Understanding the iPhone’s Image View Frame Serialization

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In this article, we will delve into the world of iOS development and explore how to serialize the frame of an image view when saving its state using encodeWithCoder and initWithCoder. We will also examine why the frame size and origin may appear absurd in the console output.

Introduction

When developing iOS applications, it’s essential to save the state of UI elements, such as images, to ensure that they maintain their appearance even after the application is terminated or when the user navigates away from a view. The encodeWithCoder and initWithCoder methods are used to serialize objects and store them in a file for later retrieval.

In this article, we’ll focus on how to save and load the state of an image view, including its frame, using these methods.

Subclassing UIImageView

We will begin by creating a subclass of UIImageView called CustomImageView. This subclass will have a custom implementation for handling touches and storing its state when the application is terminated.

#import <UIKit/UIKit.h>

@interface CustomImageView : UIImageView

@property (nonatomic, assign) CGRect frame;

@end

Implementing Encode WithCoder

To store the image view’s state when it’s terminated, we need to implement the encodeWithCoder method. This method will serialize the image view’s properties, including its frame.

#import "CustomImageView.h"

@implementation CustomImageView

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    
    // Serialize the image view's frame
    CGRect frame = self.frame;
    aCoder.encodeCGRect(&frame, forKey:@"frame");
    
    // Store other properties in a dictionary
    NSDictionary *properties = @{
        @"transforms": NSValue(valueOfCGPoint:self.transform),
        @"image": self.image
    };
    [aCoder encodeObject:properties forKey:@"properties"];
}

@end

Implementing Init WithCoder

To load the image view’s state when it’s initialized, we need to implement the initWithCoder: method. This method will deserialize the image view’s properties from a file.

#import "CustomImageView.h"

@implementation CustomImageView

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    
    if (self) {
        // Deserialize the image view's frame
        CGRect frame = [aDecoder decodeCGRectForKey:@"frame"];
        self.frame = frame;
        
        // Load other properties from a dictionary
        NSDictionary *properties = [aDecoder decodeObjectForKey:@"properties"];
        NSValue *transform = properties[@"transforms"];
        self.transform = [transform CGPointValue];
        self.image = properties[@"image"];
    }
    
    return self;
}

@end

Serializing and Deserializing the Frame

Now that we’ve implemented encodeWithCoder and initWithCoder, let’s examine why the frame size and origin may appear absurd in the console output.

The issue arises when we serialize and deserialize the image view’s frame. When encoding, we create a new CGRect struct and copy its values to it. However, when decoding, we’re working with an existing CGRect struct that may have been modified after the original encoding.

To illustrate this, let’s look at an example:

CustomImageView *imageView = [[CustomImageView alloc] init];
// Set the image view's frame and transform
imageView.frame = CGRectMake(100, 200, 300, 400);
imageView.transform = CGAffineTransformScale(CGAffineTransformIdentity, 2, -1);

// Encode the image view's state
NSMutableDictionary *encodedData = [NSMutableDictionary dictionary];
[encodedData addObject:@"frame" forKey:@"frame"];
[encodedData setObject:@"properties" forKey:@"properties"];

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:encodedData requiringSecureCoding:NO];

// Decode the data back into an image view
CustomImageView *decodedImageView = [[CustomImageView alloc] initWithCoder:[NSKeyedUnarchiver unarchiveTopLevelObjectWithData:data requireSecureCoding:YES]];

In this example, we set the image view’s frame to CGRectMake(100, 200, 300, 400) and transform it using CGAffineTransformScale. We then encode the image view’s state, including its frame.

When decoding, we create a new CustomImageView instance and initialize it with the encoded data. The issue arises when we try to access the frame of the decoded image view. Since the original encoding created a new CGRect struct that was not persisted in memory, the decoded image view’s frame is set to an absurd value.

Solution

To avoid this issue, we need to serialize and deserialize the image view’s frame correctly. One way to do this is to store the frame’s bounds and center points separately.

Here’s an updated implementation of encodeWithCoder that stores the frame’s bounds and center points:

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    
    // Serialize the image view's bounds
    CGRect bounds = self.bounds;
    aCoder.encodeCGRect(&bounds, forKey:@"bounds");
    
    // Store the image view's center point
    CGPoint center = self.center;
    aCoder.encodeCGPoint(&center, forKey:@"center"];
}

And here’s an updated implementation of initWithCoder: that deserializes the frame’s bounds and center points:

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    
    if (self) {
        // Deserialize the image view's bounds
        CGRect bounds = [aDecoder decodeCGRectForKey:@"bounds"];
        self.bounds = bounds;
        
        // Load the image view's center point
        CGPoint center = [aDecoder decodeCGPointForKey:@"center"];
        self.center = center;
    }
    
    return self;
}

By storing and deserializing the frame’s bounds and center points separately, we can avoid the issue of absurd frame values.

Conclusion

In this article, we explored how to serialize and deserialize the state of an image view using encodeWithCoder and initWithCoder. We examined why the frame size and origin may appear absurd in the console output and provided a solution by storing and deserializing the frame’s bounds and center points separately.

By following these steps, you can ensure that your image views maintain their appearance even after serialization and deserialization.

Last modified on 2023-06-20