For a lot of writers Grammarly is an invaluable tool to help nail grammar, punctuation, and the many other pitfalls that English throws your way. There's no full-featured version for mobile, but the Grammarly Keyboard comes pretty close. It now has another feature from the desktop version: Tone Detector, which helps you send the right message at the right time. Or alternately, helps curb your less savory texting habits.
The Tone Detector analyzes the last few sentences of text input and labels them with likely adjectives, like "Joyful" or "Formal." It then assigns a confidence meter for each one. There's no way to select which tone you're going for, so you just kind of have to wing it and see what you get. But based on those results, you can go back and adjust your text to try and get a different outcome, or the same one with a greater degree of confidence.
To access the tool, tap the Grammarly logo button in the top left of the keyboard after inputting at least 150 characters of text. It's live in the version of the app I tried, which has been on the Play Store for two weeks now, so it should be available for everyone in the US at least. Oh, and Grammarly says that the feature is in beta at the time of writing.