JKSegueAction

prepareForSegue:sender: Hurts My Design

I’ve never really liked UIKIt’s prepareForSegue:sender: especially when I manually trigger a segue with performSegueWithIdentifier:sender:.  The problem, as I see it, is that prepareForSegue:sender: accumulates too much responsibility in preparing for a number of independent UIViewControllers.  Furthermore, the responsibility is implemented far away from where it is triggered.  I find, that this makes it hard to read, write, and maintain iOS code related to segues.

- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
	if ([segue.identifer isEqualToString:@"MySegue1"]) {
		MyCustomViewController *vc = segue.destinationViewController;
		vc.delegate = self;
 
	} else if ([segue.identifer isEqualToString:@"MySegue2"]) {
		MyOtherCustomViewController *vc = segue.destinationViewController;
		vc.someProperty = [self computeSomeValue];
 
	} else if ([segue.identifer isEqualToString:@"MySegue3"]) {
		MyCustomTableViewController *vc = segue.destinationViewController;
		vc.delegate = self;
		vc.dataSource = [self dataSourceForObject:someObject];
	}
}

Segue Action Blocks

What I would prefer is to be able to call performSegueWithIdentifier:sender: and pass it a block to use instead of prepareForSegue:sender:.  So, this would end up looking something like:

[self performSegueWithIdentifier:@"MySegue1" sender:self withBlock:^(UIStoryboardSegue *segue, id sender) {
    // Prepare the destination view controller
    MyCustomViewController *vc = segue.destinationViewController;
    vc.delegate = self;
}];

An API like this would really help the design and readability of my iOS code in the cases when I manually trigger a segue.  And it would improve the locality and readability of the code.  That is, I can see, all in one place, the segue being performed and the way that the destination UIViewController is being setup.  With the existing API on UIViewController I would have to go through the following steps to get all this information:

1. Remember the segue identifier
2. Find the implementation of prepareForSegue:sender:, if it exists.
3. Read through the whole implementation of prepareForSegue:sender: looking for MySegue
4. Look at the preparation for the destination UIViewController.
5. Find my way back to code I started reading.

But, what about the case of automatic segues?  I’d really like to be able to write an IBAction in code and then link the segue in Interface Builder to the action.  Instead, the API requires me to add another if clause to prepareForSegue:sender: matching the identifier for the segue.

JKSegueAction

I’ve written up a small category on UIViewController that implements something like the proposed API.  It has performSegueWithIdentifiersender:withBlock: as described above.  Unfortunately, I’m not able to extend Interface Builder to link segues to actions.  However, I’ve done something close.   I’ve used the segue identifier to name a selector for the action.  That means that you can create a segue in interface builder and set its identifier to something like segueToMySceneAction:sender: and then implement a method

-(void) segueToMySceneAction:(UIStoryboardSegue *)segue sender:(id)sender {
    // Prepare the destination view controller
    MyOtherCustomViewController *vc = segue.destinationViewController;
    vc.someProperty = [self computeSomeValue];
}

In this particular case the sender isn’t necessary and JKSegueAction allows you to omit it by setting the segue identifier to segueToMySceneAction: and writing the method as:

-(void) segueToMySceneAction:(UIStoryboardSegue *)segue {
    // Prepare the destination view controller
    MyOtherCustomViewController *vc = segue.destinationViewController;
    vc.someProperty = [self computeSomeValue];
}

As mentioned before a block based API is provided:

[self performSegueWithIdentifier:@"MySegue2" sender:self withBlock:^(UIStoryboardSegue *segue, id sender) {
    // Prepare the destination view controller
    MyCustomViewController *vc = segue.destinationViewController;
    vc.delegate = self;
}];

There is another block based API, setActionForSegueWithIdentifier:toBlock:, that allows the action block to be set ahead of time, for example:

- (void) viewDidLoad {
    [super viewDidLoad]
 
    [self setActionForSegueWithIdentifier:@"MySegue3" toBlock:^(id theSender) {
        // Prepare the destination view controller
        MyCustomTableViewController *vc = segue.destinationViewController;
 
        vc.delegate = self;
        vc.dataSource = [self dataSourceForObject:someObject];
    }];
}
 
- (void) someOtherMethod {
    // This will perform the segue and then invoke the action block registered in -viewDidLoad
    [self performSegueWithIdentifier:@"MySegue3" sender:self];
}

This version still allows the preparation and performance of the segue to be decoupled but it may still have its uses.  At the very least, performSegueWithIdentifier:sender:withBlock: is built on top of setActionForSegueWithIdentifier:toBlock:.

I’ve posted JKSegueActionVewController on Github.  The repository includes the UIViewContorller category, an example app and tests.  I hope you find it useful.