SingleMessage

interface SingleMessage : Message

charAt	`open fun charAt(index: Int): Char`
contains	`open operator fun contains(sub: String): Boolean`
eq	`open infix fun eq(other: Message): Boolean` 将 contentToString 与 other 比较 `open infix fun eq(other: String): Boolean`
length	`open fun length(): Int`
subSequence	`open fun subSequence(start: Int, end: Int): CharSequence`

asMessageChain	直接将 this 委托为一个 MessageChain `fun SingleMessage.asMessageChain(): MessageChain`
flatten	扁平化 Message `fun Message.flatten(): Sequence<SingleMessage>`
isContentEmpty	判断消息内容是否为空. `fun Message.isContentEmpty(): Boolean`
isContentNotEmpty	`fun Message.isContentNotEmpty(): Boolean`
isNotPlain	`fun Message.isNotPlain(): Boolean`
isNotPlain2	`fun Message.isNotPlain2(): Boolean`
isPlain	`fun Message.isPlain(): Boolean`
isPlain2	`fun Message.isPlain2(): Boolean`
repeat	`fun Message.repeat(count: Int): MessageChain`
repeat2	`fun Message.repeat2(count: Int): MessageChain`
sendTo	`suspend fun <C : Contact> Message.sendTo(contact: C): MessageReceipt<C>`
times	`operator fun Message.times(count: Int): MessageChain`
toForwardMessage	转换为 ForwardMessage `fun Message.toForwardMessage(sender: User, time: Int = currentTimeSeconds.toInt(), displayStrategy: DisplayStrategy = DisplayStrategy): ForwardMessage` `fun Message.toForwardMessage(senderId: Long, senderName: String, time: Int = currentTimeSeconds.toInt(), displayStrategy: DisplayStrategy = DisplayStrategy): ForwardMessage`

CustomMessage	自定义消息 `sealed class CustomMessage : SingleMessage`
MessageContent	消息内容 `interface MessageContent : SingleMessage`
MessageMetadata	消息元数据, 即不含内容的元素. `interface MessageMetadata : SingleMessage`