在多线程环境中使用CoreData

2022-12-09 10:34:10

在多线程环境中使用CoreData

BY 子非鱼 · 2014 年 10 月 13 日

 

上回书说道，其实CoreData学起来也没有很复杂，我们其实增删改查都和别的ORM大同小异。但是世界总是很复杂的，一根筋的去考虑问题很容易卡到蛋，默认情况下我们的代码都在Main Thread中执行，数据库操作一旦量多了，频繁了，势必会阻塞住主线程的其他操作，俗话说，卡住了。

这个世界天然是多线程的，所以我们操作数据也必须多线程。CoreData对多线程的支持比较奇怪（按照一般的思路来说），CoreData的NSPersistentStoreCoordinator和NSManagedObjectContext对象都是不能跨线程使用的，NSManagedObject也不行，有人想加锁不就完了。No，作为一个处女座是不能忍受这么丑陋的解决方案的。其实NSManagedObjectContext已经对跨线程提供了内置的支持，只不过方式比较特殊，需要脑洞大开才行。

在创建NSManagedObject的时候有个构造器参数initWithConcurrencyType就是解决的关键所在，这个参数是一个枚举，有三个可选值：

NSConfinementConcurrencyType (或者不加参数，默认就是这个)
NSMainQueueConcurrencyType (表示只会在主线程中执行)
NSPrivateQueueConcurrencyType (表示可以在子线程中执行)
那么究竟应该怎么使用才能无阻塞无痛呢？

经过参考： http://www.cocoanetics.com/2012/07/multi-context-coredata/ 这篇文章，我们使用三层 NSManagedObjectContext 嵌套的方式。

NSManagedObjectContext是可以基于其他的 NSManagedObjectContext的，通过 setParentContext 方法，可以设置另外一个 NSManagedObjectContext 为自己的父级，这个时候子级可以访问父级下所有的对象，而且子级 NSManagedObjectContext 的内容变化后，如果执行save方法，会自动的 merge 到父级 NSManagedObjectContext 中，也就是子级save后，变动会同步到父级 NSManagedObjectContext。当然这个时候父级也必须再save一次，如果父级没有父级了，那么就会直接向NSPersistentStoreCoordinator中写入，如果有就会接着向再上一层的父级冒泡……

那么这里如同参考的文章一样，通过三个级别的 NSManagedObjectContext， 一个负责在background更新NSPersistentStoreCoordinator。一个用在主线程，主要执行插入，修改和删除操作，一些小的查询也可以在这里同步执行，如果有大的查询，就起一个新的 NSPrivateQueueConcurrencyType 类型的 NSManagedObjectContext，然后放在后台去执行查询，查询完成后将结果返回主线程。

NSManagedObjectContext在后台线程执行是通过 performBlock 方法来实现的，在传入的匿名block中执行的代码就是在子线程中了，比如

[_bgObjectContext performBlock:^{
__block NSError *inner_error = nil;
[_bgObjectContext save:&inner_error];
}];

那么如果是查询的话，因为 NSManagedObject 也不能跨线程访问，所以在block里获取到的NSManagedObject对象只能将objectid传到主线程，主线程再通过 objectWithID 恢复对象的方法，如下面的示例：

+(void)one:(NSString*)predicate on:(ObjectResult)handler{
NSManagedObjectContext *ctx = [[mmDAO instance] createPrivateObjectContext];
[ctx performBlock:^{
NSFetchRequest *fetchRequest = [self makeRequest:ctx predicate:predicate orderby:nil offset:0 limit:1];
NSError* error = nil;
NSArray* results = [ctx executeFetchRequest:fetchRequest error:&error];
if (error) {
NSLog(@"error: %@", error);
[[mmDAO instance].mainObjectContext performBlock:^{
handler(@[], nil);
}];
}
if ([results count]<1) {
[[mmDAO instance].mainObjectContext performBlock:^{
handler(@[], nil);
}];
}
NSManagedObjectID *objId = ((NSManagedObject*)results[0]).objectID;
[[mmDAO instance].mainObjectContext performBlock:^{
handler([[mmDAO instance].mainObjectContext objectWithID:objId], nil);
}];
}];
}

最后我们将上述的要点结合起来，解决方案就呼之欲出了。

首先我们还是需要一个抽象存储和数据操作的核心

mmDAO.h
 
//
// mmDAO.h
// agent
//
// Created by LiMing on 14-6-24.
// Copyright (c) 2014年 Alexander. All rights reserved.
//
 
#import <Foundation/Foundation.h>
 
typedef void(^OperationResult)(NSError* error);
 
@interface mmDAO : NSObject
@property (readonly, strong, nonatomic) NSOperationQueue *queue;
@property (readonly ,strong, nonatomic) NSManagedObjectContext *bgObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectContext *mainObjectContext;
 
+(mmDAO*)instance;
-(void) setupEnvModel:(NSString *)model DbFile:(NSString*)filename;
- (NSManagedObjectContext *)createPrivateObjectContext;
-(NSError*)save:(OperationResult)handler;
 
@end
mmDAO.m
 
//
// mmDAO.m
// agent
//
// Created by LiMing on 14-6-24.
// Copyright (c) 2014年 Alexander. All rights reserved.
//
 
#import "mmDAO.h"
#import "mmAppDelegate.h"
 
static mmDAO *onlyInstance;
 
@interface mmDAO ()
@property (nonatomic, copy)NSString *modelName;
@property (nonatomic, copy)NSString *dbFileName;
@end
 
@implementation mmDAO
+(mmDAO*)instance{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
onlyInstance = [[mmDAO alloc] init];
});
return onlyInstance;
}
 
- (instancetype)init
{
self = [super init];
if (self) {
}
return self;
}
 
-(void) setupEnvModel:(NSString *)model DbFile:(NSString*)filename{
_modelName = model;
_dbFileName = filename;
[self initCoreDataStack];
}
 
- (void)initCoreDataStack
{
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_bgObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_bgObjectContext setPersistentStoreCoordinator:coordinator];
 
_mainObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[_mainObjectContext setParentContext:_bgObjectContext];
}
 
}
 
- (NSManagedObjectContext *)createPrivateObjectContext
{
NSManagedObjectContext *ctx = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[ctx setParentContext:_mainObjectContext];
 
return ctx;
}
 
- (NSManagedObjectModel *)managedObjectModel
{
NSManagedObjectModel *managedObjectModel;
NSURL *modelURL = [[NSBundle mainBundle] URLForResource:_modelName withExtension:@"momd"];
managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
return managedObjectModel;
}
 
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
NSPersistentStoreCoordinator *persistentStoreCoordinator = nil;
NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:_dbFileName];
 
NSError *error = nil;
persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
abort();
}
return persistentStoreCoordinator;
}
 
- (NSURL *)applicationDocumentsDirectory
{
return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}
 
-(NSError*)save:(OperationResult)handler{
NSError *error;
if ([_mainObjectContext hasChanges]) {
[_mainObjectContext save:&error];
[_bgObjectContext performBlock:^{
__block NSError *inner_error = nil;
[_bgObjectContext save:&inner_error];
if (handler){
//handler(error);
//这里需要返回主线程，不然在回调里操作界面会出错
[_mainObjectContext performBlock:^{
handler(error);
}];
}
}];
}
return error;
}
 
@end

然后我们扩展 NSManagedObject，把操作注入到类里，这样子有个好处是可以自动获取类名，然后就不用子级写字符串的类名了。

NSManagedObject+helper.h
 
//
//  NSManagedObject+helper.h
//  agent
//
//  Created by LiMing on 14-6-24.
//  Copyright (c) 2014年 bangban. All rights reserved.
//
 
typedef void(^ListResult)(NSArray* result, NSError *error);
typedef void(^ObjectResult)(id result, NSError *error);
 
#import <CoreData/CoreData.h>
#import "mmDAO.h"
 
@interface NSManagedObject (helper)
 
+(id)createNew;
 
+(NSError*)save:(OperationResult)handler;
 
+(NSArray*)filter:(NSString *)predicate orderby:(NSArray *)orders offset:(int)offset limit:(int)limit;
 
+(void)filter:(NSString *)predicate orderby:(NSArray *)orders offset:(int)offset limit:(int)limit on:(ListResult)handler;
 
+(id)one:(NSString*)predicate;
 
+(void)one:(NSString*)predicate on:(ObjectResult)handler;
 
+(void)delobject:(id)object;
@end
NSManagedObject+helper.m
 
//
//  NSManagedObject+helper.m
//  agent
//
//  Created by LiMing on 14-6-24.
//  Copyright (c) 2014年 bangban. All rights reserved.
//
 
#import "NSManagedObject+helper.h"
 
@implementation NSManagedObject (helper)
+(id)createNew{
    NSString *className = [NSString stringWithUTF8String:object_getClassName(self)];
    return [NSEntityDescription insertNewObjectForEntityForName:className inManagedObjectContext:[mmDAO instance].mainObjectContext];
}
 
+(NSError*)save:(OperationResult)handler{
    return [[mmDAO instance] save:handler];
}
 
+(NSArray*)filter:(NSString *)predicate orderby:(NSArray *)orders offset:(int)offset limit:(int)limit{
 
    NSManagedObjectContext *ctx = [mmDAO instance].mainObjectContext;
    NSFetchRequest *fetchRequest = [self makeRequest:ctx predicate:predicate orderby:orders offset:offset limit:limit];
 
    NSError* error = nil;
    NSArray* results = [ctx executeFetchRequest:fetchRequest error:&error];
    if (error) {
        NSLog(@"error: %@", error);
        return @[];
    }
    return results;
}
 
+(NSFetchRequest*)makeRequest:(NSManagedObjectContext*)ctx predicate:(NSString*)predicate orderby:(NSArray*)orders offset:(int)offset limit:(int)limit{
    NSString *className = [NSString stringWithUTF8String:object_getClassName(self)];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:[NSEntityDescription entityForName:className inManagedObjectContext:ctx]];
    [fetchRequest setPredicate:[NSPredicate predicateWithFormat:predicate]];
    NSMutableArray *orderArray = [[NSMutableArray alloc] init];
    if (orders!=nil) {
        for (NSString *order in orders) {
            NSSortDescriptor *orderDesc = nil;
            if ([[order substringToIndex:1] isEqualToString:@"-"]) {
                orderDesc = [[NSSortDescriptor alloc] initWithKey:[order substringFromIndex:1]
                                                        ascending:NO];
            }else{
                orderDesc = [[NSSortDescriptor alloc] initWithKey:order
                                                        ascending:YES];
            }
        }
        [fetchRequest setSortDescriptors:orderArray];
    }
    if (offset>0) {
        [fetchRequest setFetchOffset:offset];
    }
    if (limit>0) {
        [fetchRequest setFetchLimit:limit];
    }
    return fetchRequest;
}
 
+(void)filter:(NSString *)predicate orderby:(NSArray *)orders offset:(int)offset limit:(int)limit on:(ListResult)handler{
 
    NSManagedObjectContext *ctx = [[mmDAO instance] createPrivateObjectContext];
    [ctx performBlock:^{
        NSFetchRequest *fetchRequest = [self makeRequest:ctx predicate:predicate orderby:orders offset:offset limit:limit];
        NSError* error = nil;
        NSArray* results = [ctx executeFetchRequest:fetchRequest error:&error];
        if (error) {
            NSLog(@"error: %@", error);
            [[mmDAO instance].mainObjectContext performBlock:^{
                handler(@[], nil);
            }];
        }
        if ([results count]<1) {
            [[mmDAO instance].mainObjectContext performBlock:^{
                handler(@[], nil);
            }];
        }
        NSMutableArray *result_ids = [[NSMutableArray alloc] init];
        for (NSManagedObject *item  in results) {
            NSLog(@"id=%@", item.objectID);
            [result_ids addObject:item.objectID];
        }
        [[mmDAO instance].mainObjectContext performBlock:^{
            NSMutableArray *final_results = [[NSMutableArray alloc] init];
            for (NSManagedObjectID *oid in result_ids) {
                [final_results addObject:[[mmDAO instance].mainObjectContext objectWithID:oid]];
            }
            handler(final_results, nil);
        }];
    }];
}
 
+(id)one:(NSString*)predicate{
    NSManagedObjectContext *ctx = [mmDAO instance].mainObjectContext;
    NSFetchRequest *fetchRequest = [self makeRequest:ctx predicate:predicate orderby:nil offset:0 limit:1];
    NSError* error = nil;
    NSArray* results = [ctx executeFetchRequest:fetchRequest error:&error];
    if ([results count]!=1) {
        raise(1);
    }
    return results[0];
}
 
+(void)one:(NSString*)predicate on:(ObjectResult)handler{
    NSManagedObjectContext *ctx = [[mmDAO instance] createPrivateObjectContext];
    [ctx performBlock:^{
        NSFetchRequest *fetchRequest = [self makeRequest:ctx predicate:predicate orderby:nil offset:0 limit:1];
        NSError* error = nil;
        NSArray* results = [ctx executeFetchRequest:fetchRequest error:&error];
        if (error) {
            NSLog(@"error: %@", error);
            [[mmDAO instance].mainObjectContext performBlock:^{
                handler(@[], nil);
            }];
        }
        if ([results count]<1) {
            [[mmDAO instance].mainObjectContext performBlock:^{
                handler(@[], nil);
            }];
        }
        NSManagedObjectID *objId = ((NSManagedObject*)results[0]).objectID;
        [[mmDAO instance].mainObjectContext performBlock:^{
            handler([[mmDAO instance].mainObjectContext objectWithID:objId], nil);
        }];
    }];
}
 
+(void)delobject:(id)object{
    //[[mmDAO instance].mainObjectContext delete:object];
    //这里方法名写错了
    [[mmDAO instance].mainObjectContext deleteObject:object];
}
 
@end

本文内容已经打包放在了github，有兴趣的可以fork下来直接用 https://github.com/ipconfiger/asyncCoreDataWrapper

另，文中代码有两处错误，已经加上了注释，提前看了的同学记得以github为准。

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