App Stops at Breakpoint with no Reason

 Published On July 20, 2015

Sometimes when you are debugging an app with an “All Exceptions” breakpoint enabled, you may get your app stops at a breakpoint without a reason. Instead of showing a exception that helps you to debug, LLDB doesn’t tell you anything about the error, and if you try to run bt you can only see something like breakpoint 1.1. And you can even continue the app and everything works fine.

Normally it’s caused by some internal assertion failure, which doesn’t print any logs while it should have. This is not really a Swift issue, but since it’s more likely to be seen in Swift since it’s not as mature as Objective-C. In this case, you will have to use your imagination to solve the problem.

In my case, my app stopped at setValue:forKey. I thought it was caused by invalid key or object, the log printed them correctlly:

LF.log("key", key)
LF.log("value", object[key])
self.setValue(object[key], forKey:key)

The reason was pretty stupid: the self object I set was immutable. Obviously you need to make it mutable to call setValue:forKey, but the thing is that firstly there should be some named exception raised, and secondly, after continue it should crash anyway. As this problem has been spotted in Swift 1.2, it can be considered as an existing Syntax Trap, which means something doesn’t work as the way it should. Since Swift is still a new language, I’ll write blogs about it as I discover them and report them to Apple in hope that they can be fixed in the next release.

Other Syntax Traps

I’m listing some other syntax traps I found here just for fun.

Update 1: Expression too Complex

While I’m working on elastic search API, the following code results an error: “Expression was too complex to be solved in reasonable time; consider breaking up the expression into distinct sub-expressions”:

let query_bool = [

As it’s said, breaking down the expression fixes the problem:

let query_must = [["term":["approval":"confirmed"]],["filtered":["filter":["exists":["field":"id"]]]]]
let query_should = [["match_phrase_prefix":["name":query]],["match":["name":["query":query,"operator":"and"]]],["match":["_all":["query":query,"operator":"and"]]]]
let query_bool = [

It’s not really a “trap” since the error message is quite clear. However, it’s interesting for us to guess how Swift compiler works when it’s dealing with dictionary expressions. Anyway, it’s not THAT complex :)

Update 2: Failed table.dequeueReusableCellWithIdentifier

In short, if you copy-paste a UITableViewCell and change the class name and reuse identifiler correctly, but forgot to remove some IBOutlet, table.dequeueReusableCellWithIdentifier will fail silently. If a global exception breakpoint has been set, it stops running without telling you anything.

Tags: Swift Breakpoint Exception Debug LLDB


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