QFileSelectoris a convenience for selecting file variants based on platform or device characteristics. This allows you to develop and deploy one codebase containing all the different variants more easily in some circumstances, such as when the correct variant cannot be determined during the deploy step.
If you always use the same file you do not need to use
Consider the following example usage, where you want to use different settings files on different locales. You might select code between locales like this:QString defaultsBasePath = "data/"; QString defaultsPath = defaultsBasePath + "defaults.conf"; QString localizedPath = defaultsBasePath + QString("%1/defaults.conf").arg(QLocale().name()); if (QFile::exists(localizedPath)) defaultsPath = localizedPath; QFile defaults(defaultsPath);
Similarly, if you want to pick a different data file based on target platform, your code might look something like this:QString defaultsPath = "data/defaults.conf"; #if defined(Q_OS_ANDROID) defaultsPath = "data/android/defaults.conf"; #elif defined(Q_OS_IOS) defaultsPath = "data/ios/defaults.conf"; #endif QFile defaults(defaultsPath);
QFileSelectorprovides a convenient alternative to writing such boilerplate code, and in the latter case it allows you to start using an platform-specific configuration without a recompile.
QFileSelectoralso allows for chaining of multiple selectors in a convenient way, for example selecting a different file only on certain combinations of platform and locale. For example, to select based on platform and/or locale, the code is as follows:QFileSelector selector; QFile defaultsFile(selector.select("data/defaults.conf"));
The files to be selected are placed in directories named with a
'+'and a selector name. In the above example you could have the platform configurations selected by placing them in the following locations:data/defaults.conf data/+android/defaults.conf data/+ios/+en_GB/defaults.conf
To find selected files,
QFileSelectorlooks in the same directory as the base file. If there are any directories of the form +<selector> with an active selector,
QFileSelectorwill prefer a file with the same file name from that directory over the base file. These directories can be nested to check against multiple selectors, for example:images/background.png images/+android/+en_GB/background.png
With those files available, you would select a different file on the android platform, but only if the locale was en_GB.
For error handling in the case no valid selectors are present, it is recommended to have a default or error-handling file in the base file location even if you expect selectors to be present for all deployments.
In a future version, some may be marked as deploy-time static and be moved during the deployment step as an optimization. As selectors come with a performance cost, it is recommended to avoid their use in circumstances involving performance-critical code.
Selectors normally available are
platform, any of the following strings which match the platform the application is running on (list not exhaustive): android, ios, osx, darwin, mac, macos, linux, qnx, unix, windows. On Linux, if it can be determined, the name of the distribution too, like debian, fedora or opensuse.
locale, same as QLocale().name().
Further selectors will be added from the
QT_FILE_SELECTORSenvironment variable, which when set should be a set of comma separated selectors. Note that this variable will only be read once; selectors may not update if the variable changes while the application is running. The initial set of selectors are evaluated only once, on first use.
You can also add extra selectors at runtime for custom behavior. These will be used in any future calls to
select(). If the extra selectors list has been changed, calls to
select()will use the new list and may return differently.
Conflict Resolution when Multiple Selectors Apply¶
When multiple selectors could be applied to the same file, the first matching selector is chosen. The order selectors are checked in are:
Selectors set via
setExtraSelectors(), in the order they are in the list
Selectors in the
QT_FILE_SELECTORSenvironment variable, from left to right
Here is an example involving multiple selectors matching at the same time. It uses platform selectors, plus an extra selector named “admin” is set by the application based on user credentials. The example is sorted so that the lowest matching file would be chosen if all selectors were present:images/background.png images/+linux/background.png images/+windows/background.png images/+admin/background.png images/+admin/+linux/background.png
Because extra selectors are checked before platform the
+admin/background.pngwill be chosen on Windows when the admin selector is set, and
+windows/background.pngwill be chosen on Windows when the admin selector is not set. On Linux, the
+admin/+linux/background.pngwill be chosen when admin is set, and the
+linux/background.pngwhen it is not.
- param parent
If supplied, it will have the given
- Return type
list of strings
Returns the complete, ordered list of selectors used by this instance
- Return type
list of strings
Returns the list of extra selectors which have been added programmatically to this instance.
filePath – unicode
- Return type
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