Virtual file-system layer

To improve cross platform support, all file access (and path manipulation) is now done through a well known interface (FileSystem).

Note that FileSystem extends ReadonlyFileSystem, which itself extends PathManipulation. If you are using a file-system object you should only ask for the type that supports all the methods that you require. For example, if you have a function (foo()) that only needs to resolve paths then it should only require PathManipulation: foo(fs: PathManipulation). This allows the caller to avoid implementing unneeded functionality.

For testing, a number of MockFileSystem implementations are supplied. These provide an in-memory file-system which emulates operating systems like OS/X, Unix and Windows.

The current file system is always available via the helper method, getFileSystem(). This is also used by a number of helper methods to avoid having to pass FileSystem objects around all the time. The result of this is that one must be careful to ensure that the file-system has been initialized before using any of these helper methods. To prevent this happening accidentally the current file system always starts out as an instance of InvalidFileSystem, which will throw an error if any of its methods are called.

Generally it is safer to explicitly pass file-system objects to constructors or free-standing functions if possible. This avoids confusing bugs where the global file-system has not been set-up correctly before calling functions that expect there to be a file-system configured globally.

You can set the current file-system by calling setFileSystem(). During testing you can call the helper function initMockFileSystem(os) which takes a string name of the OS to emulate, and will also monkey-patch aspects of the TypeScript library to ensure that TS is also using the current file-system.

Finally there is the NgtscCompilerHost to be used for any TypeScript compilation, which uses a given file-system.

All tests that interact with the file-system should be tested against each of the mock file-systems. A series of helpers have been provided to support such tests:

• runInEachFileSystem() - wrap your tests in this helper to run all the wrapped tests in each of the mock file-systems, it calls initMockFileSystem() for each OS to emulate.
• loadTestFiles() - use this to add files and their contents to the mock file system for testing.
• loadStandardTestFiles() - use this to load a mirror image of Angular test files on disk into the in-memory mock file-system.
• loadAngularCore() - use this to load the npm package of @angular/core into the mock file-system.

All ngtsc source and tests now use this virtual file-system setup.