dim.red
新出的design:23.2.0 引入了一个新的Behavior :BottomSheetBehavior

问题

设置 BottomSheetBehavior 没有设置 PeekHeight 或者 PeekHeight为0 的时候. 底部的BottomSheetBehavior 视图滑动不出来.

分析

根据进入源码,具体的问题是

@Override
public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {
    // First let the parent lay it out
    if (mState != STATE_DRAGGING && mState != STATE_SETTLING) {
        parent.onLayoutChild(child, layoutDirection);
    }
    // Offset the bottom sheet
    mParentHeight = parent.getHeight();
    mMinOffset = Math.max(0, mParentHeight - child.getHeight());
    mMaxOffset = mParentHeight - mPeekHeight;
    if (mState == STATE_EXPANDED) {
        ViewCompat.offsetTopAndBottom(child, mMinOffset);
    } else if (mHideable && mState == STATE_HIDDEN) {
        ViewCompat.offsetTopAndBottom(child, mParentHeight);
    } else if (mState == STATE_COLLAPSED) {
        ViewCompat.offsetTopAndBottom(child, mMaxOffset);
    }
    if (mViewDragHelper == null) {
        mViewDragHelper = ViewDragHelper.create(parent, mDragCallback);
    }
    mViewRef = new WeakReference<>(child);
    mNestedScrollingChildRef = new WeakReference<>(findScrollingChild(child));
    return true;
}

一开始view 的位置是屏幕中的.因为 mState 的初始状态为STATE_COLLAPSED,mPeekHeight 为0,
mMaxOffset 变成了 rv 的高度,所以view 马上就被移出了屏幕.
然后后面View就全程透明了.很奇葩.

解决方案

解决方案 1

主动发起requestLayout 强制重绘.

behavior.setBottomSheetCallback(new BottomSheet.BottomSheetCallback() {
    public boolean hasRequest;

    @Override
    public void onStateChanged(@NonNull View bottomSheet, int newState) {
    }

    @Override
    public void onSlide(@NonNull View bottomSheet, float slideOffset) {

        if (!hasRequest && behavior.getPeekHeight() == 0 && slideOffset > 0) {
            hasRequest = true;
            bottomSheet.requestLayout();
        }
    }
});

缺点是使用了requestLayout 方法.

解决方案2

这是幸运Science提供的

 

bottomSheet.setTranslationY(statusbarheight);

behavior.setBottomSheetCallback(new BottomSheet.BottomSheetCallback() {
    public boolean hasRequest;

    @Override
    public void onStateChanged(@NonNull View bottomSheet, int newState) {
    }

    @Override
    public void onSlide(@NonNull View bottomSheet, float slideOffset) {

        if (!hasRequest && behavior.getPeekHeight() == 0 && slideOffset > 0) {
            hasRequest = true;
            bottomSheet.setTranslationY(0);
        }
    }
});

通过设置TranslationY的差值.让他重绘出来.
恍惚间记得google Design 包下有类似的解决方案,同样使用的是TranslationY差值的方式.

类:android.support.design.widget.ViewOffsetHelper
方法:updateOffsets

 
private void updateOffsets() {
    ViewCompat.offsetTopAndBottom(mView, mOffsetTop - (mView.getTop() - mLayoutTop));
    ViewCompat.offsetLeftAndRight(mView, mOffsetLeft - (mView.getLeft() - mLayoutLeft));

    // Manually invalidate the view and parent to make sure we get drawn pre-M
    if (Build.VERSION.SDK_INT < 23) {
        tickleInvalidationFlag(mView);
        final ViewParent vp = mView.getParent();
        if (vp instanceof View) {
            tickleInvalidationFlag((View) vp);
        }
    }
}

private static void tickleInvalidationFlag(View view) {
    final float y = ViewCompat.getTranslationY(view);
    ViewCompat.setTranslationY(view, y + 1);
    ViewCompat.setTranslationY(view, y);
}

可以看出这是一个23以下都有可能存在的bug.
同时需要注意的是ViewOffsetHelper是一个私有的类,updateOffsets是一个私有的方法.所以你只能copy,不能直接使用.

最终的方案

就是在方案2的基础上做一点小修改,改使用google的方案.

 
behavior.setBottomSheetCallback(new BottomSheet.BottomSheetCallback() {
    public boolean hasRequest;

    @Override
    public void onStateChanged(@NonNull View bottomSheet, int newState) {
    }

    @Override
    public void onSlide(@NonNull View bottomSheet, float slideOffset) {

        if (!hasRequest && behavior.getPeekHeight() == 0 && slideOffset > 0) {
            hasRequest = true;
            updateOffsets(bottomSheet);
        }
    }
});

private void updateOffsets(View view) {

    // Manually invalidate the view and parent to make sure we get drawn pre-M
    if (Build.VERSION.SDK_INT < 23) {
        tickleInvalidationFlag(view)
        final ViewParent vp = view.getParent();
        if (vp instanceof View) {
            tickleInvalidationFlag((View) vp);
        }
    }
}

private static void tickleInvalidationFlag(View view) {
    final float y = ViewCompat.getTranslationY(view);
    ViewCompat.setTranslationY(view, y + 1);
    ViewCompat.setTranslationY(view, y);
}

尾巴

google 在开发这个的时候应该并没有考虑这样的应用场景. 如果场景是这样的话可以是使用BottomSheetDialog 这个代替.