Kotlin-简约之美-进阶篇（十七）：DSL的简单应用

@[toc]
当接口的方法过多，而我们只需要其中一个方法时，例如TextView.addTextChangedListener，我们只需要它的onTextChanged方法时

1.原始方式

tv.addTextChangedListener(object : TextWatcher {
    override fun afterTextChanged(s: Editable?) {
    }
    override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {
    }
    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
        print("文字修改为：$s")
    }
})

显然，我们这里还需要空实现afterTextChanged和beforeTextChanged，有点繁琐

2. 空适配器模式

tv.addTextChangedListener(object : EmptyTextWatcher() {
    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
        print("文字修改为：$s")
    }
})

这个其实很简单，就是加了个接口空实现的中间类而已，这也是我们通常的操作，但是在kotlin里面，我们还可以有下面这两种高级玩法

open class EmptyTextWatcher : TextWatcher {
    override fun afterTextChanged(s: Editable?) {

    }

    override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {
    }

    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
    }

}

3. 高阶函数方式

tv.addTextChangedListenerClosure(
    onTextChanged = { charSequence, start, after, count ->
        print("文字修改为：$charSequence")
    }
)

这个就是加了一个内联的拓展方法addTextChangedListenerClosure，这个方法的参数都是函数

inline fun TextView.addTextChangedListenerClosure(
    crossinline afterTextChanged: (Editable?) -> Unit = {},
    crossinline beforeTextChanged: (CharSequence?, Int, Int, Int) -> Unit = { charSequence, start, count, after -> },
    crossinline onTextChanged: (CharSequence?, Int, Int, Int) -> Unit = { charSequence, start, after, count -> }
) {
    val listener = object : TextWatcher {
        override fun afterTextChanged(s: Editable?) {
            afterTextChanged.invoke(s)
        }

        override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {
            beforeTextChanged.invoke(s, start, count, after)
        }

        override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
            onTextChanged.invoke(s, start, before, count)
        }
    }
    this.addTextChangedListener(listener)
}

4. DSL方式

tv.addTextChangedListenerDsl {
    onTextChanged { charSequence, start, after, count ->
        print("文字修改为：$charSequence")
    }
}

这个呢，就是先通过拓展方法传入方法体init,在init方法里面，初始化afterTextChanged等函数，然后在回调的方法里面调用保存的函数

fun TextView.addTextChangedListenerDsl(init: TextChangedListenerDsl.() -> Unit) {
    val listener = TextChangedListenerDsl()
    listener.init()
    this.addTextChangedListener(listener)
}

class TextChangedListenerDsl : TextWatcher {

    var afterTextChanged: ((Editable?) -> Unit)? = null

    var beforeTextChanged: ((CharSequence?, Int, Int, Int) -> Unit)? = null

    var onTextChanged: ((CharSequence?, Int, Int, Int) -> Unit)? = null

    /**
     * DSL方法
     */
    fun afterTextChanged(method: (Editable?) -> Unit) {
        afterTextChanged = method
    }

    fun beforeTextChanged(method: (CharSequence?, Int, Int, Int) -> Unit) {
        beforeTextChanged = method
    }

    fun onTextChanged(method: (CharSequence?, Int, Int, Int) -> Unit) {
        onTextChanged = method
    }

    /**
     * 原始方法
     */
    override fun afterTextChanged(s: Editable?) {
        afterTextChanged?.invoke(s)
    }

    override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {
        beforeTextChanged?.invoke(s, start, count, after)
    }

    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
        onTextChanged?.invoke(s, start, before, count)
    }

}

以上只是DSL的一个简单示例，主要是示范怎么写DSL，如果你有更好的使用DSL的场景，欢迎补充。