Cocoa: Problem Passing a CGPoint with NSNotification and NSDictionary

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As a developer working on iPhone applications, we often encounter issues when dealing with notifications and dictionaries. In this article, we will explore the problem of passing a CGPoint value in a dictionary using NSNotifications and provide solutions to resolve this issue.

Introduction

In Cocoa, NSNotifications are used to notify objects about specific events or changes in the application’s state. The userInfo parameter of the postNotification: method allows us to pass additional data with the notification, such as custom values like CGPoint. However, when working with CGPoint, we often encounter compatibility issues that prevent us from passing it directly into a dictionary.

Problem Description

The problem arises because CGPoint is not compatible with dictionaries. When we try to add a CGPoint value to a dictionary using the initWithObjectsAndKeys: method, Xcode throws an error indicating that the initializer is incompatible.

### The Error

Error: Incompatible type for argument 1 of "Value With Point"

Solution 1: Using NSValue

One solution to this problem is to use NSValue instead of CGPoint. NSValue provides a way to store and retrieve value objects, including points.

### Using NSValue

-(void)setPosition:(CGPoint)point
{
    // Create an NSValue object with the point
    NSValue *pointAsObject = [NSValue valueWithPoint:point];
    
    // Add the point to a dictionary
    NSDictionary *dict = [[NSDictionary alloc] 
                         initWithObjectsAndKeys:@"sp", 
                         pointAsObject, 
                         nil];

    [[NSNotificationCenter defaultCenter] 
    postNotificationName:@"SpriteDidSetPosition" 
    object:self 
    userInfo:dict];

    [super setPosition:point];
}

However, this solution has a drawback. When we retrieve the CGPoint value from the dictionary using [notification userInfo] valueForKey:@"sp"], it will throw an error because NSValue is not compatible with the value type expected by valueForKey.

### Error

Error: 'NSValue' object cannot be converted to the required type

Solution 2: Using NSPoint

Another solution is to use NSPoint instead of CGPoint. NSPoint provides a similar set of methods for working with points, but it is not compatible with dictionaries either.

### Using NSPoint

-(void)setPosition:(NSPoint)point
{
    // Create an NSValue object with the point
    NSValue *pointAsObject = [NSValue valueWithPoint:point];
    
    // Add the point to a dictionary
    NSDictionary *dict = [[NSDictionary alloc] init];
    [dict initWithObjectsAndKeys:@"sp", pointAsObject, nil];

    [[NSNotificationCenter defaultCenter] 
    postNotificationName:@"SpriteDidSetPosition" 
    object:self 
    userInfo:dict];

    [super setPosition:NSPointMake(point.x, point.y)];
}

However, this solution also has a drawback. When we retrieve the NSPoint value from the dictionary using [notification userInfo] valueForKey:@"sp"], it will throw an error because NSValue is not compatible with the value type expected by valueForKey.

Solution 3: Using CGFloat

A third solution is to use CGFloat values instead of CGPoint. CGFloat provides a way to store floating-point values that can be used in place of CGPoint.

### Using CGFloat

-(void)setPosition:(CGPoint)point
{
    // Create two CGFloat values for the point's x and y coordinates
    CGFloat x = point.x;
    CGFloat y = point.y;

    // Add the points to a dictionary
    NSDictionary *dict = [[NSDictionary alloc] 
                         initWithObjectsAndKeys:@"spX", @(x), @"spY", @(y), nil];

    [[NSNotificationCenter defaultCenter] 
    postNotificationName:@"SpriteDidSetPosition" 
    object:self 
    userInfo:dict];

    [super setPosition:point];
}

When we retrieve the CGFloat values from the dictionary using [notification userInfo] valueForKey:@"spX"], we can access the x and y coordinates separately.

### Retrieving CGFloat Values

-(void) setViewPointCenter:(NSNotification *)notification 
{
    // Get the x coordinate value
    CGFloat x = [[notification userInfo] valueForKey:@"spX"];
    
    // Use the x coordinate to update your view...
}

Conclusion

Passing a CGPoint value in a dictionary using NSNotifications can be challenging. However, by using NSValue, NSPoint, or CGFloat values, we can work around this issue and successfully pass custom data with our notifications.

In conclusion, when working with Cocoa and NSNotifications, it’s essential to understand the compatibility issues between different value types and dictionaries. By choosing the right solution for your specific use case, you can effectively communicate with other parts of your application or even with other developers.

References

Last modified on 2025-03-06