Application of kotlin - higher order function

Higher order functions are very suitable for simplifying various functions API Call to , some API The original usage of is simplified by using higher-order functions , Whether in terms of ease of use or readability , May be greatly improved .

simplify SharedPreferences Usage of

The most primitive direction I learned before SharedPreferences The code for storing data in is as follows ：

// obtain SharedPreferences.Editor object 
val editor = getSharedPreferences("data",Context.MODE_PRIVATE).edit()
editor.putString("name","ZLS") // add to String Type data 
editor.putInt("age",18)
editor.putBoolean("married",false)
editor.apply() // Submit data 

But in the extension library indicated by the red arrow , Added his simplified usage

It contains a code similar to the following , Can greatly facilitate SharedPreferences Use

fun SharedPreferences.edit(block: SharedPreferences.Editor.() -> Unit){
    
	val editor = edit()
	editor.block()
	editor.apply()
}

Extend the function to SharedPreferences Class added a edit function , And it also receives a function type parameter , therefore edit Function is naturally a higher-order function . because edit Owned in function SharedPreferences The context of , Therefore, it can be called directly here edit() Method to get SharedPreferences.Editor object . in addition edit The function receives a SharedPreferences.Editor Function type parameters for , So we need to call editor.block() Calling a function type parameter , We can add data to the specific implementation of function type parameters . Finally, call editor.apply() Method to submit data , So as to complete the data storage operation .
The simplified code is as follows ：

getSharedPreferences("data",Context.MODE_PRIVATE).edit{
    
    putString("name","ZLS")
    putInt("age",18)
    putBoolean("married",false)
}

simplify ContentValues Usage of

ContentValues Mainly used for combination SQLiteDatabase Of API Storing and modifying data in a database , Specific basic usage, such as Next ：

val values = ContentValues()
values.put("name", "Game of Thrones")
values.put("author", "George Martin")
values.put("pages", 720)
values.put("price", 20.85)
db.insert("Book", null, values)

The following describes how to make it simple by creating a new method ：
First new ⼀ individual ContentValues.kt file , And then define a cvOf() Method ：

fun cvOf(vararg pairs: Pair<String, Any?>): ContentValues {
    
}

The purpose of this method is to build a ContentValues object .
First ,cvOf() Method receives a Pair Parameters , That is to use A to B Parameter type created by syntax structure , But a parameter is added in front of it vararg keyword , It corresponds to Java Variable parameter list in , Allow passing... To this method 0 individual 、1 individual 、2 One or even any number Pair Parameters of type , These parameters are assigned to use vararg Above this variable declared , And then use for-in Loop can traverse all the parameters passed in .
Let's look at the statement Pair type . because Pair Is a data structure of key value pairs , Therefore, you need to specify the type of data corresponding to its key and value through generics .ContentValues All keys of are string type , Here you can put Pair The generic type of the key is specified as String. but ContentValues There can be many types of values （ String type 、 integer 、 floating-point , Even null）, So we need to Pair The generic type of the value is specified as Any?. This is because Any yes Kotlin Common base class for all classes in , amount to Java Medium Object, and Any? It means that null values are allowed to be passed in .

fun cvOf(vararg pairs: Pair<String, Any?>): ContentValues {
    
	val cv = ContentValues()
 	for (pair in pairs) {
    
    	val key = pair.first
		val value = pair.second
    	when (value) {
    
	    	is Int -> cv.put(key, value)
			is Long -> cv.put(key, value)
			is Short -> cv.put(key, value)
			is Float -> cv.put(key, value)
			is Double -> cv.put(key, value)
	 		is Boolean -> cv.put(key, value)
 			is String -> cv.put(key, value)
 			is Byte -> cv.put(key, value)
 			is ByteArray -> cv.put(key, value)
			null -> cv.putNull(key)
    	}
	}
	return cv
}

when Statement entry Int After conditional branch , Under this condition value Will be automatically converted to Int type , Instead of Any? type , So you don't need to be like Java In that way, another downward transformation , This function is if The same applies in the statement .
With this cvOf() After method , Use ContentValues It becomes easier when , For example, you can insert a piece of data into the database and write ：

val values = cvOf("name" to "Game of Thrones", "author" to "George Martin", "pages" to 720, "price" to 20.85)
db.insert("Book", null, values)

Besides , You can also use apply Function for secondary optimization

fun cvOf(vararg pairs: Pair<String, Any?>) = ContentValues().apply {
    
	for (pair in pairs) {
    
		val key = pair.first
		val value = pair.second
		when (value) {
    
			is Int -> put(key, value)
			is Long -> put(key, value)
			is Short -> put(key, value)
			is Float -> put(key, value)
			is Double -> put(key, value)
			is Boolean -> put(key, value)
			is String -> put(key, value)
			is Byte -> put(key, value)
			is ByteArray -> put(key, value)
			null -> putNull(key)
		}
	}
}

As mentioned above KTX A library with the same function is also provided in the library contentValuesOf() Method , Usage is as follows ：

val values = contentValuesOf("name" to "Game of Thrones", "author" to "George Martin", "pages" to 720, "price" to 20.85)
db.insert("Book", null, values)

Usually when writing code , Use it directly KTX Provided contentValuesOf() The method is ok .