how to create disappearing message

You can use a combination of textCache plugin with refreshvariable. 
You can set true in a cache key and set the expiry to 28 seconds. In the next line, you can check the value of that variable in refresh variable and refresh the value of it in 30 seconds (0.5 minute). After the first refresh, the value will be set to null and the message can be hidden by using that variable in the showWhen of it. 

text message will disappear in few second

a!localVariables(
local!timeFlag: second(now() + second() * 58),
local!show: a!refreshVariable(
value: second(now()) < local!timeFlag,
refreshInterval: 1
),
{
a!richTextDisplayField(
value: "text will disappear in few seconds",
showWhen: not(local!show),

),

}
)