Using the View Hierarchy and Console for easier debugging

At some point in life, you are gonna work with a codebase that you didn't write from the very beginning. Specially if you are maintaing it, one of the biggest questions often is 'where to start looking in order to fix bug Y?'.

Most of the times, there will be a strong relationship with what you see in the screen and the code you need to get fixed. For those cases, I've wrote three strategies that involve the Xcode's 'view hierarchy debug' and the console. I've used them often and they have proved handy, so I hope they can be of help for you.

For this guide I'm using Xcode's view hierarchy debug, a knock-off of an original app, Reveal. If you haven't tried it, I really recomend that you download Reveal's trial and that you ask your company / boss / parents to buy it since it makes debugging so much less painful.

1. Find the delegate and dataSource of a UITableView

  1. Get the memory address of the UITableView you want to debug. If you are using the debug view hierarchy tool from Xcode, you will find the address on the top of the Object Inspector: Memory Address of the cell in the object inspector In this example, we observe the address 0x7fc4ddf403c0 for the UITableView.

  2. Knowing this address we can simply print its delegate or dataSource in Xcode's Terminal:

    po [0x7fc4ddf403c0 delegate]
    

    Memory address and class of the UITableView's delegate

    Aha! We got the delegate and datasource for this UITableView and we know where we need to start adding breakpoints!

2. Find the dataSource and delegate of a UICollectionViewCell

If you can get the memory address of a UICollectionViewCell, you can get its UICollectionView and then find its dataSource and delegate.

  1. Start by finding the address of the UICollectionViewCell by using the debug view hierarcy tool. In this example 0x7f83068767f0.

  2. Then, look for the nextResponder of it.

    po [0x7f83068767f0 nextResponder]
    

    UICollectionView print

  3. With the address of the UICollectionView now we simply print its dataSource and delegate like we did for the UITableView.

    po [[0x7f83068767f0 nextResponder] dataSource]
    

    UICollectionView delegate print

3. Find the method called by a UIButton (or UIControl)

The 'Debug View Hierarcy' in Xcode, can tell us the class name of a button's target. This is very helpful, but perhaps you really want to nail down the name of the method that is being called when you press that button.

  1. First, get the memory address of the UIButton (or UIControl) you want to debug.

  2. Then, print the targets of the button.

    po [0x7fc3277025d0 allTargets]
    

    This will print all the targets of your UIControl with their memory addresses.

    Memory addresses and class of the button's targets

  3. We got the target of this UIControl, now we can get the selector being called during the control event. Here I'm looking for UIControlEventTouchUpInside. Enum values are not defined in the console so we will need to use the actual numeric value for UIControlEventTouchUpInside, 64.

    po [0x7fc3277025d0 actionsForTarget:0x7fc322e99070 forControlEvent:64]
    

    This command will print an array with selector(s) we are looking for. In this example, the selectors for the event userDidTouchUpInsideFeedbackButton in the button.

    Selectors for `userDidTouchUpInsideFeedbackButton`

    Now we narrowed the event of this control down to the method and class.

Happy debugging!