Displaying formatted text

So far, we've been happily printing formatted text to the console, but how do we do that on the LCD screen?

format!

The Rust standard library's alloc crate provides the format! macro, which gives us a String output. Let's try it quickly.

Add the alloc crate and import the format macro.

#![allow(unused)]
fn main() {
extern crate alloc;
use alloc::format;
}

Then add this code to the main loop, after frame_buffer.clear():

#![allow(unused)]
fn main() {
let message = format!(
    "Current timestamp: {} ms",
    Instant::now().duration_since_epoch().as_millis()
);
let text2 = Text::new(&message, Point::new(30, y + 20), character_style);
text2.draw(&mut frame_buffer).ok();
}

If we try to compile our program, we'll get an error:

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait

In order to create a String, we need dynamic memory allocation (hence the alloc crate). However, on embedded systems, dynamic memory allocation is often not desirable and is not enabled by default.

For ESP32 projects, we can enable it at project creation time using esp-generate, but we can also set it up after the fact by adding the esp-alloc crate:

cargo add esp-alloc --features esp32c6

Lastly, declare the allocator by adding this line near the beginning of the main function.

#![allow(unused)]
fn main() {
esp_alloc::heap_allocator!(size: 1024);
}

The size can be adjusted based on project needs, but we just want to format a short string, so 1 kB should be plenty!

If we run our program now, the message should successfully show up on the screen.