How to set a conditional breakpoint in Xcode based on an object string property?

Question

I'm looking to be able to have the debugger break when it reaches a particular string match. As an example, I might have something like this:

Foo myObj = [self gimmeObj];

myObj might have a property called name. I want the debugger to stop on the assignment when

[myObj.name isEqualToString:@"Bar"];

How can I set my conditional breakpoint in Xcode to do that?

Overdraw

Answer 1

You can set a conditional break point in Xcode by setting the breakpoint normally, then control-click on it and select Edit Breakpoint (choose Run -> Show -> Breakpoints).

In the breakpoint entry, there is a Condition column.

Now, there are several issues to keep in mind for the condition. Firstly, gdb does not understand dot syntax, so instead of myObj.name, you must use [myObj name] (unless name is an ivar).

Next, as with most expressions in gdb, you must tell it the type of return result, namely "BOOL". So set a condition like:

(BOOL)[[myObj name] isEqualToString:@"Bar"]

Often it is actually easier to just do this in code by temporarily adding code like:

if ( [myObj.name isEqualToString:@"Bar"] ) {
    NSLog( @"here" );
}

and then setting the break point on the NSLog. Then your condition can be arbitrarily complex without having to worry about what gdb can and can't parse.

Joggle

Except that by altering your code you run the risk of forgetting to remove your logging or altering behaviour – Sinking

That's true. I often mitigate this by adding "NYI" (Not Yet Implemented) to the string, and then my pre-release check search for NYI will catch it. – Joggle

To get this working I had to make (bool) uppercase as (BOOL), probably an LLDB thing. – Overstock

bool didn't work for me in GDB, I had to use BOOL or int– the difference is explained here https://mcmap.net/q/127319/-objective-c-bool-vs-bool. – Foxtail

I've found that putting it in code is a lot better. I had a for loop with a breakpoint stopping at a certain case of the for loop, it took 2 minutes before it finally stopped at the breakpoint. Setting it in code is a lot faster. – Mercaptide

Symbolic links in the debugger seems more appropriate. – Dever

You can't put it in code if you have a once-every 200 game bug which has finally come up, and now you need to do a conditional breakpoint. Stopping the program to alter the code is not an option. – Bulbul

I found that I did not even have to add the NSLog placeholder in C++ – Comportment

sleep(0) is a great ad-hoc break point (NSLog(@"") is more complex) – Nephrolith

I had to recompile and re-run the app to get the condition to work. – Unthankful

@justin-vallely yes, for in-application debugging lines like I mention, you have to rebuild and rerun your application, but it does mean you can use any code to set the condition. – Joggle

Answer 2

Here is how you do using XCode lldb conditional breakpoints.

First, double click the breakpoint (or right-click edit breakpoint), you can see a dialogue popup.

Updated 2021-04-22 for Xcode 12:

Here is what these options means:

1. Condition: The breakpoint will only fire under this condition.
2. Ignore: The amount of times the condition needs to meet before fire the breakpoint
3. Action: Action that runs after the breakpoint breaks.
4. Options: Automatically continue after evaluating actions

Here is a summary. For the above example in the image, it means that when the variable testedString is equal to "Testing", break here. If I add ignore time to 1, then it will ignore the first time when testedString is equal to "Testing" and break at the second time the condition is met.

For actions, when you press add actions, there will be a list of choices. Usually what I do is to use the Debugger Command po to print variables that I need to check and I believe that there are better ways using the actions than I do.

It seems that you have to recompile and run the app if you change the conditions at runtime

Tisdale

Probably because the question was about stopping at breakpoint based on a string value [I wasn't the down-voter] – Lavolta

Thanks, quite helpful. This answer deserves more votes. – Dodger

Updated the above example to break based on a string value now with Xcode 12 screenshot. – Tisdale

Answer 3

I'm not sure if this will work, but you can try setting a breakpoint at that line of code, open up the debugger console (Cmd+Shift+R), and type

condition N (int)[[myObj name] isEqualToString:@"Bar"]

Where N is replaced by the number of the breakpoint (an integer).

Procathedral

Answer 4

If you mutate myObj.name using the setter, you can add a symbolic breakpoint on -[MyObjClass setName:] either from the Debugger Console or from the Run->Manage Breakpoints->Add Symbolic Breakpoint menu in Xcode. If not (why not? you probably shouldn't be modifying the instance variable directly except in the designated initializer or dealloc) you can set a watchpoint in gdb (use the Debugger Console in Xcode once the debugger is running). This page explains how. I don't believe Xcode exposes a UI for setting watchpoints without using the Debugger Console.

Forfar

Answer 5

At times when working with Frameworks (debug builds) and need to put a breakpoint in certain file/location that is hard to navigate or isn't exposed publically in framework under development. One option is to write a helper class to trigger conditional breakpoints & make step-in/step-out easier.

- (void)invokeFrameworkMethod {
    ...
    [DebugConditionalBreakPointHelper breakPointCondition:YES comment:@"from invokeFrameworkMethod."];
    ...
}

Header declaration in framework under development.

#import <Foundation/Foundation.h>

@interface DebugConditionalBreakPointHelper : NSObject
+ (void)breakPointCondition:(BOOL)enabled comment:(NSString *)comment;
@end

And implementation file:

#import "DebugConditionalBreakPointHelper.h"

@implementation DebugConditionalBreakPointHelper
+ (void)breakPointCondition:(BOOL)enabled comment:(NSString *)comment {
    if (enabled)
    {
        NSLog(@"Triggerred Conditional Break Point. Comment: %@");
    }
}
@end

Godiva