Android中当item数量超过一定大小时，将RecyclerView高度固定

重写LayoutManger的onMeasure方法，这种方式可以获取到各个item的不同高度，从而可以设置变动的高度。

在使用这种方式时，有一点需要注意的是，不要将RecyclerView的android:layout_height属性设置为wrap_content，不然是不会成功的。

小于四高度自适应，大于四高度就固定

recyclerView.setLayoutManager(new LinearLayoutManager(this) {
    @Override
    public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
        int count = state.getItemCount();

        if (count > 0) {
            if(count>4){
                count =4;
            }
            int realHeight = 0;
            int realWidth = 0;
            for(int i = 0;i < count; i++){
                View view = recycler.getViewForPosition(0);
                if (view != null) {
                    measureChild(view, widthSpec, heightSpec);
                    int measuredWidth = View.MeasureSpec.getSize(widthSpec);
                    int measuredHeight = view.getMeasuredHeight();
                    realWidth = realWidth > measuredWidth ? realWidth : measuredWidth;
                    realHeight += measuredHeight;
                }
                setMeasuredDimension(realWidth, realHeight);
            }
        } else {
            super.onMeasure(recycler, state, widthSpec, heightSpec);
        }
    }
});