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记Kotlin类型安全构建器的一次运用

 3 years ago
source link: https://mingdroid.github.io/2020/06/21/%E8%AE%B0Kotlin%E7%B1%BB%E5%9E%8B%E5%AE%89%E5%85%A8%E6%9E%84%E5%BB%BA%E5%99%A8%E7%9A%84%E4%B8%80%E6%AC%A1%E8%BF%90%E7%94%A8/
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记Kotlin类型安全构建器的一次运用

在android官方指导的相关应用框架中，用到一个Resource类来表示网络请求的状态与结果

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// A generic class that contains data and status about loading this data.
sealed class Resource<T>(
   val data: T? = null,
   val message: String? = null
) {
   class Success<T>(data: T) : Resource<T>(data)
   class Loading<T>(data: T? = null) : Resource<T>(data)
   class Error<T>(message: String, data: T? = null) : Resource<T>(data, message)
}

大多数情况下我们在activity里面是这样用的

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private val testViewModel : TestViewModel by viewModels()

private fun getUserInfo(uid: String) {
    testViewModel.userInfoData.observe(this, Observer {
        when (it.status) {
            Status.SUCCESS -> TODO()
            Status.ERROR -> TODO()
            Status.LOADING -> TODO()
        }
    })
    testViewModel.setUserId(uid)
}

这样写多了感觉好烦，每次都是when()，有没有更爽的写法呢？比如这样？

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private fun getUserInfo(uid: String) {
    testViewModel.userInfoData.observe(this, Observer {
        success {

}
        error {

}
        loading {

}
    })
    testViewModel.setUserId(uid)
}

当我只需要处理success的时候，我可以不写error/loading情况。

kotlin的类型安全构建器可以做到，我们先看下官方的示例

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class HTML {
    fun body() { …… }
}

fun html(init: HTML.() -> Unit): HTML {
    val html = HTML()  // 创建接收者对象
    html.init()        // 将该接收者对象传给该 lambda
    return html
}

html {       // 带接收者的 lambda 由此开始
    body()   // 调用该接收者对象的一个方法
}

先分析下，我们需要的是一个实现了Observer接口的对象。

所以我们先定义一个类来实现Observer接口

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class ResourceObserver<T: Any> : Observer<Resource<T>> {
    override fun onChanged(t: Resource<T>) {
        when(t) {
            is Resource.Success -> TODO()
            is Resource.Error -> TODO()
            is Resource.Loading -> TODO()
        }
    }
}

实现一个顶层函数，返回一个ResourceObserver对象

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fun <T: Any> resourceObserver(init: ResourceObserver<T>.() -> Unit): ResourceObserver<T> {
    val observer = ResourceObserver<T>()
    observer.init()
    return observer 
}

调用该函数即可得到ResourceObserver对象

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resourceObserver {
	//在此处可以调用对象内的成员函数
}

所以我的实现是

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class ResourceObserver<T: Any> : Observer<Resource<T>> {
    private var success: (Resource.Success<T>.() -> Unit)? = null
    private var error: (Resource.Error.() -> Unit)? = null
    private var loading: (Resource.Loading<T>.() -> Unit)? = null

fun success(s: (Resource.Success<T>.() -> Unit)) {
        success = s
    }

fun error(e: Resource.Error.() -> Unit) {
        error = e
    }

fun loading(l: Resource.Loading<T>.() -> Unit) {
        loading = l
    }

override fun onChanged(t: Resource<T>) {
        when(t) {
            is Resource.Success -> success?.invoke(t)
            is Resource.Error -> error?.invoke(t)
            is Resource.Loading -> loading?.invoke(t)
        }
    }
}

report

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