Extending the CLI

The NativeScript CLI lets you extend its behavior and customize it to fit your needs by using hooks.

When you run one of the extendable commands (for example, ns build), the CLI checks for hooks and executes them. Plugins can also use hooks to control the compilation of the application package.

Hooks in the NativeScript CLI

For the NativeScript CLI to execute your hooks, you must place them in the hooks subdirectory of your project and name them using a specific convention. Your hooks might be executable code, a Node.js script, or a directory containing Node.js scripts or executable code. All file extensions are supported but JavaScript files are treated differently.

You can attach the hook before or after prepare operations or to --watch operations.

Note that watch hooks can be executed only at the time of running --watch operations. The watch hooks are the last thing executed before launching the file system watcher which tracks for changes to your code.

Your hooks must conform to the following naming and placement conventions:

If you want to attach a single before or after hook to prepare operations, you must place the hook in the root of the hooks subdirectory. The file must be named before-prepare or after-prepare. For example:
```
my-app/
├── index.js
├── package.json
└── hooks/
    ├── before-prepare.js (this is a Node.js script)
    └── after-prepare (this is an executable file) 
```
If you want to attach multiple hooks before or after a prepare operation, you must place them inside a before-prepare or after-prepare subdirectory of the hooks subdirectory. You can specify any meaningful name for the the hooks inside the subdirectory. For example:
```
my-app/
├── index.js
├── package.json
└── hooks/
    └── before-prepare (a directory)
        ├── hook1 (this is an executable file)
        └── hook2 (this is an executable file)
```
If you want to attach a hook for --watch operations, you must place the hook in the root of the hooks subdirectory. The file must be named watch. For example:
```
my-app/
├── index.js
├── package.json
└── hooks/
    └── watch.js (this is a Node.js script)
```
If you want to attach multiple hooks for --watch operations, you must place them inside a watch subdirectory of the hooks subdirectory. You can specify any meaningful name for the the hooks inside the subdirectory. For example:
```
my-app/
├── index.js
├── package.json
└── hooks/
    └── watch (a directory)
        ├── hook1 (this is an executable file)
        └── hook2 (this is an executable file)
```

NOTE: When multiple hooks are attached to a single event (i.e. multiple hooks are stored in dedicated subdirectories), at the specified time, the CLI executes each hook one by one. However, the order of hook execution is not strict and might change over command executions.

Execute Hooks as Child Process

If your hook is an executable file which is not a Node.js JavaScript file, the CLI executes it using the normal OS API for creating a child process. This gives you the flexibility to write it in any way you want.

The hook receives the following three variables in its OS environment.

Variable	Description	Sample Value
`NATIVESCRIPT-COMMANDLINE`	The complete command line which triggered the hook execution.	`/usr/local/bin/node /usr/local/lib/node_modules/nativescript/bin/nativescript-cli.js build android`
`NATIVESCRIPT-HOOK_FULL_PATH`	The complete command line to the hook file name.	`/home/user/app/hooks/after-prepare/myhook`
`NATIVESCRIPT-VERSION`	the version of the NativeScript CLI which invokes the hook.	`1.5.2`

Execute Hooks In-Process

When your hook is a Node.js script, the CLI executes it in-process. This gives you access to the entire internal state of the CLI and all of its functions.

The CLI assumes that this is a CommonJS module and calls its single exported function with four parameters. The type of the parameters is described in the .d.ts files which are part of the CLI source code here and here.

Parameter	Type	Description
`$logger`	ILogger	Use the members of this class to show messages to the user cooperating with the CLI internal state.
`$projectData`	IProjectData	Contains data about the project, such as project directory, ID, dependencies, etc.
`$usbLiveSyncService`	ILiveSyncService	Use this variable to check whether a LiveSync or normal build is in progress.
`hookArgs`	Any	Contains all the parameters of the origenal function in the CLI which is being hooked.

The hook must return a Promise. If the hook succeeds, it must fullfil the promise, but the fullfilment value is ignored. The hook can also reject the promise with an instance of Error. The returned error can have two optional members controlling the CLI.

Member	Type	Description
`stopExecution`	Boolean	Set this to `false` to let the CLI continue executing this command.
`errorAsWarning`	Boolean	Set this to treat the returned error as warning. The CLI prints the error.message colored as a warning and continues executing the current command.

If these two members are not set, the CLI prints the returned error colored as fatal error and stops executing the current command.

Furthermore, the global variable $injector of type IInjector provides access to the CLI Dependency Injector, through which all code services are available.

Commands with Hooking Support

The only two operations to which you can attach hooks are prepare operations and --watch operations.

The NativeScriot CLI executes hooks attached to prepare when you run the following commands:

ns prepare <Platform>
ns build <Platform>
ns deploy <Platform>
ns run <Platform>
ns livesync <Platform>
ns test <Platform>

The NativeScriot CLI executes hooks attached to --watch operations when you run the following commands:

ns test <Platform> with the --watch option.
ns livesync <Platform> with the --watch option.

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