如何在 NSMutableArray 上添加观察者？

Question

我搜索了很多，但没有找到有用的代码或教程。

在我的应用程序中，我有一个每 60 秒更新一次的可变数组。

数组中的对象被多个视图控制器中的表视图显示。

我想仅在数组中的值更改或更新时自动重新加载表格视图。

为此，我想在可变数组上添加观察者，即当数组中的值发生变化时，它应该调用特定的方法，例如

-(void)ArrayUpdatedNotification:(NSMutableArray*)array
{
    //Reload table or do something
} 

提前致谢。

Answer 1

您可以将数组抽象为具有访问器方法的数据容器类，然后使用键值观察来观察支持容器对象的数组何时更改（您不能直接在NSArray上使用KVO）。

下面是一个简单的类示例，该类用作数组顶部的抽象。您使用它的insertObject:inDataAtIndex: 和removeObjectFromDataAtIndex: 方法，而不是直接使用addObject: 和removeObject: 访问。

// DataContainer.h
@interface DataContainer : NSObject

// Convenience accessor
- (NSArray *)currentData;

// For KVC compliance, publicly declared for readability
- (void)insertObject:(id)object inDataAtIndex:(NSUInteger)index;
- (void)removeObjectFromDataAtIndex:(NSUInteger)index;
- (id)objectInDataAtIndex:(NSUInteger)index;
- (NSArray *)dataAtIndexes:(NSIndexSet *)indexes;
- (NSUInteger)countOfData;

@end

// DataContainer.m

@interface DataContainer ()

@property (nonatomic, strong) NSMutableArray *data;

@end

@implementation DataContainer

//  We'll use automatic notifications for this example
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    if ([key isEqualToString:@"data"]) {
        return YES;
    }
    return [super automaticallyNotifiesObserversForKey:key];
}

- (id)init
{
    self = [super init];
    if (self) {
        // This is the ivar which provides storage
        _data = [NSMutableArray array];
    }
    return self;
}

//  Just a convenience method
- (NSArray *)currentData
{
    return [self dataAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, [self countOfData])]];
}

//  These methods enable KVC compliance
- (void)insertObject:(id)object inDataAtIndex:(NSUInteger)index
{
    self.data[index] = object;
}

- (void)removeObjectFromDataAtIndex:(NSUInteger)index
{
    [self.data removeObjectAtIndex:index];
}

- (id)objectInDataAtIndex:(NSUInteger)index
{
    return self.data[index];
}

- (NSArray *)dataAtIndexes:(NSIndexSet *)indexes
{
    return [self.data objectsAtIndexes:indexes];
}

- (NSUInteger)countOfData
{
    return [self.data count];
}

@end

我们这样做的原因是我们现在可以观察对底层数组所做的更改。这是通过Key Value Observing 完成的。显示了一个实例化和观察数据控制器的简单视图控制器：

// ViewController.h
@interface ViewController : UIViewController

@end

// ViewController.m

@interface ViewController ()

@property (nonatomic,strong) DataContainer *dataContainer;

@end

@implementation ViewController

static char MyObservationContext;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        //  Instantiate a DataContainer and store it in our property
        _dataContainer = [[DataContainer alloc] init];
        //  Add self as an observer. The context is used to verify that code from this class (and not its superclass) started observing.
        [_dataContainer addObserver:self
                         forKeyPath:@"data"
                            options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew)
                            context:&MyObservationContext];
    }

    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    //  Check if our class, rather than superclass or someone else, added as observer
    if (context == &MyObservationContext) {
        //  Check that the key path is what we want
        if ([keyPath isEqualToString:@"data"]) {
            //  Verify we're observing the correct object
            if (object == self.dataContainer) {
                NSLog(@"KVO for our container property, change dictionary is %@", change);
            }
        }
    }
    else {
        //  Otherwise, call up to superclass implementation
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    //  Insert and remove some objects. Console messages should be logged.
    [self.dataContainer insertObject:[NSObject new] inDataAtIndex:0];
    [self.dataContainer insertObject:[NSObject new] inDataAtIndex:1];
    [self.dataContainer removeObjectFromDataAtIndex:0];
}

- (void)dealloc
{
    [_dataContainer removeObserver:self forKeyPath:@"data" context:&MyObservationContext];
}

@end

当这段代码运行时，视图控制器会观察到数据的三个变化并记录到控制台：

KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x8557d40>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
        kind = 2;
        new =     (
            "<NSObject: 0x8557d10>"
        );
    }
KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x715d2b0>[number of indexes: 1 (in 1 ranges), indexes: (1)]";
        kind = 2;
        new =     (
            "<NSObject: 0x71900c0>"
        );
    }
KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x8557d40>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
        kind = 3;
        old =     (
            "<NSObject: 0x8557d10>"
        );
    }

虽然这有点复杂（并且可能涉及更多），但这是自动通知可变数组的内容已更改的唯一方法。

Answer 2

可以做的是 - 更新您的阵列后发送通知 (NSNotificationCenter)，所有控制器都会收到此通知。收到通知后，控制器应该执行 [tableview reloaddata]。

代码示例：

// Adding an observer
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateTable:) name:@"arrayUpdated" object:nil];

// Post a notification
[[NSNotificationCenter defaultCenter] postNotificationName:@"arrayUpdated" object:nil]; 

// the void function, specified in the same class where the Notification addObserver method has defined
- (void)updateTable:(NSNotification *)note { 
    [tableView reloadData]; 
}

Answer 3

如果你想使用闪亮的方块，你可以这样做

// Create an instance variable for your block holder in your interface extension
@property (strong) id notificationHolder;

// Listen for notification events (In your TableView class.
self.notificationHolder = [[NSNotificationCenter defaultCenter] addObserverForName:@"NotificationName"
                             object:nil
                              queue:[NSOperationQueue mainQueue]
                         usingBlock:^(NSNotification *note) {

        NSLog(@"Received notification");
}];

然后在 dealloc 中（或者当你不再使用它时）

- (void)dealloc {
     [[NSNotificationCenter defaultCenter] removeObserver:self.notificationHolder];
}

然后在其他类中

// Send a notification
[[NSNotificationCenter defaultCenter] postNotificationName:@"NotificationName" object:nil];

如果有不清楚的地方请教！希望对您有所帮助！

因评论而编辑

“YourEvent”是通知的名称，这意味着您可以随意命名。 （也许“UpdateArrayNotification 可能是个好名字？）

需要考虑的事项：请注意，同一通知可以有多个观察者。这意味着一个“帖子”将被所有观察者抢购一空。