2019-08-18 kotlin泛型相关 -> reified

kotlin泛型相关 -> reified

1.reified的作用

https://github.com/JetBrains/kotlin/blob/master/spec-docs/reified-type-parameters.md

支持通过方法传递的类型运行时可用可达

由于java泛型是伪泛型,为了兼容java1.5以前的版本,java运行时,会泛型擦除 会擦除为泛型上界,如果没有泛型上界会擦除为Object,所以jvm在程序运行时是不知道泛型的真实类型, reifield 能保证运行时依然能拿到泛型的具体类型.(当前只限制支持内联函数可用)

# Reified Type Parameters

Goal: support run-time access to types passed to functions, as if they were reified (currently limited to inline functions only).

## Syntax

A type parameter of a function can be marked as `reified`:



inline fun foo() {}

2.实际使用

Activity定义扩展函数

inline fun  Activity.startActivityKtx() {
    this.startActivity(Intent(this, T::class.java))
}

inline fun  Activity.startActivityKtxWithParam(block: (intent: Intent) -> Any) {
    val intent = Intent(this, T::class.java)
    block.invoke(intent)
    this.startActivity(intent)
}

调用

//跳转Activity 带参数
startActivityKtxWithParam {
                            it.putExtra(TemplateListActivity.KEY_CATEGORY_NAME, datas[position].categoryName)
                        }
                        
                        
 //跳转Activity   不带参数                    
startActivityKtx()
                              

如果不用reifield 我们的写法是

fun Activity.startActivityKtx(cls: Class<*>) {
    this.startActivity(Intent(this, cls))
}

调用是

  startActivityKtx(Intent(this, MoreActivity::class.java))

String定义扩展函数

inline fun  String.jsonToObj(): T? {
    return try {
        val g = Gson()
        val type = object : TypeToken() {}.type
        g.fromJson(this, type)
    } catch (e: Exception) {
        null
    }
}


inline fun  String.jsonToObjList(): ArrayList? {
    return try {
        val g = Gson()
        val type = object : TypeToken>() {}.type
        return g.fromJson>(this, type)
    } catch (e: Exception) {
        null
    }
}

json string直接转对象

1.json 格式

[
  {
    "pic": "classic_08",
    "name": "classic"
  },
  {
    "pic": "polygon_04",
    "name": "polygon"
  },
  {
    "pic": "quote_02",
    "name": "quote"
  },
  {
    "pic": "simple_03",
    "name": "simple"
  },
  {
    "pic": "travel_07",
    "name": "travel"
  },
  {
    "pic": "marble_06",
    "name": "marble"
  },
  {
    "pic": "cloud_08",
    "name": "cloud"
  },
  {
    "pic": "freedom_03",
    "name": "freedom"
  }
]

2.bean

@Parcelize
data class TemplateCategoryItemInfo(@SerializedName("pic") var categoryCoverPic: String, @SerializedName("name") var categoryName: String) : Parcelable

3.string 2 bean (assets2String.jsonToObjList())

class CategoryModel : IModel {

    fun getData(): ArrayList? {
        val assets2String = ResourceUtils.readAssets2String("category.json")
        return assets2String.jsonToObjList()
    }
}

不使用reifeild

fun  String.jsonToObj2(): T? {
    return try {
        val g = Gson()
        val type = object : TypeToken() {}.type
        g.fromJson(this, type)
    } catch (e: Exception) {
        null
    }
}

jsonToObj不使用reifeild.png

可以看出Gson 在解析时是没有判断出泛型T的真实类型的

使用reifeild

jsonToObj使用reifeild.png

参考

reified-type-parameters.md

[译]Kotlin的独门秘籍Reified实化类型参数(上篇)

我与 Kotlin 的爱恨情仇之浅谈 reified