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Adding movementX and movementY to synthenticMouseEvent fixes #6723 #9018

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merged 3 commits into from
Jun 15, 2018
Merged

Adding movementX and movementY to synthenticMouseEvent fixes #6723 #9018

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5b2676b
adding movementX and movementY into syntheticMouseEvent
Jun 11, 2018
51bc415
fixing case mistake
jasonwilliams Jun 11, 2018
2cff055
Add test fixture for movementX/Y fields
nhunzaker Jun 15, 2018
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21 changes: 21 additions & 0 deletions packages/react-dom/src/events/SyntheticMouseEvent.js
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,9 @@
import SyntheticUIEvent from './SyntheticUIEvent';
import getEventModifierState from './getEventModifierState';

let previousScreenX = null;
let previousScreenY = null;

/**
* @interface MouseEvent
* @see http://www.w3.org/TR/DOM-Level-3-Events/
Expand All @@ -34,6 +37,24 @@ const SyntheticMouseEvent = SyntheticUIEvent.extend({
: event.fromElement)
);
},
movementX: function(event) {
if ('movementX' in event) {
return event.movementX;
}

const screenX = previousScreenX;
previousScreenX = event.screenX;
return screenX ? event.screenX - screenX : 0;
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I think the check here intends to check against null but will accidentally also be falsy if screenX is actually equal to 0. Please avoid truthy checks like this, they tend to hide bugs.

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@aweary aweary Jun 15, 2018 •
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If it's equal to 0 then it will still return 0 since that's the second expression in the ternary.

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I think that if saved screenX is 0, but then you moved to 200, you want to return 200 - 0 rather than 0.

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I believe Dan is right, and we need to correct this.

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https://github.com/tc39/proposal-nullish-coalescing would have helped a lot with this, yes we should check if equal to null

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Another thing is that using the same variable for both null and numeric values can mess with VM optimizations. I think it would be better to use a separate boolean flag here.

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Also, are we happy to assume screenX on event? If undefined we could end up with undefined - null

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Yep, screenX/Y are very well supported.

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@jasonwilliams jasonwilliams Jun 20, 2018 •
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Ok so if screenX will always be an Number, could we not just initiate previousScreenX as 0 Instead of null, and then instead of a ternary or null checking, always perform the minus operation. Because we can guarantee both operands will be Numbers by this point .

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Wouldn't make the first movementX equal to the current screen coordinate, instead of 0? I think 0 would be expected (because the cursor didn't "jump" from top left point).

},
movementY: function(event) {
if ('movementY' in event) {
return event.movementY;
}

const screenY = previousScreenY;
previousScreenY = event.screenY;
return screenY ? event.screenY - screenY : 0;
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Same.

},
});

export default SyntheticMouseEvent;