需求：在开头显示一个 @xxxx ，不可选中，不可删除

网上有很多复杂的，我这里不需要，看了下，主要是一个继承关系才能监听到光标没什么其他特别的，完整代码如下：

class AtEditText(context: Context, attrs: AttributeSet?) : AppCompatEditText(context, attrs) {

    private var userText: String? = ""

    fun init(userText: String, onLengthChange: (Int) -> Unit) {
        this.userText = userText
        setText(userText)
        setSelection(userText.length)
        addTextChangedListener {
            val curLength = text?.length ?: 0
            if (curLength < userText.length) {
                setText(userText)
                setSelection(userText.length)
            }
            val i = (curLength - userText.length).coerceAtLeast(0)
            onLengthChange.invoke(i)
        }

    }


    override fun onSelectionChanged(selStart: Int, selEnd: Int) {
        val length = userText?.length ?: 0
        if (selStart < length) {
            setSelection(length)
            super.onSelectionChanged(length, length)
        } else {
            super.onSelectionChanged(selStart, selEnd)
        }
    }
}