关于 BaseNode 与 BaseExpandNode 的设计

abstract class BaseNode {
    abstract val childNode: MutableList<BaseNode>?
}

abstract class BaseExpandNode : BaseNode() {
    var isExpanded: Boolean = true
}

提一个小小建议，个人觉得这两个节点类没必要设计成抽象类，改为接口更好一些。特别是当场景需要实现其他一些接口以提供能力后，由于 BaseNodeAdapter 只接受 BaseNode ，因此在业务代码中充斥着强转，感觉不是太好。

假设我有一个 Model 接口，我希望每个节点都实现它，如果按照目前框架的设计，是这样的：

interface Model {
    val id: String?
    val name: String?
}

class Student : BaseNode(), Model {
    override val childNode: MutableList<BaseNode>? = null

    override val id: String? = null
    override val name: String? = null
}

class Clazz : BaseExpandNode(), Model {
    override val childNode: MutableList<BaseNode>? = null

    override val id: String? = null
    override val name: String? = null
}

fun test() {
    val baseNodeAdapter = baseNodeAdapter
    val models = mutableListOf<Model>()

    // Compile error, type mismatch.
    baseNodeAdapter.setList(models)
    
    val node = baseNodeAdapter.getItem(0)
    
    // Compile error, unresolved reference: name.
    val name = node.name

    // It work, but complex.
    val castName = (node as Model).name
}

可以看到在转型上面比较繁琐，但如果设计成接口，我认为是比较合理的，解决单继承的痛点，如下：

interface BaseNode {
    val childNode: MutableList<BaseNode>?
}

interface BaseExpandNode : BaseNode {
    var isExpanded: Boolean
}

interface Model : BaseNode {
    val id: String?

    val name: String?

    val children: MutableList<Model>?

    override val childNode: MutableList<BaseNode>?
        get() = children as? MutableList<BaseNode>?
}

class Student : Model {
    override val id: String? = null
    
    override val name: String? = null
    
    override val children: MutableList<Model>? = null
}

class Clazz : Model, BaseExpandNode {
    override var isExpanded: Boolean = true
    
    override val id: String? = null
    
    override val name: String? = null
    
    override val children: MutableList<Model>? = null
}

如果改成接口，那么 NaseNodeAdapter 也需要改为支持指定泛型，否则在 getItem() 等这些泛型方法的处境仍然没变。

abstract class BaseNodeAdapter<T : BaseNode>(nodeList: MutableList<T>? = null) : BaseProviderMultiAdapter<T>(null) {
    override fun setList(list: Collection<T>?) {
        super.setList(flatData(list ?: arrayListOf()))
    }
}

以上是我的一些想法，如有错误或不足请指出，感谢作者提供这么便捷的框架。RESPECT 👍

Start by reviewing BaseNode, BaseExpandNode, and BaseNodeAdapter, including its BaseProviderMultiAdapter inheritance. Evaluate the proposed interface and generic changes against the adapter APIs shown in the issue. Done would require a maintainer-approved design and corresponding implementation, with compatibility and usage effects resolved.