前言
本文的demo更新在github上。
客户端请求服务器，经常使用的时JSON方式传递数据。一些第三方开源库帮助我们将JSON转化为Model对象，其中比较有名的有：YYModel,JSONModel,Mantle,MJExtension等。今天主要讲一下JSONModel和相应的源代码。 (以下代码都是建立在release 1.20版本的基础上。）
常规解析
解析JSON数据的最基础的方法是使用NSJSONSerialization，比如下面的一个最简单的网络请求
NSData* ghData = [NSData dataWithContentsOfURL: [NSURL URLWithString:@"http://xxxx"]];
NSDictionary* json = nil;
if (ghData) {
json = [NSJSONSerialization
JSONObjectWithData:ghData
options:kNilOptions
error:nil];
}
最后通过NSJSONSerialization去将数据解析成了一个dictionary
如果有这样一组json数据：
{
"number":"13612345678",
"name":"Germany",
"age": 49
}
那我们会去建立相应的Object对象
@interface TestObject : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *number;
@property (nonatomic, assign) NSInteger age;
@end
然后进行设置
TestObject *testObject = [[TestObject alloc]init];
testObject.name = json[@"name"];
testObject.number = json[@"number"];
testObject.age = [json[@"age"] integerValue];
这么做虽然正确，但如果所有数据都这么处理，会有一些麻烦：
- 1.你需要很小心的处理model property类型与dictionary中的数据对应类型
比如有一个NSURL *url的值，你需要在json[@"url"]这个NSString *类型进行一次转化成NSURL *，但编译器并不会提示你这样的错误，很多时候你如果忘记了就会犯错 - 2.如果你的赋值地点过于的多，你每一次修改model的property，就需要把所有赋值地方进行一次整体的更改，会比较麻烦
- 3.很多时候json数据如果有遗漏或者变化，比较难发现
比如对应上面的age这个值，json数据中如果不包含age，通过[json[@"age"] integerValue]的写法，就会把值设置为0，这在很多时候容易被忽略，以为json数据中包含这样的值。
JSONModel解析
我们只需要建立这样一个JSONModel对象
#import <JSONModel/JSONModel.h>
@interface TestJSONModel : JSONModel
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *number;
@property (nonatomic, assign) NSInteger age;
@end
并调用
JSONModelError *error = nil;
TestJSONModel *testJSONModel = [[TestJSONModel alloc]initWithDictionary:json error:&error];
就可以将model的值进行自行设置，相对于常规方法，大大简化了代码量和难度。
JSONModel源代码分析
目录结构
我们先来看一下JSONModel的目录结构
JSONModel目录
可以看到，项目中其实还包括networking,transformer等有关的类，但我们这次解析主要聚焦在JSONModel.m上，也不是逐行解析，主要讲正题的思路和方法。
核心代码
初始化代码可以说是核心代码，代码如下：
-(id)initWithDictionary:(NSDictionary*)dict error:(NSError**)err
{
//check for nil input
//1.为空判断
if (!dict) {
if (err) *err = [JSONModelError errorInputIsNil];
return nil;
}
//invalid input, just create empty instance
//2.类型判断
if (![dict isKindOfClass:[NSDictionary class]]) {
if (err) *err = [JSONModelError errorInvalidDataWithMessage:@"Attempt to initialize JSONModel object using initWithDictionary:error: but the dictionary parameter was not an 'NSDictionary'."];
return nil;
}
//create a class instance
//3.核心，初始化映射property
self = [self init];
if (!self) {
//super init didn't succeed
if (err) *err = [JSONModelError errorModelIsInvalid];
return nil;
}
//check incoming data structure
//4.检查映射结构是否能够从dictionary中找到相应的数据
if (![self __doesDictionary:dict matchModelWithKeyMapper:self.__keyMapper error:err]) {
return nil;
}
//import the data from a dictionary
//5.进行数据赋值
if (![self __importDictionary:dict withKeyMapper:self.__keyMapper validation:YES error:err]) {
return nil;
}
//run any custom model validation
//6.本地数据检查
if (![self validate:err]) {
return nil;
}
//model is valid! yay!
return self;
}
主要分为以下6块：
- 1.空值判断
- 2.输入类型dictionary判断
- 3.初始化：解析model对象，并且映射property
- 4.查值：检查model property名与数据来源json字典中数据名，判断是否所有property都有值
- 5.赋值：进行赋值
- 6.本地数据正确性检查
以下我将主要解析3，4，5这三部分的主代码
初始化
以下是初始化的调用函数
-(void)__setup__
{
//if first instance of this model, generate the property list
//使用AssociateObject进行映射property的缓存，判断是否映射过
if (!objc_getAssociatedObject(self.class, &kClassPropertiesKey)) {
[self __inspectProperties];
}
//if there's a custom key mapper, store it in the associated object
//获取对象的keyMapper影射，同样使用AssociateObject进行映射property的缓存
id mapper = [[self class] keyMapper];
if ( mapper && !objc_getAssociatedObject(self.class, &kMapperObjectKey) ) {
objc_setAssociatedObject(
self.class,
&kMapperObjectKey,
mapper,
OBJC_ASSOCIATION_RETAIN // This is atomic
);
}
}
-(id)init
{
self = [super init];
if (self) {
//do initial class setup
[self __setup__];
}
return self;
}
这段代码使用AssociateObject的缓存判断kClassPropertiesKey就知道该model对象是否有进行过解析property，没有的话进行解析，同时取出model的key mapper，也同样进行缓存。
key mapper主要是用来针对某些json字段名和model数据名不一致的情况。
比如"com.app.test.name":"xxx","test_name":"xxx"这样的情况，可能对应的model数据字段名为name，那如何讲着两个值进行映射，就通过key mapper来完成。
主体的解析代码如下：
//inspects the class, get's a list of the class properties
//解析property结构主体
-(void)__inspectProperties
{
//JMLog(@"Inspect class: %@", [self class]);
NSMutableDictionary* propertyIndex = [NSMutableDictionary dictionary];
//temp variables for the loops
Class class = [self class];
NSScanner* scanner = nil;
NSString* propertyType = nil;
// inspect inherited properties up to the JSONModel class
while (class != [JSONModel class]) {
//JMLog(@"inspecting: %@", NSStringFromClass(class));
unsigned int propertyCount;
//赋值所有property列表，进行循环判断
objc_property_t *properties = class_copyPropertyList(class, &propertyCount);
//loop over the class properties
for (unsigned int i = 0; i < propertyCount; i++) {
//JSONModelClassProperty包涵解析与赋值时候的所有判断
JSONModelClassProperty* p = [[JSONModelClassProperty alloc] init];
//get property name
objc_property_t property = properties[i];
const char *propertyName = property_getName(property);
p.name = @(propertyName);
//JMLog(@"property: %@", p.name);
//get property attributes
//核心，通过property_getAttributes获取property的encode string，解析encode string可以解析出具体property的类型
const char *attrs = property_getAttributes(property);
NSString* propertyAttributes = @(attrs);
NSArray* attributeItems = [propertyAttributes componentsSeparatedByString:@","];
//ignore read-only properties
if ([attributeItems containsObject:@"R"]) {
continue; //to next property
}
//check for 64b BOOLs
if ([propertyAttributes hasPrefix:@"Tc,"]) {
//mask BOOLs as structs so they can have custom converters
p.structName = @"BOOL";
}
scanner = [NSScanner scannerWithString: propertyAttributes];
//JMLog(@"attr: %@", [NSString stringWithCString:attrs encoding:NSUTF8StringEncoding]);
[scanner scanUpToString:@"T" intoString: nil];
[scanner scanString:@"T" intoString:nil];
//check if the property is an instance of a class
//解析一个类，包括自己创建的类和oc自带类NSString等
if ([scanner scanString:@"@\"" intoString: &propertyType]) {
[scanner scanUpToCharactersFromSet:[NSCharacterSet characterSetWithCharactersInString:@"\"<"]
intoString:&propertyType];
//JMLog(@"type: %@", propertyClassName);
p.type = NSClassFromString(propertyType);
p.isMutable = ([propertyType rangeOfString:@"Mutable"].location != NSNotFound);
p.isStandardJSONType = [allowedJSONTypes containsObject:p.type];
//read through the property protocols
//解析protocol的string
while ([scanner scanString:@"<" intoString:NULL]) {
NSString* protocolName = nil;
[scanner scanUpToString:@">" intoString: &protocolName];
if ([protocolName isEqualToString:@"Optional"]) {
p.isOptional = YES;
} else if([protocolName isEqualToString:@"Index"]) {
p.isIndex = YES;
objc_setAssociatedObject(
self.class,
&kIndexPropertyNameKey,
p.name,
OBJC_ASSOCIATION_RETAIN // This is atomic
);
} else if([protocolName isEqualToString:@"ConvertOnDemand"]) {
p.convertsOnDemand = YES;
} else if([protocolName isEqualToString:@"Ignore"]) {
p = nil;
} else {
p.protocol = protocolName;
}
[scanner scanString:@">" intoString:NULL];
}
}
//check if the property is a structure
//解析structure
else if ([scanner scanString:@"{" intoString: &propertyType]) {
[scanner scanCharactersFromSet:[NSCharacterSet alphanumericCharacterSet]
intoString:&propertyType];
p.isStandardJSONType = NO;
p.structName = propertyType;
}
//the property must be a primitive
//其他类型都是基本类型，比如int float等
else {
//the property contains a primitive data type
[scanner scanUpToCharactersFromSet:[NSCharacterSet characterSetWithCharactersInString:@","]
intoString:&propertyType];
//get the full name of the primitive type
propertyType = valueTransformer.primitivesNames[propertyType];
if (![allowedPrimitiveTypes containsObject:propertyType]) {
//type not allowed - programmer mistaken -> exception
@throw [NSException exceptionWithName:@"JSONModelProperty type not allowed"
reason:[NSString stringWithFormat:@"Property type of %@.%@ is not supported by JSONModel.", self.class, p.name]
userInfo:nil];
}
}
NSString *nsPropertyName = @(propertyName);
//本地覆盖方法去判断是不是Optional
if([[self class] propertyIsOptional:nsPropertyName]){
p.isOptional = YES;
}
if([[self class] propertyIsIgnored:nsPropertyName]){
p = nil;
}
//本地覆盖方法去判断是不是有protocol
NSString* customProtocol = [[self class] protocolForArrayProperty:nsPropertyName];
if (customProtocol) {
p.protocol = customProtocol;
}
//few cases where JSONModel will ignore properties automatically
if ([propertyType isEqualToString:@"Block"]) {
p = nil;
}
//add the property object to the temp index
//通过kvc去设置相应的值
if (p && ![propertyIndex objectForKey:p.name]) {
[propertyIndex setValue:p forKey:p.name];
}
}
free(properties);
//ascend to the super of the class
//(will do that until it reaches the root class - JSONModel)
class = [class superclass];
}
//finally store the property index in the static property index
//使用AssociateObject进行缓存
objc_setAssociatedObject(
self.class,
&kClassPropertiesKey,
[propertyIndex copy],
OBJC_ASSOCIATION_RETAIN // This is atomic
);
}
看上去比较长，其实我们只需要明白以下几个概念就可以比较容易理解：
- 1.runtime
The Objective-C language defers as many decisions as it can from compile time and link time to runtime. Whenever possible, it does things dynamically. This means that the language requires not just a compiler, but also a runtime system to execute the compiled code. The runtime system acts as a kind of operating system for the Objective-C language; it’s what makes the language work.
- 2.
objc_property_t *properties = class_copyPropertyList(Class cls, unsigned int *count);
You can use the functions class_copyPropertyList and protocol_copyPropertyList to retrieve an array of the properties associated with a class (including loaded categories) and a protocol respectively
- 3.
const char *property_getAttributes(objc_property_t property)
You can use the property_getAttributes function to discover the name and the @encode type string of a property.
以上几个概念都可以从Objective-C Runtime Programming Guide上找到更加具体的解释，尤其是对应encode string每一个字符的含义。
**简单来说就是：
使用runtime的class_copyPropertyList方法去获得所有model对象的property列表，再使用
property_getAttributes获得property的encode string，通过解析encode string去获得property对象的正确含义。
在解析的过程中，使用NSScanner去扫描encode string,并使用JSONModelClassProperty的结构体去保存相关信息。
其中对于protocol的使用较为特殊，在这里的protocol并非我们平常当作接口抽象的作用，而是单纯的为了让encode string中增加相应的字段，可以在解析与赋值的时候给予特定的含义。
**
举个解析的例子：
这个是JSONModel自带demo中的一个结构体，可以看到他的相关property
@protocol LoanModel @end
@interface LoanModel : JSONModel
@property (strong, nonatomic) NSString* name;
@property (strong, nonatomic) NSString* status;
@property (strong, nonatomic) NSString* use;
@property (strong, nonatomic) LocationModel* location;
@end
当解析到最后一行的property@property (strong, nonatomic) LocationModel* location;我设置了一个断点，查看结果
propertyAttributes
JSONModelClassProperty
可以看到，对于location来说，它的类为"与"中的LocationModel，并且它还是&(retain),N(nonatomic)的。
而protocol则会在encode string的<>中，JSONModel通过这样的方式，可以让我们快速设置一个property的一些属性，比如
@interface KivaFeed : JSONModel
@property (strong, nonatomic) NSArray<LoanModel, ConvertOnDemand>* loans;
@end
中的loans，它不仅代表着loans这个array中包含的元素为LoanModel,而且它还有JSONModel特别设置的几个特性ConvertOnDemand（懒加载），这些特性包括
- 可选择：isOptional
- 懒加载：convertsOnDemand
- 索引key：isIndex
通过protocol就可以达到标明array与dictionary中对应元素的类型，和一些对于property解析的时候有用的表示。
查值
-(BOOL)__doesDictionary:(NSDictionary*)dict matchModelWithKeyMapper:(JSONKeyMapper*)keyMapper error:(NSError**)err
{
//check if all required properties are present
//将输入dictionary的keys装入set，将映射的property的keys装入set
NSArray* incomingKeysArray = [dict allKeys];
NSMutableSet* requiredProperties = [self __requiredPropertyNames].mutableCopy;
NSSet* incomingKeys = [NSSet setWithArray: incomingKeysArray];
//transform the key names, if necessary
//如果存在keyMapper映射，在对应set中找到相应key进行替换
if (keyMapper || globalKeyMapper) {
NSMutableSet* transformedIncomingKeys = [NSMutableSet setWithCapacity: requiredProperties.count];
NSString* transformedName = nil;
//loop over the required properties list
for (JSONModelClassProperty* property in [self __properties__]) {
transformedName = (keyMapper||globalKeyMapper) ? [self __mapString:property.name withKeyMapper:keyMapper importing:YES] : property.name;
//check if exists and if so, add to incoming keys
id value;
@try {
value = [dict valueForKeyPath:transformedName];
}
@catch (NSException *exception) {
value = dict[transformedName];
}
if (value) {
[transformedIncomingKeys addObject: property.name];
}
}
//overwrite the raw incoming list with the mapped key names
incomingKeys = transformedIncomingKeys;
}
//check for missing input keys
//判断property解析的set是不是dictionary所有key的子set来判断是否全部包含
if (![requiredProperties isSubsetOfSet:incomingKeys]) {
//get a list of the missing properties
[requiredProperties minusSet:incomingKeys];
//not all required properties are in - invalid input
JMLog(@"Incoming data was invalid [%@ initWithDictionary:]. Keys missing: %@", self.class, requiredProperties);
if (err) *err = [JSONModelError errorInvalidDataWithMissingKeys:requiredProperties];
return NO;
}
//not needed anymore
incomingKeys= nil;
requiredProperties= nil;
return YES;
}
查值的作用主要就是为了能够检查是否model的所有property是否都能够被赋值，如果不能则说明缺少值则抛出错误。这边主要的亮点就是使用了NSSet，将dictionary的所有key存入一个set：incomingKeys，并且将key mapper映射名进行替换。将刚解析出来的model所有property的name也存入一个set：requiredProperties，判断两者是不是包含关系。
赋值
-(BOOL)__importDictionary:(NSDictionary*)dict withKeyMapper:(JSONKeyMapper*)keyMapper validation:(BOOL)validation error:(NSError**)err
{
//loop over the incoming keys and set self's properties
//循环遍历映射出来的JSONModelClassProperty结构体
for (JSONModelClassProperty* property in [self __properties__]) {
//convert key name ot model keys, if a mapper is provided
//keyMapper映射，获取镇真正的值
NSString* jsonKeyPath = (keyMapper||globalKeyMapper) ? [self __mapString:property.name withKeyMapper:keyMapper importing:YES] : property.name;
//JMLog(@"keyPath: %@", jsonKeyPath);
//general check for data type compliance
id jsonValue;
@try {
jsonValue = [dict valueForKeyPath: jsonKeyPath];
}
@catch (NSException *exception) {
jsonValue = dict[jsonKeyPath];
}
//check for Optional properties
if (isNull(jsonValue)) {
//skip this property, continue with next property
if (property.isOptional || !validation) continue;
if (err) {
//null value for required property
NSString* msg = [NSString stringWithFormat:@"Value of required model key %@ is null", property.name];
JSONModelError* dataErr = [JSONModelError errorInvalidDataWithMessage:msg];
*err = [dataErr errorByPrependingKeyPathComponent:property.name];
}
return NO;
}
Class jsonValueClass = [jsonValue class];
BOOL isValueOfAllowedType = NO;
//判断数据输入类型是不是允许的json类型
for (Class allowedType in allowedJSONTypes) {
if ( [jsonValueClass isSubclassOfClass: allowedType] ) {
isValueOfAllowedType = YES;
break;
}
}
if (isValueOfAllowedType==NO) {
//type not allowed
JMLog(@"Type %@ is not allowed in JSON.", NSStringFromClass(jsonValueClass));
if (err) {
NSString* msg = [NSString stringWithFormat:@"Type %@ is not allowed in JSON.", NSStringFromClass(jsonValueClass)];
JSONModelError* dataErr = [JSONModelError errorInvalidDataWithMessage:msg];
*err = [dataErr errorByPrependingKeyPathComponent:property.name];
}
return NO;
}
//check if there's matching property in the model
if (property) {
// check for custom setter, than the model doesn't need to do any guessing
// how to read the property's value from JSON
// 使用对象相应的setter方法进行set
if ([self __customSetValue:jsonValue forProperty:property]) {
//skip to next JSON key
continue;
};
// 0) handle primitives
// 代表基础类型，比如int float等，直接使用kvc赋值
if (property.type == nil && property.structName==nil) {
//generic setter
if (jsonValue != [self valueForKey:property.name]) {
[self setValue:jsonValue forKey: property.name];
}
//skip directly to the next key
continue;
}
// 0.5) handle nils
if (isNull(jsonValue)) {
if ([self valueForKey:property.name] != nil) {
[self setValue:nil forKey: property.name];
}
continue;
}
// 1) check if property is itself a JSONModel
// 判断子结构是否是一个JSONModel结构，进行递归遍历，先将子结构遍历完并赋值完成
if ([self __isJSONModelSubClass:property.type]) {
//initialize the property's model, store it
JSONModelError* initErr = nil;
id value = [[property.type alloc] initWithDictionary: jsonValue error:&initErr];
if (!value) {
//skip this property, continue with next property
if (property.isOptional || !validation) continue;
// Propagate the error, including the property name as the key-path component
if((err != nil) && (initErr != nil))
{
*err = [initErr errorByPrependingKeyPathComponent:property.name];
}
return NO;
}
if (![value isEqual:[self valueForKey:property.name]]) {
[self setValue:value forKey: property.name];
}
//for clarity, does the same without continue
continue;
} else {
// 2) check if there's a protocol to the property
// ) might or not be the case there's a built in transform for it
// 是否包含protocol的字段，该字段主要用来表明array或者dictionary中的对象类型
if (property.protocol) {
//JMLog(@"proto: %@", p.protocol);
//循环遍历子内容，将对应的类型赋给相应的array或者dictionary
jsonValue = [self __transform:jsonValue forProperty:property error:err];
if (!jsonValue) {
if ((err != nil) && (*err == nil)) {
NSString* msg = [NSString stringWithFormat:@"Failed to transform value, but no error was set during transformation. (%@)", property];
JSONModelError* dataErr = [JSONModelError errorInvalidDataWithMessage:msg];
*err = [dataErr errorByPrependingKeyPathComponent:property.name];
}
return NO;
}
}
// 3.1) handle matching standard JSON types
// 判断标准的json类型，比如nsstring等
if (property.isStandardJSONType && [jsonValue isKindOfClass: property.type]) {
//mutable properties
if (property.isMutable) {
jsonValue = [jsonValue mutableCopy];
}
//set the property value
if (![jsonValue isEqual:[self valueForKey:property.name]]) {
[self setValue:jsonValue forKey: property.name];
}
continue;
}
// 3.3) handle values to transform
// 其他处理情况，主要是一些类型转换的情况，比如nsstring转换为nsurl等
if (
(![jsonValue isKindOfClass:property.type] && !isNull(jsonValue))
||
//the property is mutable
property.isMutable
||
//custom struct property
property.structName
) {
// searched around the web how to do this better
// but did not find any solution, maybe that's the best idea? (hardly)
// 获取真实的json数据类型
Class sourceClass = [JSONValueTransformer classByResolvingClusterClasses:[jsonValue class]];
//JMLog(@"to type: [%@] from type: [%@] transformer: [%@]", p.type, sourceClass, selectorName);
//build a method selector for the property and json object classes
// 通过property类型和json数据类型进行转换的判断
NSString* selectorName = [NSString stringWithFormat:@"%@From%@:",
(property.structName? property.structName : property.type), //target name
sourceClass]; //source name
SEL selector = NSSelectorFromString(selectorName);
//check for custom transformer
//是否有本地转换的方法
BOOL foundCustomTransformer = NO;
if ([valueTransformer respondsToSelector:selector]) {
foundCustomTransformer = YES;
} else {
//try for hidden custom transformer
selectorName = [NSString stringWithFormat:@"__%@",selectorName];
selector = NSSelectorFromString(selectorName);
if ([valueTransformer respondsToSelector:selector]) {
foundCustomTransformer = YES;
}
}
//check if there's a transformer with that name
if (foundCustomTransformer) {
//it's OK, believe me...
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
//transform the value
// 通过 JSONValueTransformer 进行类型转换
jsonValue = [valueTransformer performSelector:selector withObject:jsonValue];
#pragma clang diagnostic pop
if (![jsonValue isEqual:[self valueForKey:property.name]]) {
[self setValue:jsonValue forKey: property.name];
}
} else {
// it's not a JSON data type, and there's no transformer for it
// if property type is not supported - that's a programmer mistake -> exception
@throw [NSException exceptionWithName:@"Type not allowed"
reason:[NSString stringWithFormat:@"%@ type not supported for %@.%@", property.type, [self class], property.name]
userInfo:nil];
return NO;
}
} else {
// 3.4) handle "all other" cases (if any)
if (![jsonValue isEqual:[self valueForKey:property.name]]) {
[self setValue:jsonValue forKey: property.name];
}
}
}
}
}
return YES;
}
代码看上去很长，其实也比较好理解：
循环遍历model的每一个解析出来的property结构，首先从dictioanry拿出真正对应property的value，进行value一系列的值判断。value可用的情况下，就开始进行赋值，有setter方法的通过setter方法赋值，基础类型int，float等直接赋值，如果property又是一个JSONModel，就递归先将子Model进行整体解析。如果包含protocol字段，则表明内部是一个array或者dictionary，并包含这个protocol字段的对象解析。对于其他情况，应该是一种类型的转换，通过获取值类型和property类型，调用相应的转换方法进行赋值。
其中值得一提的就是JSONValueTransformer的类型转化，它解决了我们之前所说的麻烦1，将数据类型得以正确转换。
总结
至此，JSONModel主代码的作为，基本解释的差不多了。
总的来说JSONModel的源代码有以下优点：
- Runtime动态解析model数据类型
- AssociatedObject缓存
- keyMapper映射
- NSScanner扫描String
- JSONValueTransformer类型转换
- KVC附值
- 。。。
如果以上有任何我说错的地方，或者可以解释的更好的地方，也欢迎给我留言，我也会修改我的错误。Thanks。
(PS:和同事聊起这方面的话题，他表示YYModel的效率会比JSONModel高好几倍，下一份就看一下YYModel的源代码。)
参考资料
本文csdn地址
1.Objective-C Runtime Programming Guide
2.JSONModel源码解析
