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Displaying images is a fascinating subject, React Native uses some cool tricks to make it a better experience.
No Automatic Sizing
If you don't give a size to an image, the browser is going to render a 0x0 element, download the image, and then render the image based with the correct size. The big issue with this behavior is that your UI is going to jump all around as images load, this makes for a very bad user experience.
In React Native, this behavior is intentionally not implemented. It is more work for the developer to know the dimensions (or just aspect ratio) of the image in advance, but we believe that it leads to a better user experience.
Background Image via Nesting
A common feature request from developers familiar with the web is background-image. It turns out that iOS has a very elegant solution to this: you can add elements as a children to an <Image> component. This simplifies the API and solves the use case.
return (
<Image source={...}>
<Text>Inside</Text>
</Image>
);
Off-thread Decoding
Image decoding can take more than a frame-worth of time. This is one of the major source of frame drops on the web because decoding is done in the main thread. In React Native, image decoding is done in a different thread. In practice, you already need to handle the case when the image is not downloaded yet, so displaying the placeholder for a few more frames while it is decoding does not require any code change.
Static Assets
In the course of a project you add and remove images and in many instances, you end up shipping images you are not using anymore in the app. In order to fight this, we need to find a way to know statically which images are being used in the app. To do that, we introduced a marker called ix. The only allowed way to refer to an image in the bundle is to literally write ix('name-of-the-asset') in the source.
var { ix } = React;
// GOOD
<Image source={ix('my-icon')} />
// BAD
var icon = this.props.active ? 'my-icon-active' : 'my-icon-inactive';
<Image source={ix(icon)} />
// GOOD
var icon = this.props.active ? ix('my-icon-active') : ix('my-icon-inactive');
<Image source={icon} />
When your entire codebase respects this convention, you're able to do interesting things like automatically packaging the assets that are being used in your app. Note that in the current form, nothing is enforced, but it will be in the future.
Best Camera Roll Image
iOS saves multiple sizes for the same image in your Camera Roll, it is very important to pick the one that's as close as possible for performance reasons. You wouldn't want to use the full quality 3264x2448 image as source when displaying a 200x200 thumbnail. If there's an exact match, React Native will pick it, otherwise it's going to use the first one that's at least 50% bigger in order to avoid blur when resizing from a close size. All of this is done by default so you don't have to worry about writing the tedious (and error prone) code to do it yourself.
Source being an object
In React Native, one interesting decision is that the src attribute is named source and doesn't take a string but an object with an uri attribute.
<Image source={{uri: 'something.jpg'}} />
On the infrastructure side, the reason is that it allows to attach metadata to this object. For example if you are using ix, then we add a isStored attribute to flag it as a local file (don't rely on this fact, it's likely to change in the future!). This is also future proofing, for example we may want to support sprites at some point, instead of outputting {uri: ...}, we can output {uri: ..., crop: {left: 10, top: 50, width: 20, height: 40}} and transparently support spriting on all the existing call sites.
On the user side, this lets you annotate the object with useful attributes such as the dimension of the image in order to compute the size it's going to be displayed in. Feel free to use it as your data structure to store more information about your image.