UncontainedCarouselStrategy

public KeylineState onFirstChildMeasuredWithMargins(Carousel carousel, View child)

Calculates a keyline arrangement and returns a constructed KeylineState .

This method is called when Carousel measures the first item to be added to its scroll container. This method then must create a KeylineState which tells Carousel how to fill the scroll container with items - how many are visible at once, what their sizes are, and where they're placed.

For example, take a simple arrangement that fills the scroll container with two large items and one small item. As the user scrolls the first large item moves off-screen to the left, the second large item moves to position 1, the small item unmasks into a large item at position 2, and a new small item scrolls into view from the right. To create this arrangement, pick any size for the small item that will be smaller than the large item. Next, take the carousel's total space, subtract the small item size and divide the remainder by two - this is your large item size. After determining the size of our large and small items, we can now construct a KeylineState and add keylines representing each item:

// Find the centers of the items in our arrangement, aligning the first item's left with the
// left of the scroll container (0).
floatfirstLargeItemCenter=largeChildSize/2F;
floatsmallItemCenter=(largeChildSize*2F)+(smallChildSize/2F);
// Get our child margins to use when calculating mask percentage
LayoutParamschildLayoutParams=(LayoutParams)child.getLayoutParams();
floatchildMargins=childLayoutParams.leftMargin+childLayoutParams.rightMargin;
returnnewKeylineState.Builder(largeChildWidth)
.addKeylineRange(
firstLargeItemCenter,// offsetLoc
getChildMaskPercentage(largeChildSize,largeChildSize,childMargins),// mask
largeChildSize,// maskedItemSize
2,// count
true)// isFocal
.addKeyline(
smallItemCenter,// offsetLoc
getChildMaskPercentage(smallChildSize,largeChildSize,childMargins),// mask
smallChildSize);// maskedItemSize

A strategy does not need to take layout direction into account. CarouselLayoutManager automatically reverses the strategy's KeylineState when laid out in right-to-left. Additionally, CarouselLayoutManager shifts the focal keylines to the start or end of the container when at the start or end of a list in order to allow every item in the list to pass through the focal state.

For additional guidelines on constructing valid KeylineStates, see KeylineState.Builder .