textview - Android sp vs dp texts - what would adjust the 'scale' and what is the philosophy of support

Question

So we know from many other posts that we should use sp rather than dp for text in Android, and we know the reason for this is to respect a 'user's preferences'.

But precisely what are these preferences? How might a user change this setting?

I cannot find any reference through the settings on my phone (I would have expected something in 'Accessibility' or 'Display'). So what is a user setting? Is it only done through the likes of an app such as 'Big Font'?

Assuming that it is (set by something like big font) - I have played with Google Docs and some other Google apps with the font set to 130%. While most layout stays fine, some gets a bit cut off and can't be read (and that is on a big screened SGS2). So, what is the approach to developing apps with text sizes using 'sp'? Do we make sure it works on 100% scaling and then ignore other settings - call it a special case that the user can worry about, or do we go out of our way to make sure things expand or are scrollable, in case the text overflows?

One argument is that we should use 'dp' to guarantee a user has a chance of seeing the text (even if they have to use a magnifying glass)

Thoughts/comments?

Answer 1

It is exposed in the settings menu on some Android devices (manufacturer dependent). It may also be altered by some accessibility options (device-dependent).

In general, you should always used scale-independent pixels, especially for a large body of text.

However if your text has to fit into a bounding-box of known size then you should use density independent pixels in order to ensure that the text always fits properly and that all characters are visible regardless of the users' setting.

In a nutshell: would increasing the text-size by around 5sp result in the text being unreadable or mangle your UI? If so use density-independent pixels. If not, use scale-independent pixels. However you should generally aim to use scale-independent pixels wherever possible, which means designing a UI that can accommodate different text sizes.