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Learn how and where to use scope functions in kotlin. Also, explore 'with' vs 'apply' vs 'run' vs 'also' vs 'let' higher-order functions with their Lambda expression usage.  Explore each function with their properties such as whether they return 'Context Object' or 'lambda result'. And also whether the context object can be accessed by using either 'this' keyword or 'it' identifier.

Author: smartherd

src/49_with_scope_function.kt

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+class Person {
+    var name: String = "Sriyank Siddhartha"
+    var age: Int = 26
+}
+
+fun main() {
+
+    /** Scope Function: 'with'
+        Property 1: Refer to context object by using 'this'
+        Property 2: The return value is the 'lambda result'  */
+
+    val person = Person()
+
+    val bio: String = with(person) {
+        println(name)
+        println(age)
+        age + 5
+        "He is a freak who loves to teach in his own way" // will be returned and stored in 'bio' String variable
+    }
+
+//    println("Age after five years is $ageAfterFiveYears")
+    println(bio)
+}

src/50_apply_scope_function.kt

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+class Person {
+    var name: String = ""
+    var age: Int = 0
+}
+
+fun main() {
+
+    /** Scope Function: 'apply'
+    Property 1: Refer to context object by using 'this'
+    Property 2: The return value is the 'context object'  */
+
+    val person = Person().apply {
+        name = "Sriyank Siddhartha"
+        age = 26
+    }
+
+    with(person) {
+        println(name)       // prints   Sriyank Siddhartha
+        println(age)        // prints   26 
+    }
+
+    // Perform some other operations on 'person' object
+    person.also {
+        it.name = "Shreks from Smartherd"
+        println("New name: ${it.name}")     // prints   New name: Shreks from Smartherd
+    }
+}

src/51_also_scope_function.kt

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+
+
+fun main() {
+
+    /** Scope Function: 'also'              'ALSO PERFORM THE FOLLOWING EXTRA OPERATION'
+    Property 1: Refer to context object by using 'it'
+    Property 2: The return value is the 'context object'  */
+
+    // Initialise numbersList 
+    val numbersList: MutableList<Int> = mutableListOf(1, 2, 3)
+
+    // Some other code... may be a function call or program to swap numbers (doesn't matter what code)
+
+    // Operations on the 'numbersList'
+    val duplicateNumbers = numbersList.also {
+        println("The list elements are: $it")
+        it.add(4)
+        println("The list elements after adding an element: $it")
+        it.remove(2)
+        println("The list elements after removing an element: $it")
+    }
+
+    // duplicateNumbers will be same as numbersList
+    println("Original numbers: $numbersList")
+    println("Duplicate numbers: $duplicateNumbers")
+}

src/52_let_scope_function.kt

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+
+fun main() {
+
+    /** Scope Function: 'let'
+    Property 1: Refer to context object by using 'it'
+    Property 2: The return value is the 'lambda result'  */
+
+    // Use 'let' function to avoid NullPointerException
+
+    val name: String? = "Hello"
+
+    // Execute the lambda expression only if the 'name' variable is NOT NULL
+    val stringLength = name?.let {
+        println(it.reversed())
+        println(it.capitalize())
+        it.length       // Will be returned and stored within stringLength variable 
+    }
+
+    println(stringLength)
+}

src/53_run_scope_function.kt

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+
+class Person {
+    var name: String = "Sriyank Siddhartha"
+    var age: Int = 26
+}
+
+fun main() {
+
+    /** Scope Function: 'run'
+    Property 1: Refer to context object by using 'this'
+    Property 2: The return value is the 'lambda result'  */
+
+    // 'run' is combination of 'with' and 'let'
+    // If you want to operate on a Nullable object and avoid NullPointerException then use 'run'
+
+    val person: Person? = Person()
+
+    val bio = person?.run {
+        println(name)
+        println(age)
+        age + 5
+        "He is a freak who loves to teach in his own way"   // will be returned and stored in 'bio' variable
+    }
+
+    println(bio)
+}