Interception - Droidcon NYC 2023

Embrace the Interceptor pattern of software design to create cleaner abstractions, enable flexibility, and set yourself up for writing high quality tests. This versatile approach empowers engineers with unmatched control, adaptability, and seamless integration with dependency injection. Join me to explore how interfaces and protocols serve as the foundation for making code flexible, adaptable and composable. We’ll talk about high level design playbooks as well as real-world examples you can start using today to leverage this software design pattern. Examples such as utilizing reflection-based Proxy Objects, employing "Fakes" in unit tests, wrapping 3rd party libraries and modifying network traffic.

Example Code on Github: https://github.com/handstandsam/interception-examples

Transcript

1. Sam Edwards - @HandstandSam - Square
   Interception
   Droidcon NYC
   September 14, 2023
   #DCNYC23
   

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2. A
   Call
   B
   Completion
   Invocation
   

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3. A
   Call Chain
   B
   

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4. A
   Call Chain
   B
   

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5. Call Chain
   A B
   

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6. Call Chain with Interceptors
   A B
   

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7. Telephone Game ☎
   A B
   

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8. Interceptor Pattern 💻
   

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9. https://en.wikipedia.org/wiki/Interceptor_pattern
   Offer a way to change, or augment the
   
   
   usual processing cycle.
   Interceptor Pattern
   

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10. https://en.wikipedia.org/wiki/Interceptor_pattern
    The rest of the system does not have to
    
    
    know something has been added or
    
    
    changed and can keep working as before.
    Interceptor Pattern
    

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11. Call Chain with Interceptors
    A B
    

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12. Categories of Interception
    • Passthrough (NoOp)
    
    • Logging (Inspect)
    
    • Modifying/Transform (Change)
    
    • Circuit Breaker (Stop)
    

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13. OkHttp Interceptors
    

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14. OkHttp Interceptor Examples
    

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15. Adding Interceptors for OkHttp
    val okHttpClient
    =
    OkHttpClient.Builder()
    
    
    .build()
    

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16. Adding Interceptors for OkHttp
    val okHttpClient
    =
    OkHttpClient.Builder()
    
    
    .addInterceptor(MyInterceptor)
    
    
    .build()
    

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17. Adding Interceptors for OkHttp
    val okHttpClient
    =
    OkHttpClient.Builder()
    
    
    .addInterceptor(MyInterceptor)
    
    
    .addInterceptor(MyInterceptor2
    )
    
    .build()
    

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18. Adding Interceptors for OkHttp
    val okHttpClient
    =
    OkHttpClient.Builder()
    
    
    .addInterceptor(MyInterceptor)
    
    
    .addInterceptor(MyInterceptor2
    )
    
    .addInterceptor(MyInterceptor3
    )
    
    .build()
    

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19. Passthrough Interceptor
    🤷
    

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20. Passthrough Interceptor
    Interceptor { chain
    ->
    
    
    val request = chain.request()
    
    
    val response = chain.proceed(request)
    
    
    response
    
    
    }
    

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21. Logging Interceptor
    📝
    

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22. Logging Interceptor
    Interceptor { chain
    ->
    
    
    val request = chain.request()
    
    
    val response = chain.proceed(request)
    
    
    println("
    - - -
    Request Headers ---")
    
    
    println(request.headers)
    
    
    println("
    - - -
    Response Headers ---")
    
    
    println(response.headers)
    
    
    response
    
    
    }
    

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23. Modifying Interceptor
    🥷
    

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24. Modifying Interceptor
    Interceptor { chain
    ->
    
    
    val newRequest = chain.request()
    
    
    .newBuilder()
    
    
    .addHeader("Droidcon", "NYC")
    
    
    .build()
    
    
    chain.proceed(newRequest)
    
    
    }
    

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25. Short Circuit Interceptor
    ⚡
    

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26. Short Circuit Interceptor
    val ShortCircuitInterceptor
    =
    Interceptor { chain
    ->
    
    
    val request = chain.request()
    
    
    Response.Builder()
    
    
    .code(200)
    
    
    .protocol(Protocol.HTTP_2
    )
    
    .request(request)
    
    
    .message("")
    
    
    .body("Short Circuited".toResponseBody())
    
    
    .build()
    
    
    }
    

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27. Caching Interceptor
    🧠
    

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28. Caching Interceptor
    private val cache = mutableMapOf> ( )
    
    Interceptor { chain
    ->
    
    
    val request = chain.request()
    
    
    val bodyStr = cache[request.url] ?
    :
    run {
    
    
    val response = chain.proceed(request)
    
    
    response.body?.string().also {
    
    
    cache[request.url] = it
    
    
    }
    
    
    }
    
    
    buildResponseWithBodyStr(request, bodyStr)
    
    
    }
    

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29. OkHttp Interceptors are Beautiful 🌈
    • Composable
    
    • Flexible
    
    • Powerful
    

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30. Pop Quiz! 🏈
    

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31. American Football Interception 🏈
    

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32. When the ball is caught by the
    
    
    defense, instead of the offense.
    🏈
    

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33. A
    Interception
    American Football
    B
    

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34. A
    Interception
    American Football
    B
    🏈
    

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35. Interception
    American Football
    🏈 B
    A
    

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36. Recap
    

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37. Interceptor Pattern Benefits
    • Clean Abstraction
    
    • Flexible
    
    • Testable
    
    • Adaptable
    
    • Composable
    
    • Injectable
    

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38. Kotlin Flows
    

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39. Chaining Flows and Intercepting
    flowOf(1, 2, 3
    )
    
    .collect()
    

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40. Chaining Flows and Intercepting
    flowOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    
    
    .collect()
    Logged 1
    
    
    Logged 2
    
    
    Logged 3
    

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41. Chaining Flows and Intercepting
    flowOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    
    
    .map { it
    +
    100
    }
    
    .collect()
    Logged 1
    
    
    Logged 2
    
    
    Logged 3
    

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42. Chaining Flows and Intercepting
    flowOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    
    
    .map { it
    +
    100
    }
    
    .onEach { println("Collected $it") }
    
    
    .collect()
    Logged 1
    
    
    Collected 101
    
    
    Logged 2
    
    
    Collected 102
    
    
    Logged 3
    
    
    Collected 103
    

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43. Kotlin Flows are Beautiful 🌈
    • Composable
    
    • Flexible
    
    • Powerful
    

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44. Iterating Kotlin Collections
    

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45. Chaining Actions and Intercepting
    listOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    Logged 1
    
    
    Logged 2
    
    
    Logged 3
    

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46. Chaining Actions and Intercepting
    listOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    
    
    .map { it
    +
    100
    }
    Logged 1
    
    
    Logged 2
    
    
    Logged 3
    

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47. Chaining Actions and Intercepting
    listOf(1, 2, 3
    )
    
    .onEach { println("Logged $it") }
    
    
    .map { it
    +
    100
    }
    
    .onEach { println("Collected $it") }
    Logged 1
    
    
    Logged 2
    
    
    Logged 3
    
    
    Collected 101
    
    
    Collected 102
    
    
    Collected 103
    

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48. Interfaces Enable Injection
    

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49. Interfaces Enable Injection
    interface Logger {
    fun log(message: String)
    }
    

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50. Interfaces Enable Injection
    interface Logger {
    fun log(message: String)
    }
    class SystemOutLogger : Logger {
    override fun log(message: String) {
    println(message)
    }
    }
    

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51. Interfaces Enable Injection
    interface Logger {
    fun log(message: String)
    }
    class SystemOutLogger : Logger {
    override fun log(message: String) {
    println(message)
    }
    }
    class AndroidLogger : Logger {
    override fun log(message: String) {
    Log.d("Logger", message)
    }
    }
    

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52. Interfaces Enable Interception
    

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53. A
    Interfaces Enable Interception
    B
    SystemOutLogger
    Logger
    

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54. A
    Interfaces Enable Interception
    B
    Logger SystemOutLogger
    Logger
    

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55. A
    Interfaces Enable Interception
    B
    Logger SystemOutLogger
    Logger Logger
    

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56. Interfaces Enable Interception
    A B
    Logger SystemOutLogger
    Logger Logger Logger
    

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57. Interfaces Enable Fakes
    

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58. A
    Interfaces Enable Fakes
    B
    Interface 3rd Party Lib
    Fake
    

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59. Kotlin Delegates
    

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60. Kotlin Delegates
    Passthrough
    class PassthroughDelegateLogger(
    
    
    private val delegate: Logger
    
    
    )
    :
    Logger {
    
    
    override fun log(message: String) {
    
    
    delegate.log(message)
    
    
    }
    
    
    override fun error(message: String) {
    
    
    delegate.error(message)
    
    
    }
    
    
    }
    

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61. Kotlin Delegates
    Logging
    class LoggingDelegateLogger(
    
    
    private val delegate: Logger
    
    
    )
    :
    Logger by delegate {
    
    
    override fun log(message: String) {
    
    
    println("Logged $message")
    
    
    delegate.log(message)
    
    
    }
    
    
    }
    

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62. Kotlin Delegates
    Modifying
    class AddTimestampLogger(
    
    
    private val delegate: Logger
    
    
    )
    :
    Logger by delegate {
    
    
    override fun log(message: String) {
    
    
    delegate.log("${Date()} $message")
    
    
    }
    
    
    }
    

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63. Kotlin Delegates
    Short Circuit
    class ShortCircuitNoErrorsDelegateLogger(
    
    
    private val delegate: Logger
    
    
    )
    :
    Logger by delegate {
    
    
    override fun log(message: String) {
    
    
    delegate.log("${Date()} $message")
    
    
    }
    
    
    override fun error(message: String) {
    
    
    // Do Not Delegate
    
    
    }
    
    
    }
    

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64. Interception Recap
    

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65. Categories of Interception
    • Passthrough (NoOp)
    
    • Logging (Inspect)
    
    • Modifying/Transform (Change)
    
    • Circuit Breaker (Stop)
    

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66. Why?
    • Clean Abstraction
    
    • Flexible
    
    • Testable
    
    • Adaptable
    
    • Composable
    
    • Injectable
    

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67. Takeaways
    • Use Interfaces
    
    • OkHttp “Interception”
    
    • Kotlin Flow “Interception”
    
    • Kotlin Delegate “Interception”
    
    • Design for “Interception”
    

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68. BONUS LIVE DEMO
    
    
    Java Reflection Dynamic Proxies
    

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69. Sam Edwards - @HandstandSam - Square #DCNYC23
    conferenceChain.proceed(talk)
    

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