對于Spinner，也是最近項目需求用的，才去了解這個控件磕潮。項目里面的出生年月日，一開始使用OS標準的DatePicker容贝，但是后來客戶想要這樣的自脯。

Spinner.png

立馬就想到了Spinner，于是就開始著手斤富，具體的代碼就不貼了膏潮。
想要解決的是選擇同一項的時候，也能回調
void onItemSelected(AdapterView<?> parent, View view, int position, long id)方法满力。先看下為什么不能回調戏罢？

AdapterView.class源碼

/**
     * Register a callback to be invoked when an item in this AdapterView has
     * been selected.
     *
     * @param listener The callback that will run
     */
    public void setOnItemSelectedListener(@Nullable OnItemSelectedListener listener) {
        mOnItemSelectedListener = listener;
    }

// 然后看下 mOnItemSelectedListener 這個監(jiān)聽對象在哪兒調用onItemSelected方法
// 也就觸發(fā)了onItemSelected事件屋谭。 

    private void fireOnSelected() {
        if (mOnItemSelectedListener == null) {
            return;
        }
        final int selection = getSelectedItemPosition();
        if (selection >= 0) {
            View v = getSelectedView();
            mOnItemSelectedListener.onItemSelected(this, v, selection,
                    getAdapter().getItemId(selection));
        } else {
            mOnItemSelectedListener.onNothingSelected(this);
        }
    }

// 那在看看是在哪里調用fireOnSelected()方法

 private void dispatchOnItemSelected() {
        fireOnSelected();
        performAccessibilityActionsOnSelected();
 }

 void selectionChanged() {
        // We're about to post or run the selection notifier, so we don't need
        // a pending notifier.
        mPendingSelectionNotifier = null;

        if (mOnItemSelectedListener != null
                || AccessibilityManager.getInstance(mContext).isEnabled()) {
            if (mInLayout || mBlockLayoutRequests) {
                // If we are in a layout traversal, defer notification
                // by posting. This ensures that the view tree is
                // in a consistent state and is able to accommodate
                // new layout or invalidate requests.
                if (mSelectionNotifier == null) {
                    mSelectionNotifier = new SelectionNotifier();
                } else {
                    removeCallbacks(mSelectionNotifier);
                }
                post(mSelectionNotifier);
            } else {
                dispatchOnItemSelected();
            }
        }
        // Always notify AutoFillManager - it will return right away if autofill is disabled.
        final AutofillManager afm = mContext.getSystemService(AutofillManager.class);
        if (afm != null) {
            afm.notifyValueChanged(this);
        }
    }

   /**
     * Called after layout to determine whether the selection position needs to
     * be updated. Also used to fire any pending selection events.
     */
    void checkSelectionChanged() {
        if ((mSelectedPosition != mOldSelectedPosition) || (mSelectedRowId != mOldSelectedRowId)) {
            selectionChanged();
            mOldSelectedPosition = mSelectedPosition;
            mOldSelectedRowId = mSelectedRowId;
        }

        // If we have a pending selection notification -- and we won't if we
        // just fired one in selectionChanged() -- run it now.
        if (mPendingSelectionNotifier != null) {
            mPendingSelectionNotifier.run();
        }
    }

我們看這個判斷條件if ((mSelectedPosition != mOldSelectedPosition) || (mSelectedRowId != mOldSelectedRowId))，比較當前選項的位置與上一次選擇的位置是否不相同龟糕，如果不相同就可以調用selectionChanged()執(zhí)行onItemSelected事件。這就是選擇一項時悔耘，必須與上一次選項的位置不相同才可以觸發(fā)選擇事件讲岁。

那該如何解決？

方法1：

利用反射的方式修改上一次位置的值衬以，只要與當前選擇的位置不同就可以出發(fā)事件了缓艳。因為mOldSelectedPosition是私有屬性，所以我們需要使用反射來修改mOldSelectedPosition的值看峻。代碼如下：

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
    try {
       Field field = AdapterView.class.getDeclaredField("mOldSelectedPosition");
       field.setAccessible(true);  //設置mOldSelectedPosition可訪問
       field.setInt(spinner, AdapterView.INVALID_POSITION); //設置mOldSelectedPosition的值
    } catch (Exception e) {
       e.printStackTrace();
    }
}

@Override
public void onNothingSelected(AdapterView<?> parent) {

}

如果頁面只有一個Spinner是沒有問題的阶淘，但是當頁面包含了兩個及以上的Spinner的時候就會出現(xiàn)一個問題：當點擊第一個spinner的item時候，會觸發(fā)其他Spinner的listener的onItemSelected方法互妓，這就比較難受了溪窒，明明是只操作一個，但是其他也有響應冯勉。

方法2

在原有Spinner基礎上自定義澈蚌，代碼不多

public class CKDateSpinner extends AppCompatSpinner {
    public CKDateSpinner(Context context) {
        super(context);
    }

    public CKDateSpinner(Context context, int mode) {
        super(context, mode);
    }

    public CKDateSpinner(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public void setSelection(int position, boolean animate) {
        boolean sameSelected = position == getSelectedItemPosition();
        super.setSelection(position, animate);
        if (sameSelected) {
            // Spinner does not call the OnItemSelectedListener if the same item is selected, so do it manually now
            getOnItemSelectedListener().onItemSelected(this, getSelectedView(), position, getSelectedItemId());
        }
    }

    @Override
    public void setSelection(int position) {
        boolean sameSelected = position == getSelectedItemPosition();
        super.setSelection(position);
        if (sameSelected) {
            // Spinner does not call the OnItemSelectedListener if the same item is selected, so do it manually now
            getOnItemSelectedListener().onItemSelected(this, getSelectedView(), position, getSelectedItemId());
        }
    }
}

以上就是兩種解決方法。