switching between languages in same file - intellij-idea

Question

I recently attended a user group meeting where the IntelliJ representative was demonstrating version 13.
He demonstrated how to switch the code completion view of a file. I do not exactly remember what the file extension of this particular file was, probably java.
The concept was that if the file is html with embedded javascript he could then switch the code completion between html and javascript with a shortcut. If he says treat the file as html then all code in file was treated for code completion purposes as html, and vice versa for javascript.
Does anybody know what shortcut he might have been using to enable the language switch?

Answer 1

Sounds like you may be referring to the IntelliLang feature. IntelliJ IDEA can be aware of other languages embedded within a file.
A simple example is in an HTML file that has CSS and JavaScript.
Notice when I am inside the HTML markup:
or inside an HTML element:
The code complete shows HTML completion options. However, when I am inside the style attribute, I get CSS code completion:
I also get CSS code completion if I am inside a <style> element. So even though I am in an HTML file, I see CSS code completion because of my location.
Same case with JavaScript. When I invoke code completion inside a <script> element, I get JavaScript completion, even though I am in an HTML file.
Anytime IntelliJ IDEA can determine that another embedded language is present, it provides, via IntelliLang, the appropriate syntax highlighting, error highlighting, and code completion. The same holds true for Java. Notice here that IDEA knows the method I am competing takes an SQL statement and therefore highlights the String value using SQL highlighting, and provides SQL code completion:
So even though I am in a .java file, I get SQL code completion. The reason is that IntelliLang comes pre-configured knowing the embedded language of some methods. You modify them, or add more, in File > Settings > [Project Settings] > Language Injections.
In addition, you can use an annotation to tell IntelliJ IDEA (as well as developers looking at the code) that a String must be valid in a particular language. For example, I can annotate a String field, variable, or parameter, to indicate it must be valid HTML:
Notice I get HTML syntax highlighting, HTML code completions, and the CSS color shows in the left gutter. If I annotate a method parameter, then any time I call the method, I get the appropriate syntax highlighting, code completion, and error/warning highlighting:
The #Language annotation is inside the annotations.jar that is contained in the redist directory inside the IntelliJ IDEA installation directory. It is also available in maven central, or IDEA will offer to attach it as a Library if you use the annotation without it being attached.
IntelliLang and the #Language annotation supports a large number of languages. Just use code Completion inside the quotes after typing #Language("") to see a list. (Inline search works in the list as well.) One of the most useful is Regexp. For example, if you have a method that expects the string passed in to be a valid Regular Expression, annotating it as such will give anyone that calls it Regex code completion and error highlighting if they are passing in an invalid Regex pattern. Even for developers using other IDEs it is useful as a form of documentation.
As for a shortcut to change the the language on the fly for code completion, the only thing I can think that you might be referring to is the "Inject Language" intention. If I am entering a String value, and I bring up the quick-fix/intention menu via Alt+Enter, I am given an option to inject a language:
If I select that, IntelliJ IDEA will ask me what language I want to use:
After making my selection, IntelliJ IDEA will give me temporary language injection (including code completion) for the selected language.
It also gives me an option to add the #Language annotation for permanent injection.
To the best of my knowledge (as a 10 year IntelliJ IDEA user) that is the only way to switch code completion language types. So hopefully that is what you are looking for. To me, IntelliLang is one of the coolest features in IntelliJ. (It actually started as a third party plug-in and JetBrains then absorbed it into the product.)