Protocol composition in Swift - Step-by-Step Execution

Concept Flow - Protocol composition

Define Protocol A

↓

Define Protocol B

↓

Create variable with Protocol A & B

↓

Assign instance conforming to both

↓

Use combined protocol methods/properties

Protocol composition lets you combine multiple protocols into one type requirement, so a variable must conform to all listed protocols.

Execution Sample

Swift

protocol A {
    func foo()
}
protocol B {
    func bar()
}
struct S: A, B {
    func foo() { print("foo") }
    func bar() { print("bar") }
}
let x: A & B = S()
x.foo()
x.bar()

This code defines two protocols A and B, a struct S that conforms to both, then uses a variable x typed as A & B to call both methods.

Execution Table

Step	Action	Evaluation	Result
1	Define protocol A with method foo()	protocol A created	A protocol exists
2	Define protocol B with method bar()	protocol B created	B protocol exists
3	Define struct S conforming to A and B	S has foo() and bar()	S type ready
4	Create variable x of type A & B	x can hold any instance conforming to both	x declared
5	Assign S() instance to x	S conforms to A and B	x = S()
6	Call x.foo()	Calls foo() from S	Output: foo
7	Call x.bar()	Calls bar() from S	Output: bar
8	End of execution	No more code	Program ends

💡 All steps executed; program ends after calling both methods on x

Variable Tracker

Variable	Start	After Step 5	After Step 6	After Step 7	Final
x	undefined	S instance assigned	S instance (foo called)	S instance (bar called)	S instance

Key Moments - 3 Insights

Why can variable x call both foo() and bar() methods?

Because x is declared as type A & B, it must conform to both protocols, so it can call methods from both. See execution_table steps 4-7.

Can x hold an instance that conforms to only one protocol?

No, x requires an instance conforming to both A and B protocols. Step 5 shows assignment of S which conforms to both.

What happens if struct S only conforms to one protocol?

Assignment to x would fail because x expects both protocols. This is why S must conform to both, as in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of variable x after step 5?

A An instance conforming only to protocol A

B Undefined

C An instance of struct S

D An instance conforming only to protocol B

💡 Hint

Check variable_tracker column 'After Step 5' and execution_table step 5

At which step does the program call the bar() method?

A Step 7

B Step 6

C Step 4

D Step 3

💡 Hint

Look at execution_table rows describing method calls

If struct S did not conform to protocol B, what would happen at step 5?

A Assignment would succeed

B Assignment to x would fail

C x could call bar() method

D Program would print 'bar'

💡 Hint

Refer to key_moments about protocol conformance and execution_table step 5

Concept Snapshot

Protocol composition syntax: let x: ProtocolA & ProtocolB
- Variable x must conform to all protocols combined
- Allows calling methods/properties from all protocols
- Useful to require multiple behaviors in one variable

Full Transcript

This example shows how to combine two protocols A and B using protocol composition in Swift. We define protocols A and B, each with one method. Then we create a struct S that conforms to both protocols by implementing both methods. We declare a variable x with type A & B, meaning it must conform to both protocols. We assign an instance of S to x. Then we call both methods foo() and bar() on x, which works because S implements both. The execution table traces each step from defining protocols, creating struct, declaring variable, assigning instance, and calling methods. The variable tracker shows how x changes from undefined to holding an instance of S. Key moments clarify why x can call both methods and why S must conform to both protocols. The quiz tests understanding of variable state and method calls during execution. The snapshot summarizes protocol composition usage in Swift.