ReactiveCocoa

为什么RAC更加适合编写Cocoa App？说这个之前，我们先来看下Web前端编程，因为有些相似之处。目前很火的AngularJS有一个很重要的特性：数据与视图绑定。就是当数据变化时，视图不需要额外的处理，便可正确地呈现最新的数据。而这也是RAC的亮点之一。RAC与Cocoa的编程模式，有点像AngularJS和jQuery。所以要了解RAC，需要先在观念上做调整。

• 1.RAC进行数据绑定KVO的例子：

    // When self.username changes, logs the new name to the console.
    //
    // RACObserve(self, username) creates a new RACSignal that sends the current
    // value of self.username, then the new value whenever it changes.
    // -subscribeNext: will execute the block whenever the signal sends a value.
    [RACObserve(self, username) subscribeNext:^(NSString *newName) {
        NSLog(@"%@", newName);
    }];
  

当每次self.username改变，都会自动执行subscribeNext，打出内容到控制台

• 2.RAC不仅仅是kvo，它还可以对信号进行过滤，筛选正确的信号，例如：

    // Only logs names that starts with "j".
    //
    // -filter returns a new RACSignal that only sends a new value when its block
    // returns YES.
    [[RACObserve(self, username)
        filter:^(NSString *newName) {
            return [newName hasPrefix:@"j"];
        }]
        subscribeNext:^(NSString *newName) {
            NSLog(@"%@", newName);
        }];
  

筛选前缀是j的username，打出到控制台

##Swift 与 Objective-C 中的使用区别:

设置请看 RACSwift

RAC In Objective－C:

RAC(self.collectionView,page) = RACObserve(self.collectionView,page);

In Swift:

RAC(self.collectionView,"page") <= RACObserve(self.collectionView,"page")
RACObserve(self.collectionView,"page") => RAC(self.collectionView,"page")

---

RACObserve In Objective－C:

    [[[RACObserve(self.collectionView, page)
     map:^NSString*(NSNumber* newPage) {
         return @"123";
     }]
     filter:^BOOL(NSString* newPage) {
         return false;
     }]
     subscribeNext:^(NSString* text) {
         NSLog(@"%@",text);
     }];

In Swift:

    RACObserve(self.collectionView,"page")
    .mapAs{
        (newpage:NSNumber) -> NSString in
        return "123"
    }
    .filterAs{
        (newpage:NSString) -> Bool in
        return false
    }
    .subscribeNextAs{
        (text:String) -> () in
        println(text)
    }