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53 lines
1.5 KiB
TypeScript
53 lines
1.5 KiB
TypeScript
/**
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* Use y = mx + b to
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* figure out the backdrop value
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* at a particular x coordinate. This
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* is useful when the backdrop does
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* not begin to fade in until after
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* the 0 breakpoint.
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*/
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export const getBackdropValueForSheet = (x: number, backdropBreakpoint: number) => {
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/**
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* We will use these points:
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* (backdropBreakpoint, 0)
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* (maxBreakpoint, 1)
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* We know that at the beginning breakpoint,
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* the backdrop will be hidden. We also
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* know that at the maxBreakpoint, the backdrop
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* must be fully visible. maxBreakpoint should
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* always be 1 even if the maximum value
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* of the breakpoints array is not 1 since
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* the animation runs from a progress of 0
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* to a progress of 1.
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* m = (y2 - y1) / (x2 - x1)
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*
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* This is simplified from:
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* m = (1 - 0) / (maxBreakpoint - backdropBreakpoint)
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*/
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const slope = 1 / (1 - backdropBreakpoint);
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/**
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* From here, compute b which is
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* the backdrop opacity if the offset
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* is 0. If the backdrop does not
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* begin to fade in until after the
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* 0 breakpoint, this b value will be
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* negative. This is fine as we never pass
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* b directly into the animation keyframes.
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* b = y - mx
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* Use a known point: (backdropBreakpoint, 0)
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* This is simplified from:
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* b = 0 - (backdropBreakpoint * slope)
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*/
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const b = -(backdropBreakpoint * slope);
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/**
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* Finally, we can now determine the
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* backdrop offset given an arbitrary
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* gesture offset.
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*/
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return (x * slope) + b;
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}
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