F-Droid has long lacked a working Chinese IME (input method engine), as stated in this topic. It now packages Trime, which can use many input methods, such as cangjie or pinyin. Unfortunately, it currently is quite a pain to configure, hence this post, whose objectives are to provide basic help to make it work and aggregate experiences. There is a wiki for this project, mainly in Chinese, which seems quite logical, but how about learners?
Basically, the app is useless just after it is installed, but this is somehow “normal”. First of all, you might need to manually grant permissions (app info › permissions › storage). Also, you have to manually enable the keyboard provided by the application (configuration › system › languages and input › virtual keyboard › manage keyboards).
In Trime configuration, go to Keyboard › Input to know where the “share” and “user” directories are located (e.g. /sdcard/rime). You will have to manually upload there (using adb push, or by mounting your phone as external storage, etc.) a handful of files in the build/ directory (or only in the root directory, or both, this is quite unclear to me), found in this repo. prelude and essay are essential (without essay there will be no predictive model and you will have to browse through all existing sinograms for a given sound or set of strokes). I mainly use pinyin-simp. In Trime configuration, in Input › Schemas, enable all the schemas you wish to use, and tap “OK”. Trime will now need to sync the configuration (go through this step every time you alter configuration files).
You can toggle between CJK input and Latin script with 中文/西文 and between simplified and traditional Chinese with 汉字/漢字。半角 and 全角 just seem to respectively mean half-width and full-width.
You should diagnose any issue with adb logcat, which will show for instance any missing file.