combination of -countOf<Key>, -indexIn<Key>OfObject:, -objectIn<Key>AtIndex:, and -<key>AtIndexes: messages being sent to the original receiver of -valueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -get<Key>:range: that method will be used when appropriate for best performance.
    3. Otherwise (no simple accessor method or set of ordered set access methods is found), searches the class of the receiver for methods whose names match the patterns -countOf<Key> and -objectIn<Key>AtIndex: (corresponding to the primitive methods defined by the NSArray class) and (introduced in Mac OS 10.4) also -<key>AtIndexes: (corresponding to -[NSArray objectsAtIndexes:]). If a count method and at least one of the other two possible methods are found, a collection proxy object that responds to all NSArray methods is returned. Each NSArray message sent to the collection proxy object will result in some combination of -countOf<Key>, -objectIn<Key>AtIndex:, and -<key>AtIndexes: messages being sent to the original receiver of -valueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -get<Key>:range: that method will be used when appropriate for best performance.
    4 (introduced in Mac OS 10.4). Otherwise (no simple accessor method or set of ordered set or array access methods is found), searches the class of the receiver for a threesome of methods whose names match the patterns -countOf<Key>, -enumeratorOf<Key>, and -memberOf<Key>: (corresponding to the primitive methods defined by the NSSet class). If all three such methods are found a collection proxy object that responds to all NSSet methods is returned. Each NSSet message sent to the collection proxy object will result in some combination of -countOf<Key>, -enumeratorOf<Key>, and -memberOf<Key>: messages being sent to the original receiver of -valueForKey:.
    5. Otherwise (no simple accessor method or set of collection access methods is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key>, _is<Key>, <key>, or is<Key>, in that order. If such an instance variable is found, the value of the instance variable in the receiver is returned, with the same sort of conversion to NSNumber or NSValue as in step 1.
    6. Otherwise (no simple accessor method, set of collection access methods, or instance variable is found), invokes -valueForUndefinedKey: and returns the result. The default implementation of -valueForUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Compatibility notes:
    - For backward binary compatibility, an accessor method whose name matches the pattern -_get<Key>, or -_<key> is searched for between steps 1 and 3. If such a method is found it is invoked, with the same sort of conversion to NSNumber or NSValue as in step 1. KVC accessor methods whose names start with underscores were deprecated as of Mac OS 10.3 though.
    - The behavior described in step 5 is a change from Mac OS 10.2, in which the instance variable search order was <key>, _<key>.
    - For backward binary compatibility, -handleQueryWithUnboundKey: will be invoked instead of -valueForUndefinedKey: in step 6, if the implementation of -handleQueryWithUnboundKey: in the receiver's class is not NSObject's.
*/
- (nullable id)valueForKey:(NSString *)key;

1、先搜索对应的getter
2、再检查是否响应NSOrderedSet的方法
3、再检查是否响应NSArray的方法
4、再检查是否响应NSSet的方法
5、再搜索成员变量
6、调用-valueForUndefinedKey:，抛出异常

二、Notification

1、通知出发的事件是同步还是异步？是否是主线程？
同步，在发出通知的线程执行

转载于:https://www.jianshu.com/p/49257ee47e0b

iOS知识总结

基础