【Forwarded】How stringsdict and NSLocalizedString Works

Original url: How stringsdict and NSLocalizedString Works

When I wrote everything about iOS localization, I covered on plural support, but was brief.

This post I will explain the use of the powerful .stringsdict with a simple use, explaining the basics, then an advanced use. Lastly I will try to explain some magic under the hood.

Simple Case

It is easier to use an example and explain.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>%d file(s) are selected</key>
    <dict>
        <key>NSStringLocalizedFormatKey</key>
        <string>%#@[email protected] selected</string>
        <key>num_files_are</key>
        <dict>
            <key>NSStringFormatSpecTypeKey</key>
            <string>NSStringPluralRuleType</string>
            <key>NSStringFormatValueTypeKey</key>
            <string>d</string>
            <key>zero</key>
            <string>No file is</string>
            <key>one</key>
            <string>A file is</string>
            <key>other</key>
            <string>%d files are</string>
        </dict>
    </dict>
</dict>
</plist>

The code to use:

let numberOfFiles = 1
let format = NSLocalizedString("%d file(s) are selected", comment: "")
let localizedString = String(format: format, numberOfFiles)

There are 2 distinct calls here:

1. You need to get the “format” using the macro NSLocalizedString. In the code above, the variable format will be %#@[[email protected]](/cdn-cgi/l/email-protection) selected. Note: This is not yet localized, unlike the usual localization!
2. Init String with this format and the actual arguments, and you will get the localized string.

Basics of the XML:

• %d file(s) are selected is simply a key to refer to this 1 string to localize. But it is NOT necessary to have the %d in the key. You can rename the key as eg files and it will work. Having the %d is more of a good convention, telling us 1 number should be provided as an argument.
• %#@[[email protected]](/cdn-cgi/l/email-protection) selected is known as a format, which is made up of 1 variable num_files_are and the text “ selected”.
• num_files_are is a key (to the variable) with a dict to explain the rules:
  • NSStringPluralRuleType is plural rules (there could be others in the future)
  • NSStringFormatValueTypeKey describe what type of argument works with this variable eg. d for Int
  • zero is a rule. When the argument is 0, it will use “No file is” to replace the variable num_files_are, therefore finally forming “No file is selected”
  • one is another rule
  • other is another rule. This time, it will use “%d files are”, and the argument is used here, finally forming eg. “2 files are selected”

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Advanced Case

I like the example used in objc.io:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>scope.%lu out of %lu runs</key>
    <dict>
        <key>NSStringLocalizedFormatKey</key>
        <string>%1$#@[email protected]</string>
        <key>lu_completed_runs</key>
        <dict>
            <key>NSStringFormatSpecTypeKey</key>
            <string>NSStringPluralRuleType</string>
            <key>NSStringFormatValueTypeKey</key>
            <string>lu</string>
            <key>zero</key>
            <string>No runs completed yet</string>
            <key>one</key>
            <string>One %2$#@[email protected]</string>
            <key>other</key>
            <string>%lu %2$#@[email protected]</string>
        </dict>
        <key>lu_total_runs</key>
        <dict>
            <key>NSStringFormatSpecTypeKey</key>
            <string>NSStringPluralRuleType</string>
            <key>NSStringFormatValueTypeKey</key>
            <string>lu</string>
            <key>one</key>
            <string>run completed</string>
            <key>other</key>
            <string>of %lu runs completed</string>
        </dict>
    </dict>
</dict>
</plist>

The localized string requires 2 arguments:

let completedRuns = 1
let totalRuns = 2
let format = NSLocalizedString("scope.%lu out of %lu runs", comment: "")
let str = String(format: format, completedRuns, totalRuns)

Let’s look at the output first.

Completed runs    Total Runs    Output
------------------------------------------------------------------
0                 0+            No runs completed yet
1                 1             One run completed
1                 2+            One of x runs completed
2+                2+            x of y runs completed

It is an advanced use because the output depends on both the arguments.

The biggest difference is that this time, there are 2 variables, each with their set of rules, and one variable refer to another!

[圖片上傳失敗...(image-7c7029-1555568209760)]

• The format is %1$#@[[email protected]](/cdn-cgi/l/email-protection), which uses 1 variable lu_completed_runs (yes, just 1 variable in the format is okay), and that it is the first argument as specified by the 1$
• In some of the rules for lu_completed_runs, it uses a format %2$#@[[email protected]](/cdn-cgi/l/email-protection), which uses another variable lu_total_runs. This variable is the 2nd argument as specified by the 2$.
• The rules in lu_total_runs will produce part of the text for lu_completed_runs

There is some recursive lookup going on :)

The Puzzling String

Let’s look at the simple case again to explain something strange that goes on.

let format = NSLocalizedString("%d file(s) are selected", comment: "")
// format is "%#@[email protected] selected"
let str = String(format: format, 1)
// str is "1 file is selected"

If you have been using String(format:_:), you know it can format the string and replace with the arguments.

But it is hard to digest for the format %#@[[email protected]](/cdn-cgi/l/email-protection) selected, a mere string.. How does a mere string have access to the rules in stringsdict?

The string %#@[[email protected]](/cdn-cgi/l/email-protection) selected is not a mere string. It knows the rules, for a particular locale.

Try this and it will NOT work:

// let format = NSLocalizedString("%d file(s) are selected", comment: "")
let format = "%#@[email protected] selected"
let str = String(format: format, 1)

This proves that the macro NSLocalizedString, which return a String via Bundle.localizedString, is not a pure string.

I am puzzled and I hope someone can explain.

I can see NSString use CFStringCreateWithFormatAndArguments. In CFString, it mentions stringsDictConfig. It seems like the system refer to this dictionary to look up the rules and format the final string.