結合載入檢視使用

[RACObserve(self.viewModel, loading) subscribeNext:^(NSNumber *loading){
        if (loading.boolValue) {
            [SVProgressHUD show];
        } else {
            [SVProgressHUD dismiss];
        }
    }];
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繫結loading屬性，YES的時候載入檢視出現，NO的時候家在檢視消失。

-(void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    
    self.viewModel.active = YES;
}

-(void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    
    self.viewModel.active = NO;
}
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有active屬性； 使用這個屬性需要Model繼承自RVMViewModel。

@weakify(self);
    [self.didBecomeActiveSignal subscribeNext:^(id x) {
        @strongify(self);
        [self downloadPhotoModelDetails];
    }];



-(void)downloadPhotoModelDetails {
    self.loading = YES;
    
    @weakify(self);
    [[FRPPhotoImporter fetchPhotoDetails:self.model] subscribeError:^(NSError *error) {
        NSLog(@"Could not fetch photo details: %@", error);
    } completed:^{
        @strongify(self);
        self.loading = NO;
        NSLog(@"Fetched photo details.");
    }];
}
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