【Android】AdapterView源码部分解析（二）

注：AdapterView继承于ViewGroup，所以这里标记为（二）。

同样官方解释：

An AdapterView is a view whose children are determined by an {@link Adapter}.

很容易就想到ListView和GridView两个View。类定义如下：

public abstract class AdapterView<T extends Adapter> extends ViewGroup 

贴一下源码的部分属性，看上去有好多之前不知道的信息：

/**
     * The position of the first child displayed
     */
    @ViewDebug.ExportedProperty
    int mFirstPosition = 0;

    /**
     * The offset in pixels from the top of the AdapterView to the top
     * of the view to select during the next layout.
     */
    int mSpecificTop;

    /**
     * Position from which to start looking for mSyncRowId
     */
    int mSyncPosition;

    /**
     * Row id to look for when data has changed
     */
    long mSyncRowId = INVALID_ROW_ID;

    /**
     * Height of the view when mSyncPosition and mSyncRowId where set
     */
    long mSyncHeight;

    /**
     * True if we need to sync to mSyncRowId
     */
    boolean mNeedSync = false;

    /**
     * Indicates whether to sync based on the selection or position. Possible
     * values are {@link #SYNC_SELECTED_POSITION} or
     * {@link #SYNC_FIRST_POSITION}.
     */
    int mSyncMode;

    /**
     * Our height after the last layout
     */
    private int mLayoutHeight;

    /**
     * Sync based on the selected child
     */
    static final int SYNC_SELECTED_POSITION = 0;

    /**
     * Sync based on the first child displayed
     */
    static final int SYNC_FIRST_POSITION = 1;

    /**
     * Maximum amount of time to spend in {@link #findSyncPosition()}
     */
    static final int SYNC_MAX_DURATION_MILLIS = 100;

    /**
     * Indicates that this view is currently being laid out.
     */
    boolean mInLayout = false;

    /**
     * The listener that receives notifications when an item is selected.
     */
    OnItemSelectedListener mOnItemSelectedListener;

    /**
     * The listener that receives notifications when an item is clicked.
     */
    OnItemClickListener mOnItemClickListener;

    /**
     * The listener that receives notifications when an item is long clicked.
     */
    OnItemLongClickListener mOnItemLongClickListener;

    /**
     * True if the data has changed since the last layout
     */
    boolean mDataChanged;

比如这个：

/**
     * The offset in pixels from the top of the AdapterView to the top
     * of the view to select during the next layout.
     */
    int mSpecificTop;

    /**
     * Position from which to start looking for mSyncRowId
     */
    int mSyncPosition;

还有这个：

/**
     * The listener that receives notifications when an item is selected.
     */
    OnItemSelectedListener mOnItemSelectedListener;

接口定义如下：

    /**
     * Interface definition for a callback to be invoked when
     * an item in this view has been selected.
     */
    public interface OnItemSelectedListener {
        /**
         * Callback method to be invoked when an item in this view has been
         * selected.
         *
         * Impelmenters can call getItemAtPosition(position) if they need to access the
         * data associated with the selected item.
         *
         * @param parent The AdapterView where the selection happened
         * @param view The view within the AdapterView that was clicked
         * @param position The position of the view in the adapter
         * @param id The row id of the item that is selected
         */
        void onItemSelected(AdapterView<?> parent, View view, int position, long id);

        /**
         * Callback method to be invoked when the selection disappears from this
         * view. The selection can disappear for instance when touch is activated
         * or when the adapter becomes empty.
         *
         * @param parent The AdapterView that now contains no selected item.
         */
        void onNothingSelected(AdapterView<?> parent);
    }

重载的时候，当方法不支持的时候，可以选择以下方案：直接抛出异常

/**
     * This method is not supported and throws an UnsupportedOperationException when called.
     *
     * @param child Ignored.
     *
     * @throws UnsupportedOperationException Every time this method is invoked.
     */
    @Override
    public void addView(View child) {
        throw new UnsupportedOperationException("addView(View) is not supported in AdapterView");
    }

看一个方法：

/**
     * @return The data corresponding to the currently selected item, or
     * null if there is nothing selected.
     */
    public Object getSelectedItem() {
        T adapter = getAdapter();
        int selection = getSelectedItemPosition();
        if (adapter != null && adapter.getCount() > 0 && selection >= 0) {
            return adapter.getItem(selection);
        } else {
            return null;
        }
    }

重载Adapter的时候，经常疑惑getItem这个方法是用来干嘛的，这里有解。

还有下面这个有意思的方法：

/**
     * Sets the view to show if the adapter is empty
     */
    public void setEmptyView(View emptyView) {
        mEmptyView = emptyView;

        final T adapter = getAdapter();
        final boolean empty = ((adapter == null) || adapter.isEmpty());
        updateEmptyStatus(empty);
    }

当adapter为空的时候显示什么，而不用自己去判断～

下面这个类乃是数据集监听者，在Adapter里面有一个方法叫做notifyDataSetChanged()，是用来刷新AdapterView的显示数据的，有关么？

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;
            checkSelectionChanged();

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

源码中有一大段是用来控制数据集变化，这里就不具体贴出来的，检几个：

 /**
     * Remember enough information to restore the screen state when the data has
     * changed.
     *
     */
    void rememberSyncState() {}

/**
     * Searches the adapter for a position matching mSyncRowId. The search starts at mSyncPosition
     * and then alternates between moving up and moving down until 1) we find the right position, or
     * 2) we run out of time, or 3) we have looked at every position
     *
     * @return Position of the row that matches mSyncRowId, or {@link #INVALID_POSITION} if it can't
     *         be found
     */
    int findSyncPosition() {}

具体用法有时间探讨。我要继续往下走。