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// 監聽 UITextField 當內文有改變的時候，會執行內部 function
[[self.textView rac_signalForControlEvents:UIControlEventEditingChanged] subscribeNext:^(__kindof UIControl * _Nullable x) {
        NSLog(@"text change");
}];

// 監聽 button UIControlEventTouchUpInside 狀態
[[self.btnButton rac_signalForControlEvents:UIControlEventTouchUpInside] subscribeNext:^(__kindof UIControl * _Nullable x) {
    NSLog(@"button click");
}];

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// create signnal
RACSignal *signal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
    [subscriber sendNext:@"message"];
    return nil;
}];

// subscribe notification
RACDisposable *disposable = [signal subscribeNext:^(id  _Nullable x) {
    NSLog(@"content：%@", x);
}];

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// create a signal
RACSubject *subject = [RACSubject subject];

// subscribe notification
[subject subscribeNext:^(id  _Nullable x) {
    NSLog(@"content 2：%@", x);
}];

// send message
[subject sendNext:@"message 2"];

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// loop all element
NSArray *array = @[@"1", @"2", @"3", @"4", @"5"];
[array.rac_sequence.signal subscribeNext:^(id  _Nullable x) {
    NSLog(@"element：%@", x);
}];

// output
// element：1
// element：2
// element：3

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[RACObserve(objA, strContent) subscribeNext:^(id x) {
    NSLog(@"objA : %@",x);
    [self.labelView setText:x];
}];

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RAC(_btnButton, hidden) = [RACSignal
                            combineLatest:@[_textView.rac_textSignal]
                            reduce:^id _Nullable(NSString * text) {
    return @(text.length==0);
}];

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[[[NSNotificationCenter defaultCenter] rac_addObserverForName:UIKeyboardDidShowNotification object:nil] subscribeNext:^(NSNotification * _Nullable x) {
    NSLog(@"keyboard - \n%@", x);
}];

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 [[self.btnButton rac_signalForSelector:@selector(actionButton:)] subscribeNext:^(RACTuple * _Nullable x) {
    NSLog(@"button click..");
}];

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@interface Model:NSObject
@property (strong, nonatomic) NSString *content;
@end

@implementation Model
@end

@interface ViewModel:NSObject
@property (strong, nonatomic) NSString *content;
- (void) execute:(NSString *)content;

SunXiaoShan

// JAVA 
// QUESTION 1
static String getReverseString(String input) {
    char[] in = input.toCharArray();
    char buff;
    for (int i=0,j=input.length()-1; i<input.length()/2;i++,j--) {
        buff = in[i];
        in[i] = in[j];
        in[j] = buff;
    }