完成对FetchRequest完成的阻止/回调 [英] Completion block/callback on FetchRequest completion

问题描述

我在名为 ContentManager 的模型类中有一个 FetchRequest 。它获取了大量的数据，所以屏幕是空白的，直到它完成。在显示获取结果的 ViewController 中，我想显示一个加载指示器，所以我想获得一个回调当 FetchRequest 已完成并传递到 ViewController 以停止加载指示器。这是可能吗？

I have a FetchRequest in my model class named ContentManager. It fetches quite a lot of data, so the screen is blank until it's completion. In the ViewController that displays the fetched results, I would like to show a loading indicator, so I would like to get a callback when the FetchRequest has completed and pass that to the ViewController to stop the loading indicator. Is this possible?

这是来自 ContentManager FetchRequest c> class：

Here is the FetchRequest from the ContentManager class:

- (NSArray*)recipesForMagazine
{
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Recipe" inManagedObjectContext:managedObjectContext];
    [request setEntity:entity];

    NSString* path = [[NSBundle mainBundle] pathForResource:@"magazine_recipe_guids" ofType:@"plist"];
    NSArray* recipeGuids = [NSArray arrayWithContentsOfFile:path];

    NSPredicate* predicate = [NSPredicate predicateWithFormat:@"guid IN %@",recipeGuids];
    [request setPredicate:predicate];

    NSSortDescriptor* sort = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES selector:@selector(localizedCaseInsensitiveCompare:)];
    [request setSortDescriptors:[NSArray arrayWithObject:sort]];

    [sort release];

    NSError* error = nil;
    NSArray* fetchResults = [managedObjectContext executeFetchRequest:request error:&error];
    [request release];

    return fetchResults;        
}

这里我设置在 ViewController

Here I set it up in the ViewController

self.magazineRecipes = [[ContentManager defaultManager] recipesForMagazine];

我想像这样设置fetchrequest方法：

I want to set up the fetchrequest method like this:

- （NSArray *）recipesForMagazine :( void（^）（BOOL success））block 或者什么，所以在 viewcontroller 我可以这样调用

- (NSArray*)recipesForMagazine:(void (^)(BOOL success))block or something, so in the viewcontroller I can call it like this

self.magazineRecipes = [[CTContentManager defaultManager] recipesForMagazine:^(BOOL success) {
    if (success) {
        //stop activity indicator view
    }
}];

我不知道我是否在正确的思维方式，谢谢你提前帮助！

I don't know if I'm at all in the right way of thinking, thanks for your help in advance!

推荐答案

我会使viewController成为 ContentManager 类的委托。所以在 ContentManager.h 中我会做类似的：

I would make the viewController a delegate of the ContentManager class. So in the ContentManager.h I would do something like:

    @protocol ContentManagerDelegate()
      -(void) didFetchResults:(NSArray *) results;
      -(void) didResultsFail: (NSError *) error;
    @end

    @interface ContentManager : <SuperClass Name>
    -(id) initWithDelegate: (id<ContentManagerDelegate>) delegate;
    @property (nonatomic, strong) id<ContentManagerDelegate> delegate;
    @end

并在实施中：

   -(id) initWithDelegate: (id<ContentManagerDelegate>) delegate
    {
        self = [super init];
        if(self)
        {
           _delegate = delegate;
         }
        return self;
    }

并在 recipesForMagazine 方法，你可以使用委托 [_ delegate didFetchResults：fetchResults] ，你可以实现一个方法来将错误传递给委托。在您的 ViewController.h do @interface ViewController.h：UIViewController< ContentManagerDelegate> 能够实现 didFetchResults：方法，将给你的结果，在该方法中，你可以停止活动指示器的动画。

and in your recipesForMagazine method you can use the delegate [_delegate didFetchResults: fetchResults], you can implement a method to pass errors to the delegate if you want as well. In your ViewController.h do @interface ViewController.h : UIViewController <ContentManagerDelegate> and in the implementation you should be able to implement the didFetchResults: method that will give you the results and in that method you can stop the activity indicator from animating.

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