Android ListView的理解（一）

一、概述

这部分主要是讲解ListView在初始时，即setAdapter之后，如何将Adapter中取得的view添加到ListView中，主要涉及到几个方法：

layoutChildren，fillFromTop，fillDown / fillUp，makeAndAddView，obtainView，setupChild；

二、讲解

layoutChildren是个最重要的方法，也很复杂，本篇略过，会在以后中提到，不过，为了讲解其它方法，还是提一下：

    @Override
    protected void layoutChildren() {
            .........

            boolean dataChanged = mDataChanged;
            if (dataChanged) {
                handleDataChanged();
            }

            .........

            // Clear out old views
            detachAllViewsFromParent();

            switch (mLayoutMode) {
            .........

            case LAYOUT_FORCE_BOTTOM:
                sel = fillUp(mItemCount - 1, childrenBottom);
                adjustViewsUpOrDown();
                break;
            case LAYOUT_FORCE_TOP:
                mFirstPosition = 0;
                sel = fillFromTop(childrenTop);
                adjustViewsUpOrDown();
                break;

            .........
            }

	   .........
    }

setAdapter之后，dataChanged是true，然后走到handleDataChanged方法，在这个方法中会设置mLayoutMode，通常是LAYOUT_FORCE_TOP，即从顶部开始一个一个的往下添加childview：

    @Override
    protected void handleDataChanged() {
        .........

        // Nothing is selected. Give up and reset everything.
        mLayoutMode = mStackFromBottom ? LAYOUT_FORCE_BOTTOM : LAYOUT_FORCE_TOP;
        mSelectedPosition = INVALID_POSITION;
        mSelectedRowId = INVALID_ROW_ID;
        mNextSelectedPosition = INVALID_POSITION;
        mNextSelectedRowId = INVALID_ROW_ID;
        mNeedSync = false;
        mSelectorPosition = INVALID_POSITION;
        checkSelectionChanged();
    }

这个方法的最后，就会去设置mLayoutMode，并设置其它position为INVALID属性；
        回到layoutChildren中，设置mFirstPosition=0后，将childrenTop(=0或padding top 后的值)，进入fillFromTop方法：

    /**
     * Fills the list from top to bottom, starting with mFirstPosition
     *
     * @param nextTop The location where the top of the first item should be
     *        drawn
     *
     * @return The view that is currently selected
     */
    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);
    }

该方法，第一个child的position起始点，然后调用fillDown；

    /**
     * Fills the list from pos down to the end of the list view.
     *
     * @param pos The first position to put in the list
     *
     * @param nextTop The location where the top of the item associated with pos
     *        should be drawn
     *
     * @return The view that is currently selected, if it happens to be in the
     *         range that we draw.
     */
    private View fillDown(int pos, int nextTop) {
        View selectedView = null;

        int end = (mBottom - mTop);
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            end -= mListPadding.bottom;
        }

        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++;
        }

        return selectedView;
    }

首先，获得可视区域的高度(mBottom - mTop)；

其次，while循环中，判断：累计添加到listview中child的高度，不超过可视区域（添加最后一个child时，有可能只显示部分），且添加的child的下标不超过总的个数（否则系统会报 OutOfBounds 的异常）；

在循环中，会去调用makeAndAddView，这个方法不会真正的去添加child，但会调用之后的setupChild来真正添加到listview中：

    /**
     * Obtain the view and add it to our list of children. The view can be made
     * fresh, converted from an unused view, or used as is if it was in the
     * recycle bin.
     *
     * @param position Logical position in the list
     * @param y Top or bottom edge of the view to add
     * @param flow If flow is true, align top edge to y. If false, align bottom
     *        edge to y.
     * @param childrenLeft Left edge where children should be positioned
     * @param selected Is this position selected?
     * @return View that was added
     */
    private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        View child;

        if (!mDataChanged) {
            // Try to use an existing view for this position
            child = mRecycler.getActiveView(position);
            if (child != null) {
                if (ViewDebug.TRACE_RECYCLER) {
                    ViewDebug.trace(child, ViewDebug.RecyclerTraceType.RECYCLE_FROM_ACTIVE_HEAP,
                            position, getChildCount());
                }

                // Found it -- we're using an existing child
                // This just needs to be positioned
                setupChild(child, position, y, flow, childrenLeft, selected, true);

                return child;
            }
        }

        // Make a new view for this position, or convert an unused view if possible
        child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
    }

该方法分两种情况：
        1. adapter中的数据发生了变化，初始setAdapter，或之后我们将adapter中的数据做了新增/删除后，调用Adapter.notifyDataSetChanged；变化了，就会从RecycleBin中的mScrapView中，取之前移出的view，即convertView来复用；

2. 若没有变化，则从RecycleBin的mActiveView中取当前显示的view（为啥会有这种情况？当listview稳定后，我们不滚动它，但有可能点击或长按），这时就会走到这里。

obtainView就是从RecycleBin中，取移出去的View，传给Adapter.getView方法（convertView），我们先来看看这个方法：

    /**
     * Get a view and have it show the data associated with the specified
     * position. This is called when we have already discovered that the view is
     * not available for reuse in the recycle bin. The only choices left are
     * converting an old view or making a new one.
     *
     * @param position The position to display
     * @param isScrap Array of at least 1 boolean, the first entry will become true if
     *                the returned view was taken from the scrap heap, false if otherwise.
     *
     * @return A view displaying the data associated with the specified position
     */
    View obtainView(int position, boolean[] isScrap) {
        isScrap[0] = false;
        View scrapView;

        scrapView = mRecycler.getScrapView(position);

        View child;
        if (scrapView != null) {
            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.RECYCLE_FROM_SCRAP_HEAP,
                        position, -1);
            }

            child = mAdapter.getView(position, scrapView, this);

            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(child, ViewDebug.RecyclerTraceType.BIND_VIEW,
                        position, getChildCount());
            }

            if (child != scrapView) {
                mRecycler.addScrapView(scrapView, position);
                if (mCacheColorHint != 0) {
                    child.setDrawingCacheBackgroundColor(mCacheColorHint);
                }
                if (ViewDebug.TRACE_RECYCLER) {
                    ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP,
                            position, -1);
                }
            } else {
                isScrap[0] = true;
                child.dispatchFinishTemporaryDetach();
            }
        } else {
            child = mAdapter.getView(position, null, this);
            if (mCacheColorHint != 0) {
                child.setDrawingCacheBackgroundColor(mCacheColorHint);
            }
            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(child, ViewDebug.RecyclerTraceType.NEW_VIEW,
                        position, getChildCount());
            }
        }

        return child;
    }

这个方法，实际上是在AbsListView类中的，我们知道，ListView, GridView是继承于AbsListView的，而AbsListView是继承于AdapterView<ListAdapter>，而这个是继承于ViewGroup，所以，ListView或AbsListView中，有些变量或方法，都是继承过来的，我们看源码时，需要结合这些类一起看（比较头痛，跳来跳去），但熟悉之后，对我们写出高质量的程序是很有帮助的。

小提示：RecycleBin类，它的作用就是一个View的缓存，将移出屏幕外的view回收，并给新移入到屏幕内的view来复用，这样就能节省大量内存（之后讲解该类）。

回归正题obtainView方法：

1. 先从RecycleBin中获取一个ScrapView，如果缓存中没有，则convertView为null，在Adapter中，需要自己去LayoutInflater一个view；

2. 若有，将这个convertView传给Adapter.getView方法；

3. 判断从getView方法中返回的view是否与scrapview一致，如果一致，表明是复用的，反之，则程序又去创建了一个新的view（浪费了一块内存），且将得到的scrapview重新加入到RecycleBin.mScrapView中；

回到makeAndAddView方法，将child传给setupChild，开始真正的加入到listView中去显示：

    /**
     * Add a view as a child and make sure it is measured (if necessary) and
     * positioned properly.
     *
     * @param child The view to add
     * @param position The position of this child
     * @param y The y position relative to which this view will be positioned
     * @param flowDown If true, align top edge to y. If false, align bottom
     *        edge to y.
     * @param childrenLeft Left edge where children should be positioned
     * @param selected Is this position selected?
     * @param recycled Has this view been pulled from the recycle bin? If so it
     *        does not need to be remeasured.
     */
    private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
            boolean selected, boolean recycled) {
        final boolean isSelected = selected && shouldShowSelector();
        final boolean updateChildSelected = isSelected != child.isSelected();
        final int mode = mTouchMode;
        final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
                mMotionPosition == position;
        final boolean updateChildPressed = isPressed != child.isPressed();
        final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();

        // Respect layout params that are already in the view. Otherwise make some up...
        // noinspection unchecked
        AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
        if (p == null) {
            p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
                    ViewGroup.LayoutParams.WRAP_CONTENT, 0);
        }
        p.viewType = mAdapter.getItemViewType(position);

        if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
                p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
            attachViewToParent(child, flowDown ? -1 : 0, p);
        } else {
            p.forceAdd = false;
            if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                p.recycledHeaderFooter = true;
            }
            addViewInLayout(child, flowDown ? -1 : 0, p, true);
        }

        if (updateChildSelected) {
            child.setSelected(isSelected);
        }

        if (updateChildPressed) {
            child.setPressed(isPressed);
        }

        if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
            if (child instanceof Checkable) {
                ((Checkable) child).setChecked(mCheckStates.get(position));
            } else if (getContext().getApplicationInfo().targetSdkVersion
                    >= android.os.Build.VERSION_CODES.HONEYCOMB) {
                child.setActivated(mCheckStates.get(position));
            }
        }

        if (needToMeasure) {
            int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                    mListPadding.left + mListPadding.right, p.width);
            int lpHeight = p.height;
            int childHeightSpec;
            if (lpHeight > 0) {
                childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
            } else {
                childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
            }
            child.measure(childWidthSpec, childHeightSpec);
        } else {
            cleanupLayoutState(child);
        }

        final int w = child.getMeasuredWidth();
        final int h = child.getMeasuredHeight();
        final int childTop = flowDown ? y : y - h;

        if (needToMeasure) {
            final int childRight = childrenLeft + w;
            final int childBottom = childTop + h;
            child.layout(childrenLeft, childTop, childRight, childBottom);
        } else {
            child.offsetLeftAndRight(childrenLeft - child.getLeft());
            child.offsetTopAndBottom(childTop - child.getTop());
        }

        if (mCachingStarted && !child.isDrawingCacheEnabled()) {
            child.setDrawingCacheEnabled(true);
        }

        if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
                != position) {
            child.jumpDrawablesToCurrentState();
        }
    }

attachViewToParent 和 addViewInLayout两者大致差不多，都是将view添加到parent view的array中，区别在于，attachView是不用去requestLayout的，而addViewInLayout的最后一个参数指明（true不用requestLayout，false则需要requestLayout），这两个方法都在ViewGroup中。

flowDown ? -1 : 0 ， 不知道大家注意没有， -1 和 0 的区别？我们看下attachViewToParent代码吧：

    /**
     * Attaches a view to this view group. Attaching a view assigns this group as the parent,
     * sets the layout parameters and puts the view in the list of children so it can be retrieved
     * by calling {@link #getChildAt(int)}.
     *
     * This method should be called only for view which were detached from their parent.
     *
     * @param child the child to attach
     * @param index the index at which the child should be attached
     * @param params the layout parameters of the child
     *
     * @see #removeDetachedView(View, boolean)
     * @see #detachAllViewsFromParent()
     * @see #detachViewFromParent(View)
     * @see #detachViewFromParent(int)
     */
    protected void attachViewToParent(View child, int index, LayoutParams params) {
        child.mLayoutParams = params;

        if (index < 0) {
            index = mChildrenCount;
        }

        addInArray(child, index);

        child.mParent = this;
        child.mPrivateFlags = (child.mPrivateFlags & ~DIRTY_MASK & ~DRAWING_CACHE_VALID) |
                DRAWN | INVALIDATED;
        this.mPrivateFlags |= INVALIDATED;

        if (child.hasFocus()) {
            requestChildFocus(child, child.findFocus());
        }
    }

如果是-1，则将index = mChildrenCount，mChildrenCount就是当前parent view已经有多少个child view，这里将index设置为当前child view个数，意在表明如果是 -1 则将该view加入到parent view中的child view最后，如果是0， 则加入到child view 数组中的第一个；即：该view是插入到顶部，还是添加到底部。
        默认情况下，添加到listview中的item，即child都会measure一次高度和宽度，然后，调用child.layout，通知新添加的child，layout一下它里面的children。

然后，然后就没有了然后，整个流程走完，回到ListView.layoutChildren中，adjustViewsUpOrDown将所有child调整对齐，刷新一下RecycleBin的Active和Scrap缓存，调用updateScrollIndicators更新一下滚动条的值，若有注意OnScrollListener，也通知一下invokeOnItemScrollListener。

三、总结

虽然，只讲了一个fillDown，其实fillUp以及ListView中的其它fillXXX方法，是差不多的，本篇的目的，也只是希望大家了解ListView / GridView继承于AbsListView时，如何将child添加进去。

之后，会涉及ListView / GridView， AbsListView 以及其内部类 RecycleBin的一些讲解，也欢迎大家留言，一起讨论。