详解iOS应用开发中Core Data数据存储的使用

1.如果想创建一个带有coredata的程序,要在项目初始化的时候勾选中
 
2.创建完成之后,会发现在appdelegate里多出了几个属性,和2个方法

<span style="font-size:18px;"> 
 
@property (readonly, strong, nonatomic) nsmanagedobjectcontext *managedobjectcontext; 
@property (readonly, strong, nonatomic) nsmanagedobjectmodel *managedobjectmodel; 
@property (readonly, strong, nonatomic) nspersistentstorecoordinator *persistentstorecoordinator; 
 
- (void)savecontext; 
- (nsurl *)applicationdocumentsdirectory;</span> 

core data数据持久化是对sqlite的一个升级，它是ios集成的，在说core data之前，我们先说说在coredata中使用的几个类。

（1）nsmanagedobjectmodel(被管理的对象模型)

相当于实体，不过它包含 了实体间的关系

(2)nsmanagedobjectcontext(被管理的对象上下文)

操作实际内容

作用：插入数据  查询  更新  删除

（3）nspersistentstorecoordinator(持久化存储助理)

相当于数据库的连接器

(4)nsfetchrequest(获取数据的请求)

相当于查询语句

(5)nspredicate(相当于查询条件)

(6)nsentitydescription(实体结构)

(7)后缀名为.xcdatamodel的包

里面的.xcdatamodel文件，用数据模型编辑器编辑

编译后为.momd或.mom文件，这就是为什么文件中没有这个东西，而我们的程序中用到这个东西而不会报错的原因。

3.如果想创建一个实体对象的话,需要点击.xcdatamodel,add entity,添加想要的字段

4.生成对象文件,command+n,然后选中coredata里的nsmanagerobjectsubclass进行关联,选中实体创建

5.添加数据

person *newperson = [nsentitydescription insertnewobjectforentityforname:@"person" inmanagedobjectcontext:self.managedobjectcontext]; 
     
    if (newperson == nil){ 
        nslog(@"failed to create the new person."); 
        return no; 
    } 
     
    newperson.firstname = paramfirstname; 
    newperson.lastname = paramlastname; 
    newperson.age = [nsnumber numberwithunsignedinteger:paramage]; 
    nserror *savingerror = nil; 
     
    if ([self.managedobjectcontext save:&savingerror]){ 
        return yes; 
    } else { 
        nslog(@"failed to save the new person. error = %@", savingerror); 
    } 

nsentitydescription（实体结构）相当于表格结构

6.取出数据查询

/* create the fetch request first */ 
    nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init]; 
    /* here is the entity whose contents we want to read */ 
    nsentitydescription *entity = 
    [nsentitydescription 
     entityforname:@"person" 
     inmanagedobjectcontext:self.managedobjectcontext]; 
    /* tell the request that we want to read the
     contents of the person entity */ 
    [fetchrequest setentity:entity]; 
    nserror *requesterror = nil; 
    /* and execute the fetch request on the context */ 
    nsarray *persons = 
    [self.managedobjectcontext executefetchrequest:fetchrequest 
                                             error:&requesterror]; 
    /* make sure we get the array */ 
    if ([persons count] > 0){ 
        /* go through the persons array one by one */ 
        nsuinteger counter = 1; 
        for (person *thisperson in persons){ 
            nslog(@"person %lu first name = %@", 
                  (unsigned long)counter,  
                  thisperson.firstname);  
            nslog(@"person %lu last name = %@",  
                  (unsigned long)counter,  
                  thisperson.lastname); 
            nslog(@"person %lu age = %ld", 
                  (unsigned long)counter, 
                  (unsigned long)[thisperson.age unsignedintegervalue]); 
            counter++; 
        } 
    } else { 
        nslog(@"could not find any person entities in the context.");  
    } 

7.删除数据

/* create the fetch request first */ 
    nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init]; 
    /* here is the entity whose contents we want to read */ 
    nsentitydescription *entity = 
    [nsentitydescription 
     entityforname:@"person" 
     inmanagedobjectcontext:self.managedobjectcontext]; 
    /* tell the request that we want to read the
     contents of the person entity */ 
    [fetchrequest setentity:entity]; 
    nserror *requesterror = nil; 
    /* and execute the fetch request on the context */ 
    nsarray *persons = 
    [self.managedobjectcontext executefetchrequest:fetchrequest 
                                             error:&requesterror]; 
    if ([persons count] > 0){ 
        /* delete the last person in the array */ 
        person *lastperson = [persons lastobject]; 
        [self.managedobjectcontext deleteobject:lastperson]; 
        nserror *savingerror = nil; 
        if ([self.managedobjectcontext save:&savingerror]){ 
            nslog(@"successfully deleted the last person in the array."); 
        } else { 
            nslog(@"failed to delete the last person in the array."); 
        } 
    } else {  
        nslog(@"could not find any person entities in the context.");  
    }  

8.排序

<pre code_snippet_id="243955" snippet_file_name="blog_20140319_5_4289257" name="code" class="objc">nssortdescriptor *agesort =  
[[nssortdescriptor alloc] initwithkey:@"age"  
ascending:yes];  
nssortdescriptor *firstnamesort =  
[[nssortdescriptor alloc] initwithkey:@"firstname"  
ascending:yes];  
nsarray *sortdescriptors = [[nsarray alloc] initwithobjects:  
agesort,  
firstnamesort, nil nil];  
fetchrequest.sortdescriptors = sortdescriptors; </pre><p></p> 
<pre></pre> 
<p></p> 
<p style="background-color:rgb(255,255,255); margin:10px auto; padding-top:0px; padding-bottom:0px; font-family:verdana,'ms song',arial,helvetica,sans-serif; line-height:19.09090805053711px"> 
<span style="background-color:rgb(255,255,255)"><span style="font-size:18px"><br> 
</span></span></p> 
<span style="background-color:rgb(255,255,255)">注意</span><span style="font-size:18px; background-color:rgb(255,255,255)">ascending:yes 属性决定排序顺序</span><span style="background-color:rgb(255,255,255)"><span style="font-size:18px"><br> 
<br> 
<br> 
</span></span><br>