Right-to-left reading order: why isn't this calculated automatically?

In Visual Studio you need to set the extended window style to get a reading-order of right to left (WS_EX_LAYOUTRTL). Why is this required since if I'm using UNICODE and displaying Arabic charact开发者_JS百科ers the only possible way to display it is right-to-left? I'm surprised the system doesn't simply render it the correct way around. To note: this is on a Windows Mobile system where I've copied the Arial Unicode MS font onto it, which perhaps might explain why it can't cope.

Windows' support for RTL is more complex than just the text: WS_EX_LAYOUTRTL is actually about controling the layout of other elements in the window - from MSDN:

The window layout applies to text but also affects the other GDI elements of the window, including bitmaps, icons, the location of the origin, buttons, cascading tree controls, and whether the horizontal coordinate increases as you go left or right. For example, after an application has set RTL layout, the origin is positioned at the right edge of the window or device, and the number representing the horizontal coordinate increases as you move left.

So if you create a dialog that has this, the dialog will be "flipped" automatically (because the coordinates are reversed). If a scrollbar is present, it will be on the left side of the window, not the right. Treeviews will have the expand/collapse box and connecting lines on the right side, not the left - and so on.

In the case of a static, which doesn't contain other windows, the style may not appear to make much difference - but it likely will flip the justification: a static that is right-justified using SS_RIGHT would likely end up actually left-justified when WS_EX_LAYOUTRTL is used.

Also, as the other answer notes, not all text is spans of a single language. It's possible to have a single string that mixes scripts: you can have L-to-R spans within R-to-L, and vice versa, so having Windows "do the right thing" based on the text used would be very fragile.

Also consider the case of a treeview that displays the filenames running on an Arabic system: the treeview should keep a right-to-left layout (aligned against the right side) even if the user just happens to be browsing a directory or file system that happens to have english filenames.

Long story short: WS_EX_LAYOUTRTL is really about overall window layout, not specifically text direction itself. Even without this flag, you should still get Arabic/Hebrew rendered correctly as R-to-L if using the standard APIs/controls.

Presumably because it can't be determined what you're going to display at the window level - you could be displaying nothing, a language read left to right or a language read right to left. Thus you need to set it explicitly rather than having it attempt to deduce based off incomplete information.