Add get_contrast() and get_brightness() to SSD1306 class to get protected variables

[benediktkr]

What does this implement/fix?

This adds get_contrast() and get_brightness() public methods to the SSD1306 class in the C++ API. They can then be used in lambdas or components to read the protected class variables contrast_ and brightness_, in order to represent the current brightness/contrast values without trying to keep track of the state yourself.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): Not applicable

Pull request in esphome-docs with documentation (if applicable): Not applicable

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

display: 
 - platform: ssd1306_i2c
   id: ssd1306
   model: "SSD1306 128x64"
   contrast: "33%"
   lambda: |-
      float contrast = id(ssd1306).get_contrast();
      it.printf(0, 0, id(font), TextAlign::BASELINE_LEFT, "%3.0f %%", contrast*100);

number:
  - platform: template
    id: "ssd1306_contrast"
    name: "SSD1306 contrast"
    min_value: 0
    max_value: 100
    step: 0
    entity_category: ""
    unit_of_measurement: "%"
    device_class: power_factor
    mode: slider
    lambda: |-
      float contrast = id(ssd1306).get_contrast();
      return int(contrast*100);
    set_action: 
      - lambda: |-
          id(ssd1306).set_contrast(x/100);

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.43%. Comparing base (4d8b5ed) to head (2fc03dc).
Report is 255 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6435      +/-   ##
==========================================
- Coverage   53.70%   53.43%   -0.28%     
==========================================
  Files          50       50              
  Lines        9408     9529     +121     
  Branches     1654     1684      +30     
==========================================
+ Hits         5053     5092      +39     
- Misses       4056     4128      +72     
- Partials      299      309      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[benediktkr]

Please correct me if I'm wrong, but I believe this C++ component does not have any tests.

I did find some tests under tests/components/ektf2232/ but those are all YAML configs and don't use lambdas to call the SSD1306 class.

[jesserockz]

You can add new tests for this platform if you want, but I am not going to enforce it for this PR.

[benediktkr]

Can you point me to any useful example tests?