Understanding the Crash Report and Debugging Techniques

Introduction

As a developer, receiving a crash report can be frustrating, especially when trying to diagnose issues with complex systems. In this article, we’ll delve into the details of the provided stacktrace and explore possible solutions using debugging techniques.

The Stacktrace

The provided stacktrace shows that an exception occurred in the ForthViewController class:

2016-11-29 11:57:44.987 Wellness_24x7[1400:46606] -[__NSCFNumber isEqualToString:]: unrecognized selector sent to instance 0x7a67d160
2016-11-29 11:57:45.010 Wellness_24x7[1400:46606] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber isEqualToString:]: unrecognized selector sent to instance 0x7a67d160'

Identifying the Problem

The stacktrace points to an NSInvalidArgumentException with a message indicating that an unrecognized selector was called on an instance. The selector is isEqualToString: and it’s being called on an object of type __NSCFNumber. This suggests that there’s an issue with data types or compatibility between objects.

Debugging Techniques

To debug this issue, we’ll use the following techniques:

1. Inspecting Variables: We’ll examine the values of variables involved in the crash.
2. Print Statements: We’ll add print statements to inspect variable values during runtime.
3. Console Logging: We’ll enable console logging to see output during app execution.

Step 1: Inspecting Variables

Let’s start by looking at the line where the crash occurs:

cell.textLabel.text = [menuItems objectAtIndex:indexPath.row];

We notice that menuItems is an array of objects. When we access its elements using [menuItems objectAtIndex:indexPath.row], we might encounter issues if those objects are not NSStrings.

Step 2: Adding Print Statements

To inspect variable values during runtime, let’s add print statements to the code:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    static NSString *simpleTableIdentifier = @"cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:simpleTableIdentifier];

    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:simpleTableIdentifier];
    }

    NSLog(@"menuItems count: %lu", (unsigned long)[menuItems count]);
    NSLog(@"menuItems objectAtIndex:indexPath.row: %@", [menuItems objectAtIndex:indexPath.row]);

    if ([menuItems objectAtIndex:indexPath.row] isKindOfClass:[NSString class]]) {
        cell.textLabel.text = [(NSString *)[menuItems objectAtIndex:indexPath.row] copy];
    } else {
        cell.textLabel.text = @"Unknown";
    }

    return cell;
}

Step 3: Console Logging

To see output during app execution, let’s enable console logging by modifying the Xcode launch settings:

• Open the Xcode project
• Select the target (e.g., “Wellness_24x7”)
• Go to the Build Settings
• Find the Other LDFLAGS setting and change it to -g
• Quit Xcode

Now, when you run your app in the simulator or on a physical device, it will print information about variable values during runtime.

Step 4: Analyzing Output

After adding the print statements and console logging, examine the output for clues:

menuItems count: 2
menuItems objectAtIndex:indexPath.row: <__NSCFNumber: 0x7a67d1a0> {
    count = 0,
    value = 0,
    _isa = __NSCFNumber,
}

The menuItems array contains a single element, which is an instance of __NSCFNumber. This explains the crash – we can’t call isEqualToString: on this object.

Step 5: Solving the Problem

To fix the issue, you need to convert the __NSCFNumber object to a string. In Objective-C, __NSCFNumber is an alias for NSNumber. You can use the stringValue property to get its string representation:

cell.textLabel.text = [[(NSNumber *)[menuItems objectAtIndex:indexPath.row] stringValue] copy];

Conclusion

In this article, we’ve used debugging techniques to understand the crash report and identify the root cause of the issue. By examining variable values, adding print statements, enabling console logging, analyzing output, and applying knowledge about Objective-C data types, we were able to determine that the problem lay in an incompatible data type between objects.

We fixed the issue by converting __NSCFNumber to a string using its stringValue property.

Final Code

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    static NSString *simpleTableIdentifier = @"cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:simpleTableIdentifier];

    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:simpleTableIdentifier];
    }

    NSLog(@"menuItems count: %lu", (unsigned long)[menuItems count]);
    NSLog(@"menuItems objectAtIndex:indexPath.row: %@", [menuItems objectAtIndex:indexPath.row]);

    if ([menuItems objectAtIndex:indexPath.row] isKindOfClass:[NSString class]]) {
        cell.textLabel.text = [(NSString *)[menuItems objectAtIndex:indexPath.row] copy];
    } else if ([menuItems objectAtIndex:indexPath.row] isKindOfClass:[NSNumber class]]) {
        cell.textLabel.text = [[(NSNumber *)[menuItems objectAtIndex:indexPath.row] stringValue] copy];
    } else {
        cell.textLabel.text = @"Unknown";
    }

    return cell;
}

By understanding the crash report, using debugging techniques, and applying knowledge about Objective-C data types, we were able to diagnose and fix the issue.

Last modified on 2024-05-01