What is an umbrella header?

What is basically an umbrella header? What is its use? I got a warning as shown below. What does this mean?

<module-includes>:1:1: warning: umbrella header for module 'XCTest' does not include header 'XCTextCase+AsynchronousTesting.h' [-Wincomplete-umbrella]
#import "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCTest.h"

The umbrella header is the 'master' header file for a framework. Its use is that you can write

#import <UIKit/UIKit.h>

instead of

#import <UIKit/UIViewController.h>
#import <UIKit/UILabel.h>
#import <UIKit/UIButton.h>
#import <UIKit/UIDatePicker.h>

and so on.

For me, <XCTest/XCTestCase+AsynchronousTesting.h> is included in <XCTest/XCTest.h>. Maybe it is not for you? In that case, add the

#import <XCTest/XCTestCase+AsynchronousTesting.h>

manually.

umbrella header - iOS framework or library on Objective-C or Swift can have a header file that contains references to all the other headers in that project.

When you create a framework target Xcode will automatically generate <targer_name.h> file. It should has the same name as PRODUCT_NAME

For example <umbrella_name.h> looks like

#import "header_1.h"
#import "header_2.h"

or:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

//! Project version number for SomeModule.
FOUNDATION_EXPORT double SomeModuleVersionNumber;

//! Project version string for SomeModule.
FOUNDATION_EXPORT const unsigned char SomeModuleVersionString[];

// In this header, you should import all the public headers of your framework using statements like #import <SomeModule/PublicHeader.h>

As a result you can use the next syntax

#import <umbrella_name.h>

instead of

#import <header_1.h>
#import <header_2.h>

On practice when you create a Framework on Objective-C^[Example] or Swift^[Example] this file will be created automatically with <product_name>

Using

1.umbrella header is required by [.modulemap] structure to use module(expose Objective-C headers) for Objective-C or Swift

2.umbrella header can be just used by Objective-C consumer for connivance

3.umbrella header helps to import every Objective-C header into Swift module that is why you can just skip import

>Swift sees every header you expose publicly in your umbrella header. The contents of the Objective-C files in that framework are automatically available from any Swift file within that framework target, with no import statements. Use classes and other declarations from your Objective-C code with the same Swift syntax you use for system classes.

Importing Objective-C into Swift within a Framework Target