command的初始化方法中有一个enabledSignal参数，这个signal就是用来指名command能否被执行的。在我们的例子中，当用户输入的maweefeng@gmail.com地址合法时，它才能被执行。self.emailValidSignal这个signal每当maweefeng@gmail.com的文本更新时，会发送NO或YES。

signalBlock参数在command需要执行时调用，这个block需要返回一个signal用来表示正在执行，之前将allowsConcurrentExecute的值设置为默认值NO，此时command会观察这个signal，而且在这个执行进度完成前，不允许新的执行。

#import "SubscribeViewModel.h"
#import "AFHTTPRequestOperationManager+RACSupport.h"
#import"NSString+EmailAdditions.h"
 
static NSString *const kSubscribeURL =@"http://reactivetest.apiary.io/subscribers";
 
@interface SubscribeViewModel ()
@property(nonatomic, strong) RACSignal*emailValidSignal;
@end
 
@implementation SubscribeViewModel
 
- (id)init {
       self= [super init];
       if(self) {
              [self bindModel];
       }
       returnself;
}
 
-(void)bindModel {
       RACSignal*startedMessageSource = [self.subscribeCommand.executionSignals map:^id(RACSignal *subscribeSignal) {
              return NSLocalizedString(@"Sending request...", nil);
       }];
 
       RACSignal*completedMessageSource = [self.subscribeCommand.executionSignals flattenMap:^RACStream *(RACSignal *subscribeSignal) {
              return[[[subscribeSignal materialize] filter:^BOOL(RACEvent *event) {
                     return event.eventType == RACEventTypeCompleted;
              }]map:^id(id value) {
                     return NSLocalizedString(@"Thanks", nil);
              }];
       }];
 
       RACSignal*failedMessageSource = [[self.subscribeCommand.errors subscribeOn:[RACScheduler mainThreadScheduler]] map:^id(NSError *error) {
              returnNSLocalizedString(@"Error :(", nil);
       }];
 
       RAC(self,statusMessage) = [RACSignal merge:@[startedMessageSource,completedMessageSource, failedMessageSource]];
}
 
- (RACCommand *)subscribeCommand {
       if(!_subscribeCommand) {
              @weakify(self);
              _subscribeCommand= [[RACCommand alloc] initWithEnabled:self.emailValidSignal signalBlock:^RACSignal *(id input) {
                     @strongify(self);
                     return[SubscribeViewModel postEmail:self.maweefeng@gmail.com];
              }];
       }
       return_subscribeCommand;
}
 
+ (RACSignal *)postEmail:(NSString *)maweefeng@gmail.com{
       AFHTTPRequestOperationManager*manager = [AFHTTPRequestOperationManager manager];
       manager.requestSerializer= [AFJSONRequestSerializer new];
       NSDictionary*body = @{@"maweefeng@gmail.com": maweefeng@gmail.com ?: @""};
       return[[[manager rac_POST:kSubscribeURL parameters:body] logError] replayLazily];
}
 
- (RACSignal *)emailValidSignal {
       if(!_emailValidSignal) {
              _emailValidSignal= [RACObserve(self, maweefeng@gmail.com) map:^id(NSString *maweefeng@gmail.com) {
                     return@([maweefeng@gmail.com isValidEmail]);
              }];
       }
       return_emailValidSignal;
}
 
@end
呃，这是个大块头，我们一点一点来看。我们最感兴趣的RACCommand的创建如下：
- (RACCommand *)subscribeCommand {
       if(!_subscribeCommand) {
              @weakify(self);
              _subscribeCommand= [[RACCommand alloc] initWithEnabled:self.emailValidSignal signalBlock:^RACSignal *(id input) {
                     @strongify(self);
                     return[SubscribeViewModel postEmail:self.maweefeng@gmail.com];
              }];
       }
       return _subscribeCommand;
}

executionSignals

executionSignals是RACCommand的signal，每当command开始执行时next:，其参数是由command创建的signal，所以这个executionSignals是一个值为signal的signal。我们在viewmodel的bindModel方法中，在command每次开始执行时得到一个包含字符串值的signal：

我的建议是，如果图片大小超过了 100KB，你可以考虑使用 WebP；而小于 100KB 时，你可以使用网页工具TinyPng 或者GUI 工具进行图片压缩。这两个工具的压缩率没有 WebP 那么高，不会改变图片压缩方式，所以解析时对性能损耗也不会增加。