Android ListView初始化將執行個體化多少個item

標籤：

下面是分析ListView初始化的源碼流程分析。

在AbsListView.onLayout中會調用layoutChildren（），由listview實現

@Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        super.onLayout(changed, l, t, r, b);        mInLayout = true;        final int childCount = getChildCount();        if (changed) {            for (int i = 0; i < childCount; i++) {                getChildAt(i).forceLayout();            }            mRecycler.markChildrenDirty();        }            // 由子類實現        layoutChildren();        mInLayout = false;        mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;        // TODO: Move somewhere sane. This doesn‘t belong in onLayout().        if (mFastScroll != null) {            mFastScroll.onItemCountChanged(getChildCount(), mItemCount);        }    }

ListView.layoutChildren：

@Override    protected void layoutChildren() {        final boolean blockLayoutRequests = mBlockLayoutRequests;        if (blockLayoutRequests) {            return;        }        mBlockLayoutRequests = true;        try {            super.layoutChildren();            invalidate();            if (mAdapter == null) {                resetList();                invokeOnItemScrollListener();                return;            }            final int childrenTop = mListPadding.top;            final int childrenBottom = mBottom - mTop - mListPadding.bottom;            final int childCount = getChildCount();…………default:                if (childCount == 0) {                    if (!mStackFromBottom) {                        final int position = lookForSelectablePosition(0, true);                        setSelectedPositionInt(position);                       // 初始化將執行這裡                        sel = fillFromTop(childrenTop);                    } else {                        final int position = lookForSelectablePosition(mItemCount - 1, false);                        setSelectedPositionInt(position);                        sel = fillUp(mItemCount - 1, childrenBottom);                    }                } else {                    if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {                        sel = fillSpecific(mSelectedPosition,                                oldSel == null ? childrenTop : oldSel.getTop());                    } else if (mFirstPosition < mItemCount) {                        sel = fillSpecific(mFirstPosition,                                oldFirst == null ? childrenTop : oldFirst.getTop());                    } else {                        sel = fillSpecific(0, childrenTop);                    }                }                break;            }

ListView.fillFromTop：

 private View fillFromTop(int nextTop) {        mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);        mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);        if (mFirstPosition < 0) {            mFirstPosition = 0;        }        return fillDown(mFirstPosition, nextTop);    }

ListView.fillDown：

private View fillDown(int pos, int nextTop) {        View selectedView = null;
　　　　　// listview的高度        int end = (mBottom - mTop);        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {            end -= mListPadding.bottom;        }        // 初始化時pos = 0，如果item總數少於一螢幕，執行     mItemCount - pos次      // 如果item多餘一螢幕，執行end - nextTop次          while (nextTop < end && pos < mItemCount) {            // is this the selected item?            boolean selected = pos == mSelectedPosition;            View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);            nextTop = child.getBottom() + mDividerHeight;            if (selected) {                selectedView = child;            }            pos++;        }        setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);        return selectedView;    }

 

Android ListView初始化將執行個體化多少個item