[BUG] Big txt file leads to frozen preview

[jesmrec]

Steps to reproduce

1. Open a 2MB txt file (or greater)

Actual behaviour

Blank preview. Takes long to open and ANR happens

Expected behaviour

Preview loads faster, or set an spinner/waiter

Sample file:
sample-2mb-text-file.txt

device-2022-11-23-141001.mp4

Environment data

Android version: 12

Device model: Samsung A51

ownCloud app version: 2.21.2

ownCloud server version: 10.11

[michaelstingl]

Check if there are any libraries for this?

[DeepDiver1975]

Confirmed — still present on current main. The freeze is not the file read (that already runs off the UI thread); it's rendering the whole file into a single TextView on the main thread.

In PreviewTextFragment.kt the TextLoadAsyncTask reads the entire file into one StringWriter/String with no size cap (doInBackground → String(result.buffer)), then on onPostExecute (UI thread) does textView.text = text. Measuring/laying out a multi-megabyte string in a single TextView (StaticLayout build) on the main thread is the ANR — a 2 MB file produces a 2 MB+ CharSequence for one view.

For .md files it's worse: the same full string goes through markwon.setMarkdown(textView, text) in PreviewFormatTextFragmentStateAdapter.PreviewFormatTextFragment — synchronous Markdown parsing of the whole document on the UI thread, again uncapped.

So the bug reproduces by design: background read OK, but the big-string layout/parse on the main thread blocks. A loading dialog now shows during the read, but it's dismissed right before the expensive UI-thread layout, so the freeze still surfaces.

Fix directions (any of):

• Cap preview size (read first N bytes/lines, append a "truncated, open with…" notice).
• Render incrementally / use a paginated or RecyclerView-backed view instead of one TextView.
• Move Markdown parsing off the main thread (markwon.toMarkdown(text) on a worker, then setParsedMarkdown on the UI thread) and cap input.

Files: owncloudApp/src/main/java/com/owncloud/android/ui/preview/PreviewTextFragment.kt, .../PreviewFormatTextFragmentStateAdapter.kt.

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