GridView的高度自適應

接着上面一篇文章。

當GridView不知道parent高度的時候，也就是MeasureSpec是UNSPECIFIED，這個時候，GridView高度為第一個child的高度，並顯示滾動條。

        mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
        final int count = mItemCount;
        if (count > 0) {
            final View child = obtainView(0, mIsScrap);

            AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
            if (p == null) {
                p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
                child.setLayoutParams(p);
            }
            p.viewType = mAdapter.getItemViewType(0);
            p.forceAdd = true;

            int childHeightSpec = getChildMeasureSpec(
                    MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED), 0, p.height);
            int childWidthSpec = getChildMeasureSpec(
                    MeasureSpec.makeMeasureSpec(mColumnWidth, MeasureSpec.EXACTLY), 0, p.width);
            child.measure(childWidthSpec, childHeightSpec);

            childHeight = child.getMeasuredHeight();
            childState = combineMeasuredStates(childState, child.getMeasuredState());

            if (mRecycler.shouldRecycleViewType(p.viewType)) {
                mRecycler.addScrapView(child, -1);
            }
        }
        
        if (heightMode == MeasureSpec.UNSPECIFIED) {
            heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                    getVerticalFadingEdgeLength() * 2;
        }

        if (heightMode == MeasureSpec.AT_MOST) {
            int ourSize =  mListPadding.top + mListPadding.bottom;
           
            final int numColumns = mNumColumns;
            for (int i = 0; i < count; i += numColumns) {
                ourSize += childHeight;
                if (i + numColumns < count) {
                    ourSize += mVerticalSpacing;
                }
                if (ourSize >= heightSize) {
                    ourSize = heightSize;
                    break;
                }
            }
            heightSize = ourSize;
        }

我們可以發現，確實是只設置為第一個child的高度，當MeasureSpec是AT_MOST的時候，才會讀取所有child的高度。

再網上，搜到了下面的辦法。

 heightMeasureSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE, MeasureSpec.AT_MOST); 

其實這是不對的，MeasureSpec的前兩個bits是mode，size的大小不能超過30位。於是，稍微修改，size設置為1000000，應該是足夠了，當不足夠的時候，你的app如果還沒更新早就該被淘汰了。

 heightMeasureSpec = MeasureSpec.makeMeasureSpec(1000000, MeasureSpec.AT_MOST);