# Observing Anything

As explained in the [dependency injection](/usage/injecting-dependencies.md) documentation, you can add dependencies to your `aperture` via React props. By passing observable data sources as props, you can observe anything you wish, and trigger side-effects in response.

In addition, your `aperture` is a blank slate, and allows you to observe anything which exists in the global context such as time or events.

## Events

By using your streaming library's utilities aimed at creating observables, you can respond to any events available in JavaScript.

For example, you can observe the `window.popstate` event to respond to the user clicking back/forward in the browser:

```javascript
const aperture = component => {
    const popstate$ = fromEvent(window, 'popstate').pipe(
        map(event => ({
            type: 'POPSTATE',
            state: event.state
        }))
    )
}
```

Or as another example, you could listen to the `window.resize` event in order to dynamically alter your logic depending on the user's screen size:

```javascript
const aperture = component => {
    const resize$ = fromEvent(window, 'resize').pipe(debounce(500))
}
```

## Time

Your streaming library will also provide utilities for observing the passing of time.

For example, if you want to do something once every few seconds as a kind of background process, it's easy to create an interval stream:

```javascript
const aperture = component => {
    const interval$ = interval(4000) // emits once every four seconds
}
```

## Custom Dependencies

It's also possible to create your own dependencies with custom methods similar to our `component.observe` and `store.observe`, and expose them through dependency injection via props. If you do, please publish them so that others can use it too!


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