[UML Class relationships] Phân biệt quan hệ Aggregation và Composition trong Objective C và Swift

Aggregation

ClassA có dùng lớp và giữa ClassB nhưng không cho lớp khác truy xuất ClassB thông qua ClassA.  Trong Objective C ta dùng thuộc tính là weak để thể hiện mối quan hệ này, code ví dụ như sau:

	#import <Foundation/Foundation.h>
	#import "ObjCAggregation_ClassB.h"
	@interface ObjCAggregation_ClassA : NSObject
	- (instancetype)initWithObject:(ObjCAggregation_ClassB*)inputedObject;
	- (BOOL)doAction;
	@end

view raw AggregationAssociation_h hosted with ❤ by GitHub

	#import "ObjCAggregation_ClassA.h"
	@interface ObjCAggregation_ClassA () {
	__weak ObjCAggregation_ClassB* refInstance;
	}
	@end
	@implementation ObjCAggregation_ClassA
	@synthesize classB;
	- (instancetype)initWithObject:(ObjCAggregation_ClassB*)inputedObject {
	if (self) {
	refInstance = inputedObject;
	}
	return self;
	}
	- (void)dealloc {
	NSLog(@"[Aggregation] dealloc");
	}
	- (BOOL)doAction {
	NSLog(@"[Aggregation] Action A");
	if (refInstance) {
	[refInstance doAction];
	return YES;
	}
	return NO;
	}
	@end

view raw AggregationAssociation_m hosted with ❤ by GitHub

Ngoài ra ta còn có sơ đồ Aggregation - Navigability:

Theo sơ đồ này thì ta dùng property để lớp khác có thể truy xuất ClassB thông qua ClassA.
Code ví dụ trong Objective C như:

	#import <Foundation/Foundation.h>
	#import "ObjCAggregation_ClassB.h"
	@interface ObjCAggregation_ClassA : NSObject
	@property (nonatomic, weak) ObjCAggregation_ClassB* classB;
	- (instancetype)initWithObject:(ObjCAggregation_ClassB*)inputedObject;
	- (BOOL)doAction;
	@end

view raw AggregationNavigability_h hosted with ❤ by GitHub

	#import "ObjCAggregation_ClassA.h"
	@implementation ObjCAggregation_ClassA
	@synthesize classB;
	- (instancetype)initWithObject:(ObjCAggregation_ClassB*)inputedObject {
	if (self) {
	classB = inputedObject;
	}
	return self;
	}
	- (void)dealloc {
	NSLog(@"[Aggregation] dealloc");
	}
	- (BOOL)doAction {
	NSLog(@"[Aggregation] Action A");
	if (classB) {
	[classB doAction];
	return YES;
	}
	return NO;
	}
	@end

view raw AggregationNavigability_m hosted with ❤ by GitHub

Unit Test giả sử ta huỷ đối tượng ClassB thì ClassA sẽ không gọi hàm trong ClassB được, do đó trong sơ đồ này nên dùng if để kiểm tra đối tượng khi truyền vào để tránh bị crash ứng dụng:

	/**
	* Todo list:
	// Aggregation
	* - Test aggregation, should return true.
	* - Test aggregation, should return false, when release reference object.
	* - Test aggregation, should return true.
	* - Test aggregation, should return false, when release reference object.
	**/
	- (void)testAggregation1 {
	//GIVEN
	ObjCAggregation_ClassB* objB = [[ObjCAggregation_ClassB alloc] init];
	ObjCAggregation_ClassA* objA = [[ObjCAggregation_ClassA alloc] initWithObject:objB];
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}
	- (void)testAggregation2 {
	//GIVEN
	ObjCAggregation_ClassB* objB = [[ObjCAggregation_ClassB alloc] init];
	ObjCAggregation_ClassA* objA = [[ObjCAggregation_ClassA alloc] initWithObject:objB];
	objB = nil;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertFalse(result);
	}
	- (void)testAggregation3 {
	//GIVEN
	ObjCAggregation_ClassB* objB = [[ObjCAggregation_ClassB alloc] init];
	ObjCAggregation_ClassA* objA = [[ObjCAggregation_ClassA alloc] init];
	objA.classB = objB;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}
	- (void)testAggregation4 {
	//GIVEN
	ObjCAggregation_ClassB* objB = [[ObjCAggregation_ClassB alloc] init];
	ObjCAggregation_ClassA* objA = [[ObjCAggregation_ClassA alloc] init];
	objA.classB = objB;
	objB = nil;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertFalse(result);
	}

view raw AggregationNavigabilityUnitTest hosted with ❤ by GitHub

Code ví dụ trong Swift như sau:

	class SwiftAggregation_ClassA {
	weak var classB:SwiftAggregation_ClassB?
	init() {
	}
	init(refClass: SwiftAggregation_ClassB) {
	self.classB = refClass
	}
	func doAction() -> Bool {
	print("Do action A")
	if ((classB) != nil) {
	classB?.doAction()
	return true
	}
	return false
	}
	}
	class SwiftAggregation_ClassB {
	func doAction() {
	print("Do action B")
	}
	}

view raw SwiftAggregation hosted with ❤ by GitHub

	func testAggregation1() {
	//GIVEN
	var objB:SwiftAggregation_ClassB! = SwiftAggregation_ClassB()
	let objA:SwiftAggregation_ClassA = SwiftAggregation_ClassA(refClass: objB)
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}
	func testAggregation2() {
	//GIVEN
	var objB:SwiftAggregation_ClassB! = SwiftAggregation_ClassB()
	let objA:SwiftAggregation_ClassA = SwiftAggregation_ClassA(refClass: objB)
	objB = nil
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertFalse(result)
	}
	func testAggregation3() {
	//GIVEN
	var objB:SwiftAggregation_ClassB! = SwiftAggregation_ClassB()
	let objA:SwiftAggregation_ClassA = SwiftAggregation_ClassA()
	objA.classB = objB
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}
	func testAggregation4() {
	//GIVEN
	var objB:SwiftAggregation_ClassB! = SwiftAggregation_ClassB()
	let objA:SwiftAggregation_ClassA = SwiftAggregation_ClassA()
	objA.classB = objB
	objB = nil
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertFalse(result)
	}

view raw SwiftAggregationTests hosted with ❤ by GitHub

Composition

Composition giống như Aggregation nhưng giữ đối tượng theo kiểu strong, thì sẽ không phụ thuộc vào đối tượng truyền vào.

Giống code ở trên nhưng ta thay weak thành strong thì sẽ thể hiện mối quan hệ này, ví dụ như sau:

	#import <Foundation/Foundation.h>
	#import "ObjCComposition_ClassB.h"
	@interface ObjCComposition_ClassA : NSObject
	@property (nonatomic, strong) ObjCComposition_ClassB* classB;
	- (instancetype)initWithObject:(ObjCComposition_ClassB*)inputedObject;
	- (BOOL)doAction;
	@end

view raw CompositionNavigability_h hosted with ❤ by GitHub

	#import "ObjCComposition_ClassA.h"
	@implementation ObjCComposition_ClassA
	@synthesize classB;
	- (instancetype)initWithObject:(ObjCComposition_ClassB*)inputedObject {
	if (self) {
	classB = inputedObject;
	}
	return self;
	}
	- (void)dealloc {
	NSLog(@"[Composition] dealloc");
	}
	- (BOOL)doAction {
	NSLog(@"[Composition] Action A");
	if (classB) {
	[classB doAction];
	return YES;
	}
	return NO;
	}
	@end

view raw CompositionNavigability_m hosted with ❤ by GitHub

Cũng viết UnitTest như trên nhưng kết quả khi huỷ object tham chiếu thì đối tượng đó vẫn được giữ lại để thực hiện phương thức:

	/**
	* Todo list:
	// Composition
	* - Test composition, should return true.
	* - Test composition, should return true, when release reference object.
	* - Test composition, should return true.
	* - Test composition, should return true, when release reference object.
	*/
	- (void)testComposition1 {
	//GIVEN
	ObjCComposition_ClassB* objB = [[ObjCComposition_ClassB alloc] init];
	ObjCComposition_ClassA* objA = [[ObjCComposition_ClassA alloc] initWithObject:objB];
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}
	- (void)testComposition2 {
	//GIVEN
	ObjCComposition_ClassB* objB = [[ObjCComposition_ClassB alloc] init];
	ObjCComposition_ClassA* objA = [[ObjCComposition_ClassA alloc] initWithObject:objB];
	objB = nil;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}
	- (void)testComposition3 {
	//GIVEN
	ObjCComposition_ClassB* objB = [[ObjCComposition_ClassB alloc] init];
	ObjCComposition_ClassA* objA = [[ObjCComposition_ClassA alloc] init];
	objA.classB = objB;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}
	- (void)testComposition4 {
	//GIVEN
	ObjCComposition_ClassB* objB = [[ObjCComposition_ClassB alloc] init];
	ObjCComposition_ClassA* objA = [[ObjCComposition_ClassA alloc] init];
	objA.classB = objB;
	objB = nil;
	//WHEN
	BOOL result = [objA doAction];
	//THEN
	XCTAssertTrue(result);
	}

view raw CompositionNavigabilityUnitTest hosted with ❤ by GitHub

Code ví dụ trong Swift như sau:

	class SwiftComposition_ClassA {
	var classB:SwiftComposition_ClassB? //Default: strong reference
	init() {
	}
	init(refClass: SwiftComposition_ClassB) {
	self.classB = refClass
	}
	func doAction() -> Bool {
	print("Do action A")
	if ((classB) != nil) {
	classB?.doAction()
	return true
	}
	return false
	}
	}
	class SwiftComposition_ClassB {
	func doAction() {
	print("Do action B")
	}
	}

view raw SwiftComposition hosted with ❤ by GitHub

	func testComposition1() {
	//GIVEN
	var objB:SwiftComposition_ClassB! = SwiftComposition_ClassB()
	let objA:SwiftComposition_ClassA = SwiftComposition_ClassA(refClass: objB)
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}
	func testComposition2() {
	//GIVEN
	var objB:SwiftComposition_ClassB! = SwiftComposition_ClassB()
	let objA:SwiftComposition_ClassA = SwiftComposition_ClassA(refClass: objB)
	objB = nil
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}
	func testComposition3() {
	//GIVEN
	var objB:SwiftComposition_ClassB! = SwiftComposition_ClassB()
	let objA:SwiftComposition_ClassA = SwiftComposition_ClassA()
	objA.classB = objB
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}
	func testComposition4() {
	//GIVEN
	var objB:SwiftComposition_ClassB! = SwiftComposition_ClassB()
	let objA:SwiftComposition_ClassA = SwiftComposition_ClassA()
	objA.classB = objB
	objB = nil
	//WHEN
	let result:Bool = objA.doAction()
	//THEN
	XCTAssertTrue(result)
	}

view raw SwiftCompositionTests hosted with ❤ by GitHub

Tài liệu tham khảo:

1. Stackoverflow