iOS runtime （二）（runtime学习之AutoCoding源码分析）

　　在上一篇文章中，学习了runtime中的各个重要的知识点，接下来就是要开始运用了。主要是分析一些优秀开源库是如何运用runtime,提高工作效率的。

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 　　AutoCoding

　　AutoCoding 是一个NSObject的类别，它提供归档解档（即自动序列化和反序列化）对象。在介绍这个开源库之前，先简单过一下iOS中对象归档解档，这个并不是重点，只是为了抛出问题，所以不会详讲。在iOS中对象序归档解档会使用到NSKeyedArchiver和NSKeyedUnarchiver，接下来是示例代码:

创建一个需要归档解档的对象的类,它需要遵循NSSecureCoding协议并实现相应接口，决定它是如何归档解档对象的成员变量。

//MyDataInfo.h
#import <Foundation/Foundation.h>

@interface MyDataInfo : NSObject <NSSecureCoding>
@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end

//MyDataInfo.m
#import "MyDataInfo.h"
NSString *const kNameKey = @"name";
NSString *const kAgeKey = @"age";
@implementation MyDataInfo
- (void)encodeWithCoder:(NSCoder *)aCoder
{
    [aCoder encodeObject:self.name forKey:kNameKey];
    [aCoder encodeInteger:self.age forKey:kAgeKey];
}

- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder
{
    self.name = [aDecoder decodeObjectForKey:kNameKey];
    self.age = [aDecoder decodeIntegerForKey:kAgeKey];
    return self;
}

+ (BOOL)supportsSecureCoding
{
    return YES;
}
@end

接下来就是对这个类进行归档解档的过程

//ViewController.m
.......
- (void)viewDidLoad {
    [super viewDidLoad];
    MyDataInfo *dataInfo = [[MyDataInfo alloc] init];
    dataInfo.name = @"Davi";
    dataInfo.age = 100;
    
    NSData *archivedData = [NSKeyedArchiver archivedDataWithRootObject:dataInfo];
    
    MyDataInfo *unArchiveDataInfo = [NSKeyedUnarchiver unarchiveObjectWithData:archivedData];
    
    NSLog(@"unArchiveDataInfo name:%@, age:%ld", unArchiveDataInfo.name, (long)unArchiveDataInfo.age);
}
.......

运行代码，输出结果为，结果是毫无疑问的

2016-06-30 20:16:10.482 TestAutoCoding[59320:54257381] unArchiveDataInfo name:Davi, age:100

那么要抛出的问题什么呢？在日常开发当中，我们要保存的对象，它们可能会有组合的关系，也有继承的关系，也有可能有N多属性，并且也会属于很多不同的类，那么此时如果要归档解档这些对象，都需要像MyDataInfo一样遵循NSSecureCoding协议并实现相应接initWithCoder:,encodeWithCoder:，为每个自属性定义一个key,类似于NSString *const kNameKey = @"name"，然后在encodeWithCoder:接口内写encode代码，在initWithCoder:接口内写

decode代码。当需要归档解档的类有很多很多，这部分就是一个重复的苦力活，而且会容易出错。AutoCoding就是为解决这个问题出现的。github上对它的描述是这样的

AutoCoding is a category on NSObject that provides automatic support for NSCoding to any object. This means that rather than having to implement the initWithCoder: and encodeWithCoder: methods yourself, all the model classes in your app can be saved or loaded from a file without you needing to write any additional code.

 意思就是你不需要为要归档解档的model类写任何代码，就能实现上面MyDataInfo说的归档存档。那么它是怎么做到的？

首先，在NSObject (AutoCoding) 类别中实现以下两个接口：

- (instancetype)initWithCoder:(NSCoder *)aDecoder
{
    [self setWithCoder:aDecoder];    //后续讲解
    return self;
}

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    for (NSString *key in [self codableProperties])  //后续讲解
    {
        id object = [self valueForKey:key];
        if (object) [aCoder encodeObject:object forKey:key];
    }
}

这样完成了第一步，要归档解档的model类只要没有实现这两个方法，都会被调用到NSObject 类别以上这两个方法。接下先说归档，在encodeWithCoder:接口内可以看到[self codableProperties]，它的作用是拿到对象（它所属类的继承体系中除NSObject外）的所有属性名及类型。

- (NSDictionary *)codableProperties
{
    //添加一个字典关联到这个类上，目的是做缓存，只要做一次获取成员变量名字，和类型。其中名字为key,类型为对应值value
    __autoreleasing NSDictionary *codableProperties = objc_getAssociatedObject([self class], _cmd);
    if (!codableProperties)
    {
        codableProperties = [NSMutableDictionary dictionary];
        Class subclass = [self class];
        //从当前类开始向父类遍历，一直到NSObject类就停止
        while (subclass != [NSObject class])
        {
            //获取当前类所有的变量名-类型，添加到字典codableProperties里
            [(NSMutableDictionary *)codableProperties addEntriesFromDictionary:[subclass codableProperties]/*紧跟着讲*/];
            subclass = [subclass superclass];
        }
        codableProperties = [NSDictionary dictionaryWithDictionary:codableProperties];
        
        objc_setAssociatedObject([self class], _cmd, codableProperties, OBJC_ASSOCIATION_RETAIN);
    }
    return codableProperties;
}

接下来就是[subclass codableProperties]，它的作用获取类中所有的变量名、类型，以字典作为返回值。

+ (NSDictionary *)codableProperties
{
    //deprecated
    SEL deprecatedSelector = NSSelectorFromString(@"codableKeys");
    if ([self respondsToSelector:deprecatedSelector] || [self instancesRespondToSelector:deprecatedSelector])
    {
        NSLog(@"AutoCoding Warning: codableKeys method is no longer supported. Use codableProperties instead.");
    }
    deprecatedSelector = NSSelectorFromString(@"uncodableKeys");
    if ([self respondsToSelector:deprecatedSelector] || [self instancesRespondToSelector:deprecatedSelector])
    {
        NSLog(@"AutoCoding Warning: uncodableKeys method is no longer supported. Use ivars, or synthesize your properties using non-KVC-compliant names to avoid coding them instead.");
    }
    deprecatedSelector = NSSelectorFromString(@"uncodableProperties");
    NSArray *uncodableProperties = nil;
    if ([self respondsToSelector:deprecatedSelector] || [self instancesRespondToSelector:deprecatedSelector])
    {
        uncodableProperties = [self valueForKey:@"uncodableProperties"];
        NSLog(@"AutoCoding Warning: uncodableProperties method is no longer supported. Use ivars, or synthesize your properties using non-KVC-compliant names to avoid coding them instead.");
    }
    
    unsigned int propertyCount;
    __autoreleasing NSMutableDictionary *codableProperties = [NSMutableDictionary dictionary];
    objc_property_t *properties = class_copyPropertyList(self, &propertyCount);
    for (unsigned int i = 0; i < propertyCount; i++)
    {
        //获取属性名
        objc_property_t property = properties[i];
        const char *propertyName = property_getName(property);
        __autoreleasing NSString *key = @(propertyName);

        //检查是否可coding
        if (![uncodableProperties containsObject:key])
        {
            //拿到property的Type Encodings,这部分不了解可去看官方文档Type Encodings了解下规则及runtime中对应的相关接口，这里都用到，也可以看我上一篇文章
            Class propertyClass = nil;
            char *typeEncoding = property_copyAttributeValue(property, "T");
            switch (typeEncoding[0])
            {
　　　　　　　　　　//表示是个对象
                case '@':
                {
　　　　　　　　　　　　//根据Type Encodings规则得出来的算法而已，目的截取出属性类型
                    if (strlen(typeEncoding) >= 3)
                    {
                        char *className = strndup(typeEncoding + 2, strlen(typeEncoding) - 3);
                        __autoreleasing NSString *name = @(className);
                        NSRange range = [name rangeOfString:@"<"];
                        if (range.location != NSNotFound)
                        {
                            name = [name substringToIndex:range.location];
                        }
                        propertyClass = NSClassFromString(name) ?: [NSObject class];
                        free(className);
                    }
                    break;
                }
                case 'c':
                case 'i':
                case 's':
                case 'l':
                case 'q':
                case 'C':
                case 'I':
                case 'S':
                case 'L':
                case 'Q':
                case 'f':
                case 'd':
                case 'B':
                {
　　　　　　　　　　　　//都当成是NSNumber型
                    propertyClass = [NSNumber class];
                    break;
                }
                case '{':
                {
                    propertyClass = [NSValue class];
                    break;
                }
            }
            free(typeEncoding);
            
            if (propertyClass)
            {
                //获取变量名
                char *ivar = property_copyAttributeValue(property, "V");
                if (ivar)
                {
                    //检查属性是否有编译器帮忙生成的成员变量名带_线的
                    __autoreleasing NSString *ivarName = @(ivar);
                    if ([ivarName isEqualToString:key] || [ivarName isEqualToString:[@"_" stringByAppendingString:key]])
                    {
                        codableProperties[key] = propertyClass;
                    }
                    free(ivar);
                }
                else
                {
                    //检查属性是否是 dynamic 和 readwrite的
                    char *dynamic = property_copyAttributeValue(property, "D");
                    char *readonly = property_copyAttributeValue(property, "R");
                    if (dynamic && !readonly)
                    {
                        codableProperties[key] = propertyClass;
                    }
                    free(dynamic);
                    free(readonly);
                }
            }
        }
    }
    
    free(properties);
    return codableProperties;
}

再看回encodeWithCoder:(NSCoder *)aCoder

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    for (NSString *key in [self codableProperties])  
    {
        id object = [self valueForKey:key];
        if (object) [aCoder encodeObject:object forKey:key];
    }
}

不难知道，其实就是遍历所有属性名字，并以属性名字作为key,利用KVC先把属性值拿到，再属性名字作为key调用encodeObject。原理就是这样，解档也差不多就不再写了，关键是拿到对象的所有属性名称及类型，利用KVC获取和设置属性值，再以属性名称作为Key避免手动定义。

使用AutoCoding之后，MyDataInfo直接变成下面这样就可以了，结果与手动写是一样的，大大减小工作量，提高开发效率，又减少出错率。

//MyDataInfo.h
#import <Foundation/Foundation.h>
@interface MyDataInfo : NSObject 
@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end

//MyDataInfo.m
#import "MyDataInfo.h"
@implementation MyDataInfo
@end

这里运用到runtime相关知识主要有我上篇文章介绍过的成员变量与属性、Type Encodings。

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