How to Create a Swift-like Extension to Add Constraints in Objective C

In this article, we will explore how to create an extension in Objective C that adds constraints to a UIView similar to the way you would do it with Swift.

Understanding Constraints in Objective C

Constraints are used to layout and position views within your app’s user interface. They allow you to define relationships between the edges of two or more views, such as spacing between them, aligning them horizontally or vertically, or making one view a direct child of another.

Objective C provides several classes that enable constraints to be added programmatically, including NSLayoutConstraint and its associated methods for creating constraints.

Creating Constraints Programmatically

To add constraints to a UIView programmatically, you need to create an instance of the NSLayoutConstraint class and pass it the necessary information about the constraint you want to create. This includes:

The format string that describes how the views should be arranged
An array of views that the constraint is being applied to
Any additional metrics or attributes that are needed

Here’s an example of creating a horizontal constraint between two UIViews:

// Create constraints programmatically
NSLayoutConstraint *horizontalConstraint = [NSLayoutConstraint constraintWithItem:self.view attribute:NSLayoutAttributeHorizionalToAttribute:NSLayoutAttributeMainHorizontally by:16 metric:0 length:1];
[self.view addConstraints:@[horizontalConstraint]];

However, this code will not work in Objective C as it is because you cannot pass an array of UIViews directly to the constraintsWithItem:attribute:by:metric:length: method.

Creating a Category for Constraints

One way to overcome this limitation is to create a category on UIView that adds a method called addConstraintsFormat:views:. This method takes a format string and an array of views as parameters, allowing you to programmatically add constraints between the specified views.

Here’s how you can implement this category:

Step 1: Create the category interface

Create a new file called UIView+ConstraintsFormat.h and paste the following code into it:

#import <UIKit/UIKit.h>

@interface UIView (ConstraintsFormat)

- (void)addCnstraintsFormat:(NSString *)format views:(NSArray<UIView *> *)views;

@end

Step 2: Implement the category method

Create a new file called UIView+ConstraintsFormat.m and paste the following code into it:

#import "UIView+ConstraintsFormat.h"

@implementation UIView (ConstraintsFormat)

- (void)addCnstraintsFormat:(NSString *)format views:(NSArray<UIView *> *)views {
    NSMutableDictionary *allViews = [[NSMutableDictionary alloc] init];
    for (UIView *view in views) {
        NSString *key = [NSString stringWithFormat:@"v%d", [views indexOfObject:view]];
        view.translatesAutoresizingMaskIntoConstraints = NO;
        [allViews setObject:view forKey:key];
    }
    NSArray *constraintArray = [NSLayoutConstraint constraintsWithVisualFormat:format options:0 metrics:nil views:allViews];
    [self addConstraints:constraintArray];
}

@end

Step 3: Implement the category method to handle array

As you can see, we’re using NSArray objects when calling the constraintsWithItem:attribute:by:metric:length: method. However, in Objective C, we cannot pass an array of UIViews directly to this method.

We are working around this issue by storing the view objects inside a dictionary (allViews) where each key is a string representation of the index of the corresponding view and its corresponding value is the actual view object itself. We then use NSString formatting to create the constraint format string that matches the specified order of views in our array.

Using the Category

Once you’ve implemented the category, you can add constraints to your UIView objects using the new method. Here’s an example:

// Create a UIView and add it to another UIView.
UIView *subView = [[UIView alloc] initWithFrame:CGRectZero];
[self.view addSubview:subView];

// Now we can use the addCnstraintsFormat:views: method.
[self.view addCnstraintsFormat:@"H:|[v0]|" views:@[subView]];

Example Use Cases

Constraints are useful when you need to create complex layouts in your app’s user interface. Here are a few examples of how you can use the addCnstraintsFormat:views: method:

Creating a grid layout where each cell is a UIView with different constraints applied to it.

// Create several views and add them to an array.
UIView *cell1 = [[UIView alloc] initWithFrame:CGRectZero];
UIView *cell2 = [[UIView alloc] initWithFrameCGRectMake(10, 0, 50, 30)];
UIView *cell3 = [[UIView alloc] initWithFrame:CGRectMake(60, 0, 50, 30)];

// Create an array of views.
NSArray<UIView *> *cells = @[cell1, cell2, cell3];

// Use the addCnstraintsFormat:views: method to create constraints between the cells.
[self.view addCnstraintsFormat:@"V:|-[v0]-[v1]-[v2]-|" views:cells];

Creating a layout where two views are arranged side by side.

// Create two views and add them to an array.
UIView *view1 = [[UIView alloc] initWithFrame:CGRectZero];
UIView *view2 = [[UIView alloc] initWithFrameCGRectMake(0, 0, 50, 30)];

// Use the addCnstraintsFormat:views: method to create constraints between the views.
[self.view addCnstraintsFormat:@"H:|-[v0]-[v1]|" views:@[view1, view2]];

By using this category and its addCnstraintsFormat:views: method, you can easily add constraints to your UIView objects in Objective C without having to manually create an array of NSLayoutConstraint instances.

Last modified on 2024-07-25