使用RecyclerView实现一个画廊效果，主要是使用support库中最新加入的PagerSnapHelper类，通过计算滑动偏移来计算scale的值。
基本实现
首先需要为RecyclerView添加一个滚动监听，然后为RecyclerView的第一个与最后一个itemView添加一个ItemDecoration，使位于第一个与最后一个itemView的位置居中对齐。
public void attachToRecyclerView(final RecyclerView recyclerView) {
this.recyclerView = recyclerView;
snapHelper.attachToRecyclerView(recyclerView);
recyclerView.addOnScrollListener(scrollListener);
recyclerView.addItemDecoration(new ScalableCardItemDecoration());
recyclerView.post(new Runnable() {
@Override
public void run() {
pageScrolled();
}
});
}
ScalableCardItemDecoration用于计算itemView左右剩余空间，然后为第一个itemView与最后一个itemView添加偏移量。
private static class ScalableCardItemDecoration extends RecyclerView.ItemDecoration {
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
RecyclerView.ViewHolder holder = parent.getChildViewHolder(view);
int position = holder.getAdapterPosition() == RecyclerView.NO_POSITION ? holder.getOldPosition() : holder.getAdapterPosition();
RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
int itemCount = layoutManager.getItemCount();
if(position != 0 && position != itemCount - 1){
return;
}
int peekWidth = getPeekWidth(parent, view);
boolean isVertical = layoutManager.canScrollVertically();
//移除item时adapter position为-1。
if (isVertical) {
if (position == 0) {
outRect.set(0, peekWidth, 0, 0);
} else if (position == itemCount - 1) {
outRect.set(0, 0, 0, peekWidth);
} else {
outRect.set(0, 0, 0, 0);
}
} else {
if (position == 0) {
outRect.set(peekWidth, 0, 0, 0);
} else if (position == itemCount - 1) {
outRect.set(0, 0, peekWidth, 0);
} else {
outRect.set(0, 0, 0, 0);
}
}
}
}在为item添加ItemDecoration时，需要计算两边的空间。这里需要手动测量itemView的宽高， 然后计算第一个itemView左边的偏移与最后一个itemView右边的偏移。这里碰到个问题，如果LayoutMangaer的方面是垂直或水平的且RecyclerView对应的的高度或宽度设为wrap_content时，这里计算的值不会正确。
public static int getPeekWidth(RecyclerView recyclerView, View itemView) {
RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
boolean isVertical = layoutManager.canScrollVertically();
int position = recyclerView.getChildAdapterPosition(itemView);
//TODO RecyclerView使用wrap_content时，获取的宽度可能会是0。
int parentWidth = recyclerView.getMeasuredWidth();
int parentHeight = recyclerView.getMeasuredHeight(); //有时会拿到0
parentWidth = parentWidth == 0 ? recyclerView.getWidth() : parentWidth;
parentHeight = parentHeight == 0 ? recyclerView.getHeight() : parentHeight;
int parentEnd = isVertical ? parentHeight : parentWidth;
int parentCenter = parentEnd / 2;
int itemSize = isVertical ? itemView.getMeasuredHeight() : itemView.getMeasuredWidth();
if (itemSize == 0) {
ViewGroup.LayoutParams layoutParams = itemView.getLayoutParams();
int widthMeasureSpec =
RecyclerView.LayoutManager.getChildMeasureSpec(parentWidth,
layoutManager.getWidthMode(),
recyclerView.getPaddingLeft() + recyclerView.getPaddingRight(),
layoutParams.width, layoutManager.canScrollHorizontally());
int heightMeasureSpec =
RecyclerView.LayoutManager.getChildMeasureSpec(parentHeight,
layoutManager.getHeightMode(),
recyclerView.getPaddingTop() + recyclerView.getPaddingBottom(),
layoutParams.height, layoutManager.canScrollVertically());
itemView.measure(widthMeasureSpec, heightMeasureSpec);
itemSize = isVertical ? itemView.getMeasuredHeight() : itemView.getMeasuredWidth();
}
/*
计算ItemDecoration的大小，确保插入的大小正好使view的start + itemSize / 2等于parentCenter。
*/
int startOffset = parentCenter - itemSize / 2;
int endOffset = parentEnd - (startOffset + itemSize);
return position == 0 ? startOffset : endOffset;
}
添加完ItemDecoration后，我们需要在RecyclerView每次滚动的时候计算左、中、右3个itemView的缩放比例。
private void pageScrolled() {
if (recyclerView == null || recyclerView.getChildCount() == 0)
return;
RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
View snapingView = snapHelper.findSnapView(layoutManager);
int snapingViewPosition = recyclerView.getChildAdapterPosition(snapingView);
View leftSnapingView = layoutManager.findViewByPosition(snapingViewPosition - 1);
View rightSnapingView = layoutManager.findViewByPosition(snapingViewPosition + 1);
float leftSnapingOffset = calculateOffset(recyclerView, leftSnapingView);
float rightSnapingOffset = calculateOffset(recyclerView, rightSnapingView);
float currentSnapingOffset = calculateOffset(recyclerView, snapingView);
if (snapingView != null) {
snapingView.setScaleX(currentSnapingOffset);
snapingView.setScaleY(currentSnapingOffset);
}
if (leftSnapingView != null) {
leftSnapingView.setScaleX(leftSnapingOffset);
leftSnapingView.setScaleY(leftSnapingOffset);
}
if (rightSnapingView != null) {
rightSnapingView.setScaleX(rightSnapingOffset);
rightSnapingView.setScaleY(rightSnapingOffset);
}
if(snapingView != null && currentSnapingOffset >= 1){
OnPageChangeListener listener = pageChangeListenerRef != null ? pageChangeListenerRef.get(): null;
if(listener != null)
listener.onPageSelected(snapingViewPosition);
}
Log.d(TAG, String.format("left: %f, right: %f, current: %f", leftSnapingOffset, rightSnapingOffset, currentSnapingOffset));
}
计算缩放比例时是根据左、中、右三个itemView的中间点与RecyclerView中间点的距离来计算的。
private float calculateOffset(RecyclerView recyclerView, View view) {
if (view == null)
return -1;
RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
boolean isVertical = layoutManager.canScrollVertically();
int viewStart = isVertical ? view.getTop() : view.getLeft();
int viewEnd = isVertical ? view.getBottom() : view.getRight();
int centerX = isVertical ? recyclerView.getHeight() / 2 : recyclerView.getWidth() / 2;
int childCenter = (viewStart + viewEnd) / 2;
int distance = Math.abs(childCenter - centerX);
if (distance > centerX)
return STAY_SCALE;
float offset = 1.f - (distance / (float) centerX);
return (1.f - STAY_SCALE) * offset + STAY_SCALE;
}项目地址： https://github.com/yjwfn/recyclerview-gallery