# Application Development

There are no hard and fast rules for developing applications with Wails, but there are some basic guidelines.

## Application Setup

The pattern used by the default templates are that `main.go` is used for configuring and running the application, whilst `app.go` is used for defining the application logic.

The `app.go` file will define a struct that has 2 methods which act as hooks into the main application:

```go title="app.go"
type App struct {
    ctx context.Context
}

func NewApp() *App {
    return &App{}
}

func (a *App) startup(ctx context.Context) {
    a.ctx = ctx
}

func (a *App) shutdown(ctx context.Context) {
}
```

- The startup method is called as soon as Wails allocates the resources it needs and is a good place for creating resources, setting up event listeners and anything else the application needs at startup. It is given a `context.Context` which is usually saved in a struct field. This context is needed for calling the [runtime](../reference/runtime/intro.mdx). If this method returns an error, the application will terminate. In dev mode, the error will be output to the console.

- The shutdown method will be called by Wails right at the end of the shutdown process. This is a good place to deallocate memory and perform any shutdown tasks.

The `main.go` file generally consists of a single call to `wails.Run()`, which accepts the application configuration. The pattern used by the templates is that before the call to `wails.Run()`, an instance of the struct we defined in `app.go` is created and saved in a variable called `app`. This configuration is where we add our callbacks:

```go {3,9,10} title="main.go"
func main() {

    app := NewApp()

    err := wails.Run(&options.App{
        Title:             "My App",
        Width:             800,
        Height:            600,
        OnStartup:  app.startup,
        OnShutdown: app.shutdown,
    })
    if err != nil {
        log.Fatal(err)
    }
}

```

More information on application lifecycle hooks can be found [here](../howdoesitwork.mdx#application-lifecycle-callbacks).

## Binding Methods

It is likely that you will want to call Go methods from the frontend. This is normally done by adding public methods to the already defined struct in `app.go`:

```go {16-18} title="app.go"
type App struct {
    ctx context.Context
}

func NewApp() *App {
    return &App{}
}

func (a *App) startup(ctx context.Context) {
    a.ctx = ctx
}

func (a *App) shutdown(ctx context.Context) {
}

func (a *App) Greet(name string) string {
    return fmt.Sprintf("Hello %s!", name)
}
```

In the main application configuration, the `Bind` key is where we can tell Wails what we want to bind:

```go {11-13} title="main.go"
func main() {

    app := NewApp()

    err := wails.Run(&options.App{
        Title:             "My App",
        Width:             800,
        Height:            600,
        OnStartup:  app.startup,
        OnShutdown: app.shutdown,
        Bind: []interface{}{
            app,
        },
    })
    if err != nil {
        log.Fatal(err)
    }
}

```

This will bind all public methods in our `App` struct (it will never bind the startup and shutdown methods).

### Dealing with context when binding multiple structs

If you want to bind methods for multiple structs but want each struct to keep a reference to the context so that you can use the runtime functions, a good pattern is to pass the context from the `OnStartup` method to your struct instances :

```go
func main() {

    app := NewApp()
    otherStruct := NewOtherStruct()

    err := wails.Run(&options.App{
        Title:             "My App",
        Width:             800,
        Height:            600,
        OnStartup:  func(ctx context.Context){
            app.SetContext(ctx)
            otherStruct.SetContext(ctx)
        },
        OnShutdown: app.shutdown,
        Bind: []interface{}{
            app,
            otherStruct
        },
    })
    if err != nil {
        log.Fatal(err)
    }
}
```

More information on Binding can be found [here](../howdoesitwork.mdx#method-binding).

## Application Menu

Wails supports adding a menu to your application. This is done by passing a [Menu](../reference/menus.mdx#menu) struct to application config. It's common to use a method that returns a Menu, and even more common for that to be a method on the `App` struct used for the lifecycle hooks.

```go {11} title="main.go"
func main() {

    app := NewApp()

    err := wails.Run(&options.App{
        Title:             "My App",
        Width:             800,
        Height:            600,
        OnStartup:  app.startup,
        OnShutdown: app.shutdown,
        Menu:       app.menu(),
        Bind: []interface{}{
            app,
        },
    })
    if err != nil {
        log.Fatal(err)
    }
}

```

## Assets

The great thing about the way Wails v2 handles assets is that it doesn't! The only thing you need to give Wails is an `embed.FS`. How you get to that is entirely up to you. You can use vanilla html/css/js files like the vanilla template. You could have some complicated build system, it doesn't matter.

When `wails build` is run, it will check the `wails.json` project file at the project root. There are 2 keys in the project file that are read:

- "frontend:install"
- "frontend:build"

The first, if given, will be executed in the `frontend` directory to install the node modules. The second, if given, will be executed in the `frontend` directory to build the frontend project.

If these 2 keys aren't given, then Wails does absolutely nothing with the frontend. It is only expecting that `embed.FS`.

### AssetsHandler

A Wails v2 app can optionally define a `http.Handler` in the `options.App`, which allows hooking into the AssetServer to create files on the fly or process POST/PUT requests. GET requests are always first handled by the `assets` FS. If the FS doesn't find the requested file the request will be forwarded to the `http.Handler` for serving. Any requests other than GET will be directly processed by the `AssetsHandler` if specified. It's also possible to only use the `AssetsHandler` by specifiy `nil` as the `Assets` option.

## Built in Dev Server

Running `wails dev` will start the built in dev server which will start a file watcher in your project directory. By default, if any file changes, wails checks if it was an application file (default: `.go`, configurable with `-e` flag). If it was, then it will rebuild your application and relaunch it. If the changed file was in the assets, it will issue a reload after a short amount of time.

The dev server uses a technique called "debouncing" which means it doesn't reload straight away, as there may be multiple files changed in a short amount of time. When a trigger occurs, it waits for a set amount of time before issuing a reload. If another trigger happens, it resets to the wait time again. By default this value is `100ms`. If this value doesn't work for your project, it can be configured using the `-debounce` flag. If used, this value will be saved to your project config and become the default.

## External Dev Server

Some frameworks come with their own live-reloading server, however they will not be able to take advantage of the Wails Go bindings. In this scenario, it is best to run a watcher script that rebuilds the project into the build directory, which Wails will be watching. For an example, see the default svelte template that uses [rollup](https://rollupjs.org/guide/en/). For [create-react-app](https://create-react-app.dev/), it's possible to use [this script](https://gist.github.com/int128/e0cdec598c5b3db728ff35758abdbafd) to achieve a similar result.

## Go Module

The default Wails templates generate a `go.mod` file that contains the module name "changeme". You should change this to something more appropriate after project generation.
