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當(dāng)接口的方法過多，而我們只需要其中一個(gè)方法時(shí)，例如TextView.addTextChangedListener，我們只需要它的onTextChanged方法時(shí)

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("文字修改為：$s")
    }
})

顯然，我們這里還需要空實(shí)現(xiàn)afterTextChanged和beforeTextChanged，有點(diǎn)繁瑣

2. 空適配器模式

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

這個(gè)其實(shí)很簡(jiǎn)單，就是加了個(gè)接口空實(shí)現(xiàn)的中間類而已，這也是我們通常的操作，但是在kotlin里面，我們還可以有下面這兩種高級(jí)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. 高階函數(shù)方式

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

這個(gè)就是加了一個(gè)內(nèi)聯(lián)的拓展方法addTextChangedListenerClosure，這個(gè)方法的參數(shù)都是函數(shù)

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("文字修改為：$charSequence")
    }
}

這個(gè)呢，就是先通過拓展方法傳入方法體init,在init方法里面，初始化afterTextChanged等函數(shù)，然后在回調(diào)的方法里面調(diào)用保存的函數(shù)

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的一個(gè)簡(jiǎn)單示例，主要是示范怎么寫DSL，如果你有更好的使用DSL的場(chǎng)景，歡迎補(bǔ)充。