将AppDelegate导入模型？

问题描述

我试图统一模型文件中特定类的所有函数。例如，我将在模型Contact.h / Contact.m中有一个函数fetchContactWithName：（NSString *），我的viewcontroller会调用它。

在这种情况下，将AppDelegate.h文件导入模型文件是一个坏主意，因为我需要访问其managedObjectContext？

  #importAppDelegate.h
 
 @implementation联系人
 
 ... 
 
 +（联系人*）fetchContactWithName： *）name {
 AppDelegate * delegate =（AppDelegate *）[[UIApplication sharedApplication] delegate]; 
 
 NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init]; 
 NSEntityDescription * entity = [NSEntityDescription entityForName：@ContactinManagedObjectContext：delegate.managedObjectContext]; 
 [fetchRequest setEntity：entity]; 
 
 NSPredicate * predicate = [NSPredicate predicateWithFormat：@name ==％@，name]; 
 [fetchRequest setPredicate：predicate]; 
 
 NSError * error = nil; 
 NSArray * fetchedObjects = [delegate.managedObjectContext executeFetchRequest：fetchRequest error：& error]; 
 
联系* fetchedContact; 
 for（Contact * contact in fetchedObjects）{
 fetchedContact = contact; 
} 
 
 if（fetchedContact！= nil）{
 return fetchedContact; 
} else {
 return nil; 
} 
 
} 
 @end 
  

在我看来，直接问一个不同的类，得到你的管理对象上下文是一个坏主意。因为

1. 您不能在不同的项目中重用您的课程（例如OS X应用程式）




3. 如果方法询问其他类
，则不能使用单元测试





您应该告诉此方法应该获取的上下文。

而不是 +（Contact *）fetchContactWithName：（NSString * ）name 您的方法签名应该如下所示：

  +（Contact *）fetchContactWithName： *）name inManagedObjectContext：（NSManagedObjectContext *）context 
  

每个viewController都应该引用NSManagedObjectContext你的应用程序委托。你可以在 application：didFinishLaunchingWithOptions：中为每个viewController传递一个上下文的引用，并且每次推送或呈现一个新的viewController时，都会将上下文实例传递给它。





这可能看起来像很多工作，但有一天你会受益于告诉，不要问的方法。




I'm trying to unify all of my functions of a specific class within the model file. For instance, I would have a function fetchContactWithName:(NSString *)name in the model 'Contact.h/Contact.m', which my viewcontroller would subsequently call.

In this case, would it be a bad idea to import the AppDelegate.h file into the model file as a I need to access its managedObjectContext?

#import "AppDelegate.h"

@implementation Contact

...

+ (Contact *) fetchContactWithName:(NSString *) name {
  AppDelegate *delegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];

  NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
  NSEntityDescription *entity = [NSEntityDescription entityForName:@"Contact" inManagedObjectContext:delegate.managedObjectContext];
  [fetchRequest setEntity:entity];

  NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == %@", name];
  [fetchRequest setPredicate:predicate];

  NSError *error = nil;
  NSArray *fetchedObjects = [delegate.managedObjectContext executeFetchRequest:fetchRequest error:&error];

  Contact *fetchedContact;
  for(Contact *contact in fetchedObjects) {
      fetchedContact = contact;
  }

  if(fetchedContact != nil) {
      return fetchedContact;
  } else {
      return nil;
  }

}
@end

解决方案

In my opinion it's a bad idea to directly ask a different class to get your managed object context. Because

1. You can't really reuse your classes in different projects (think OS X app)
2. You can't fetch contacts in a different context (think background import)
3. You can't use unit tests if the method asks other class

You should tell this method in which context it should fetch.
Instead of + (Contact *) fetchContactWithName:(NSString *) name your method signature should look like this:

+ (Contact *)fetchContactWithName:(NSString *)name inManagedObjectContext:(NSManagedObjectContext *)context

each viewController should have a reference to the NSManagedObjectContext used in your app delegate. You can pass a reference of the context to each viewController in application:didFinishLaunchingWithOptions:, and each time you push or present a new viewController you pass the context instance to it.

This might look like a lot of work now, but some day you will benefit from the "tell, don't ask" approach.

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