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Image: Address onLoad not being called for SSR images #3612

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merged 2 commits into from
Jun 5, 2024
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Image: Address onLoad not being called for SSR images #3612

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Image: Address onLoad not being called
liuyenwei Jun 5, 2024
99b7294
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liuyenwei Jun 5, 2024
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32 changes: 32 additions & 0 deletions packages/gestalt/src/Image.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,10 @@ type Props = {
* A comma-separated list of one or more strings indicating a set of possible image sources for the user agent to use.
*/
srcSet?: string;
/**
* Experimental prop: Fixes an issue where onLoad is not triggered if the image is already loaded by the time the component renders (i.e. SSR)
*/
_fixCompletedOnLoad?: boolean;
};

/**
Expand All @@ -102,6 +106,8 @@ export default class Image extends PureComponent<Props> {

static displayName: string | null | undefined = 'Image';

onLoadCalled: boolean = false;

componentDidMount() {
if (shouldScaleImage(this.props.fit)) {
this.loadImage();
Expand All @@ -116,6 +122,7 @@ export default class Image extends PureComponent<Props> {
}

handleLoad: (event: React.SyntheticEvent<HTMLImageElement>) => void = (event) => {
this.onLoadCalled = true;
this.props.onLoad?.({ event });
};

Expand All @@ -134,6 +141,30 @@ export default class Image extends PureComponent<Props> {
}
}

refCallback = (node: HTMLImageElement | null) => {
const { _fixCompletedOnLoad } = this.props;
// For certain scenarios, such as server-side rendering, the image may already be loaded by the time the component is rendered resulting in the onLoad event not being triggered.
// To address these, we can use a ref callback and check whether the image is already loaded - if it is, we trigger the onLoad event manually.
if (_fixCompletedOnLoad && node?.complete && !this.onLoadCalled) {
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!this.onLoadCalled

so handleLoad when image loads and we set this.onLoadCalled = true;. Then, when it renders we set the ref and we check onLoadCalled

handleLoad gets sets manually if this.onLoadCalled = true;.

"if it is, we trigger the onLoad event manually."

should && !this.onLoadCalled be && this.onLoadCalled

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!this.onLoadCalled is here to guard against a scenario where the native img load callback is fired (and onLoad called) before the ref callback

i think the only situation where this could happen is:

  • image component renders
  • image request completes very quickly
  • ref callback is triggered

i think this would be an edge case but wanted to play it safe

// Since we don't have the SyntheticEvent here,
// we must create one with the same shape.
// See https://reactjs.org/docs/events.html
const loadEvent = new Event('load');
Object.defineProperty(loadEvent, 'target', { writable: false, value: node });
this.handleLoad({
...loadEvent,
nativeEvent: loadEvent,
currentTarget: node,
target: node,
isDefaultPrevented: () => false,
isPropagationStopped: () => false,
persist: () => {},
preventDefault: () => {},
stopPropagation: () => {},
});
}
};

render() {
const {
alt,
Expand Down Expand Up @@ -183,6 +214,7 @@ export default class Image extends PureComponent<Props> {
{...(isScaledImage ? { height: '100%' } : {})}
>
<img
ref={this.refCallback}
alt={alt}
className={imageStyles}
crossOrigin={crossOrigin}
Expand Down
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