# 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.
