YYModel源码学习

• 目录
  1、模型在软件开发中的作用
  2、诞生背景
  3、整体架构
  4、核心流程

模型在软件开发中的作用

后台下发的数据格式一般是JSON格式，我们可以方便的将其转换为字典，即便如此，在使用时候仍然存在一些问题，如key值直接写在代码中，不能一眼看清下发的数据结构，维护起来极其不方便，而且通过字典数组的方式获取数据一个不小心就会crash，最主要的是会花费很多时间在字段匹配上，这时候模型的作用就体现出来了

@interface Person : NSObject
// 接口文档的链接注释在这里
@property (nonatomic, copy) NSString *name;      // 名字
@property (nonatomic, assign) NSUInterger age;   // 年龄
...

@end

如上，接口下发的数据结构一目了然，直接访问属性方便、安全，而且可以直接定位到相关文档，也便于后续拓展，这就是抽象的力量

诞生背景

将后台下发的数据结构抽象成Model，给程序员带来极大的方便，但是随着业务的爆发，这种数据转模型的代码会急剧增加，而且各个业务接口的数据结构几乎都不相同，转换的代价会越来越大，并且难于维护，每次修改数据结构都要修改数据转模型部分的代码，这大大降低了敏捷分开的迭代的速度，这时候程序员迫切需要一种数据自动转模型的工具，于是YYModel诞生了

整体架构

一共有两个实现文件NSObject+YYModel和YYClassInfo：3个分类，6个类，如下

YYClassInfo是类对象的OC封装，内部聚合了Ivar ivar、Method method、objc_property_t property
如下：

/**
 Class information for a class.
 */
@interface YYClassInfo : NSObject
@property (nonatomic, assign, readonly) Class cls; ///< class object
@property (nullable, nonatomic, assign, readonly) Class superCls; ///< super class object
@property (nullable, nonatomic, assign, readonly) Class metaCls;  ///< class's meta class object
@property (nonatomic, readonly) BOOL isMeta; ///< whether this class is meta class
@property (nonatomic, strong, readonly) NSString *name; ///< class name
@property (nullable, nonatomic, strong, readonly) YYClassInfo *superClassInfo; ///< super class's class info
@property (nullable, nonatomic, strong, readonly) NSDictionary<NSString *, YYClassIvarInfo *> *ivarInfos; ///< ivars
@property (nullable, nonatomic, strong, readonly) NSDictionary<NSString *, YYClassMethodInfo *> *methodInfos; ///< methods
@property (nullable, nonatomic, strong, readonly) NSDictionary<NSString *, YYClassPropertyInfo *> *propertyInfos; ///< properties

公有方法有四个如下：类信息更新状态存取方法，和两个初始化器

- (void)setNeedUpdate;
- (BOOL)needUpdate;
+ (nullable instancetype)classInfoWithClass:(Class)cls;
+ (nullable instancetype)classInfoWithClassName:(NSString *)className;

NSObject+YYModel中定义了两个内部类_YYModelPropertyMeta、_YYModelMeta,分别表示模型中的属性和类信息，YYClassPropertyInfo 和YYClassInfo可以理解为单纯的数据结构，而_YYModelPropertyMeta、_YYModelMeta则是组合在一起的模型单元，分别扮演树枝节点和叶子节点角色

/// A property info in object model.
@interface _YYModelPropertyMeta : NSObject {
    @package
    NSString *_name;             ///< property's name
    YYEncodingType _type;        ///< property's type
    YYEncodingNSType _nsType;    ///< property's Foundation type
    BOOL _isCNumber;             ///< is c number type
    Class _cls;                  ///< property's class, or nil
    Class _genericCls;           ///< container's generic class, or nil if threr's no generic class
    SEL _getter;                 ///< getter, or nil if the instances cannot respond
    SEL _setter;                 ///< setter, or nil if the instances cannot respond
    BOOL _isKVCCompatible;       ///< YES if it can access with key-value coding
    BOOL _isStructAvailableForKeyedArchiver; ///< YES if the struct can encoded with keyed archiver/unarchiver
    BOOL _hasCustomClassFromDictionary; ///< class/generic class implements +modelCustomClassForDictionary:
    
    /*
     property->key:       _mappedToKey:key     _mappedToKeyPath:nil            _mappedToKeyArray:nil
     property->keyPath:   _mappedToKey:keyPath _mappedToKeyPath:keyPath(array) _mappedToKeyArray:nil
     property->keys:      _mappedToKey:keys[0] _mappedToKeyPath:nil/keyPath    _mappedToKeyArray:keys(array)
     */
    NSString *_mappedToKey;      ///< the key mapped to
    NSArray *_mappedToKeyPath;   ///< the key path mapped to (nil if the name is not key path)
    NSArray *_mappedToKeyArray;  ///< the key(NSString) or keyPath(NSArray) array (nil if not mapped to multiple keys)
    YYClassPropertyInfo *_info;  ///< property's info
    _YYModelPropertyMeta *_next; ///< next meta if there are multiple properties mapped to the same key.
}
@end

/// A class info in object model.
@interface _YYModelMeta : NSObject {
    @package
    YYClassInfo *_classInfo;
    /// Key:mapped key and key path, Value:_YYModelPropertyMeta.
    NSDictionary *_mapper;
    /// Array<_YYModelPropertyMeta>, all property meta of this model.
    NSArray *_allPropertyMetas;
    /// Array<_YYModelPropertyMeta>, property meta which is mapped to a key path.
    NSArray *_keyPathPropertyMetas;
    /// Array<_YYModelPropertyMeta>, property meta which is mapped to multi keys.
    NSArray *_multiKeysPropertyMetas;
    /// The number of mapped key (and key path), same to _mapper.count.
    NSUInteger _keyMappedCount;
    /// Model class type.
    YYEncodingNSType _nsType;
    
    BOOL _hasCustomWillTransformFromDictionary;
    BOOL _hasCustomTransformFromDictionary;
    BOOL _hasCustomTransformToDictionary;
    BOOL _hasCustomClassFromDictionary;
}
@end

头文件定义了一系列的公有方法，包含初始化器和工具函数，这里不再详述

核心流程

先调用_yy_dictionaryWithJSON把json转成字典，然后调用modelWithDictionary生成model

+ (instancetype)modelWithJSON:(id)json {
    NSDictionary *dic = [self _yy_dictionaryWithJSON:json];
    return [self modelWithDictionary:dic];
}

下面这个函数用来构造类信息和参数校验

+ (instancetype)modelWithDictionary:(NSDictionary *)dictionary {
    if (!dictionary || dictionary == (id)kCFNull) return nil;
    if (![dictionary isKindOfClass:[NSDictionary class]]) return nil;
    
    Class cls = [self class];
    _YYModelMeta *modelMeta = [_YYModelMeta metaWithClass:cls];
    if (modelMeta->_hasCustomClassFromDictionary) {
        cls = [cls modelCustomClassForDictionary:dictionary] ?: cls;
    }
    
    NSObject *one = [cls new];
    if ([one modelSetWithDictionary:dictionary]) return one;
    return nil;
}

初始化模型上下文，遍历所有属性，进行赋值

- (BOOL)modelSetWithDictionary:(NSDictionary *)dic {
    if (!dic || dic == (id)kCFNull) return NO;
    if (![dic isKindOfClass:[NSDictionary class]]) return NO;
    
    // 构造_YYModelMeta 模型
    _YYModelMeta *modelMeta = [_YYModelMeta metaWithClass:object_getClass(self)];
    if (modelMeta->_keyMappedCount == 0) return NO;
    
    if (modelMeta->_hasCustomWillTransformFromDictionary) {
        dic = [((id<YYModel>)self) modelCustomWillTransformFromDictionary:dic];
        if (![dic isKindOfClass:[NSDictionary class]]) return NO;
    }
    
    ModelSetContext context = {0};
    context.modelMeta = (__bridge void *)(modelMeta);
    context.model = (__bridge void *)(self);
    context.dictionary = (__bridge void *)(dic);
    
    if (modelMeta->_keyMappedCount >= CFDictionaryGetCount((CFDictionaryRef)dic)) {
        // 字典中的每个键值对调用 ModelSetWithDictionaryFunction
        // ModelSetWithDictionaryFunction 通过字典设置模型
        CFDictionaryApplyFunction((CFDictionaryRef)dic, ModelSetWithDictionaryFunction, &context);
        if (modelMeta->_keyPathPropertyMetas) {
            // 每个映射 keyPath 的属性元执行 ModelSetWithPropertyMetaArrayFunction
            CFArrayApplyFunction((CFArrayRef)modelMeta->_keyPathPropertyMetas,
                                 CFRangeMake(0, CFArrayGetCount((CFArrayRef)modelMeta->_keyPathPropertyMetas)),
                                 ModelSetWithPropertyMetaArrayFunction,
                                 &context);
        }
        if (modelMeta->_multiKeysPropertyMetas) {
            // 每个映射多个 key 的属性元执行 ModelSetWithPropertyMetaArrayFunction
            CFArrayApplyFunction((CFArrayRef)modelMeta->_multiKeysPropertyMetas,
                                 CFRangeMake(0, CFArrayGetCount((CFArrayRef)modelMeta->_multiKeysPropertyMetas)),
                                 ModelSetWithPropertyMetaArrayFunction,
                                 &context);
        }
    } else { 
        // 每个 modelMeta 属性元执行 ModelSetWithPropertyMetaArrayFunction
        CFArrayApplyFunction((CFArrayRef)modelMeta->_allPropertyMetas,
                             CFRangeMake(0, modelMeta->_keyMappedCount),
                             ModelSetWithPropertyMetaArrayFunction,
                             &context);
    }
    
    if (modelMeta->_hasCustomTransformFromDictionary) {
        return [((id<YYModel>)self) modelCustomTransformFromDictionary:dic];
    }
    
    return YES;
}

最终调用到ModelSetValueForProperty函数，实现单个属性的赋值，通过objc_msgSend函数直接调用属性的setter方法进行赋值