今天学习了一下kotlin的属性代理，觉的挺有趣的，借助属性代理我们可以实现一些有意思的功能。
基本语法
class Student{
var name: String by Delegate()
}
class Delegate{
operator fun <T> getValue(thisRef: Any?, property: KProperty<*>): T{
...
}
operator fun <T> setValue(thisRef: Any?, property: KProperty<*>, value: T){
...
}
}
案例一：利用属性代理实现json字段的获取
class JsonDelegate(jsonStr: String) {
val jsonObject: JSONObject = JSONObject(jsonStr)
public fun <T> delegate(defaultValue: T, key: String = ""): Delegate<T> {
return Delegate(jsonObject, key, defaultValue)
}
public override fun toString(): String {
return jsonObject.toString()
}
class Delegate<T>(
var jsonObject: JSONObject,
var key: String,
val defaultValue: T
) {
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
val key = getKey(property)
jsonObject.put(key, value)
}
protected fun getKey(property: KProperty<*>): String {
if (!key.isEmpty()) {
return key
}
return property.name
}
operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
val key = getKey(property)
if (jsonObject.has(key)) {
val value = jsonObject.get(getKey(property))
return value as T
} else {
return defaultValue
}
}
}
}
使用方式：
val jsonDelegate = JsonDelegate(jsonStr)
val value1 by jsonDelegate.delegate(-1,"test")//第一个参数是默认值，第二个参数是key
val value2 by jsonDelegate.delegate(-2)//若没有第二个参数，则key为变量名
这样我们在从一段json当中取值时，就不用每次都判断json是否存在这个key，若不存在这个key，则直接返回传入的默认值。
案例二：利用属性代理实现SharedPreferences取值和存储
class SPDelegate(tag: String, context: Context) {
val sp:SharedPreferences by lazy {
context.applicationContext.getSharedPreferences(tag,Context.MODE_PRIVATE)
}
public fun <T> delegate(defaultValue: T, key: String = ""): Delegate<T> {
return Delegate(sp,key,defaultValue)
}
class Delegate<T>(
val sp:SharedPreferences,
var key: String,
val defaultValue: T
) {
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
val key = getKey(property)
with(sp.edit()) {
when (value) {
is String -> putString(key, value)
is Int -> putInt(key, value)
is Float -> putFloat(key, value)
is Long -> putLong(key, value)
is Boolean -> putBoolean(key, value)
else -> throw IllegalArgumentException("Unsupported type.")
}
}.apply()
}
protected fun getKey(property: KProperty<*>): String {
if (!key.isEmpty()) {
return key
}
return property.name
}
operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
val result = when (defaultValue) {
is String -> sp.getString(getKey(property), defaultValue as String) ?: defaultValue
is Int -> sp.getInt(getKey(property), defaultValue as Int)
is Float -> sp.getFloat(getKey(property), defaultValue as Float)
is Long -> sp.getLong(getKey(property), defaultValue as Long)
is Boolean -> sp.getBoolean(getKey(property), defaultValue as Boolean)
else -> throw IllegalArgumentException("Unsupported type.")
}
return result as T
}
}
}
使用方式：
val delegate = SPDelegate("MainActivity",this)
var test by delegate.delegate(1,"tttt")//第一个参数表示默认值，第二个参数表示key
var test1 by delegate.delegate("test")//若只有一个参数，则key为变量名称
这样我们就可以避免写出大段的SharedPreferences代码，我们的代码也非常的简洁。