RACCommand_pullback_example.m

// RACCommand α β === Kleisli RACSignal α β === α -> RACSignal β
// We can pullback along the kleisli arrow to change the input to a command.
// Likewise, we can pushout along the arrow to change the output to a command.


RACSignal *validationSignal = self.form.validationSignal.replayLast;
UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:nil action:nil];
doneButton.rac_command = [self.viewModel.doneCommand ys_pullback:^RACSignal *(id input) {
  return [validationSignal take:1];
}];

// There are two alternate approaches. The first one I did was to make a
// dummy command with `RACSignal.empty` as its signal block, and then
// sample the proper input from its execution signal, and then use that
// to execute the real command which requires input. This is a bit
// messy.

RACCommand *doneCommand = self.viewModel.doneCommand;
RACCommand *dummyCommand = [[RACCommand alloc] initWithEnabled:doneCommand.enabled signalBlock:^RACSignal *(id input) {
  return [RACSignal empty];
}];
[[self.form.validationSignal
  sample:dummyCommand.executionSignals] 
  subscribeNext:^(id value) {
    [doneCommand execute:value];
  }];

UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:nil action:nil];
doneButton.rac_command = doneCommand;

// Another option is to do basically the same thing as the pullback, but
// inline without any help:

RACSignal *validationSignal = self.form.validationSignal.replayLast;
UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:nil action:nil];
RACCommand *doneCommand = self.viewModel.doneCommand;
doneButton.rac_command = [[RACCommand alloc] initWithEnabled:doneCommand.enabled signalBlock:^RACSignal *(id input) {
  return [[validationSignal take:1] flattenMap:^RACSignal *(id validatedData) {
    return [doneCommand execute:validatedData];
  }];
}];