java.lang.IllegalStateException: Fragment already added问题

       在通过fragmentmanager管理fragment栈的时候，出现了java.lang.IllegalStateException: Fragment already added的错误，导致Email 应用程序出现FC。网上建议通过检查isAdded()函数来检查它是否已经被添加进栈中。     但在实际使用时，却发现这个检查并不准确。我在下面的show方法前调用了isAdded()方法，发现在出现此FC问题前，此方法的返回值一直是false。

    public void showDownloadingDialog() {

        final FragmentTransaction ft = mFragmentManager.beginTransaction();

        final Fragment prev = mFragmentManager.findFragmentByTag(PROGRESS_FRAGMENT_TAG);

        if (prev != null) {

            ft.remove(prev);

        }

        ft.addToBackStack(null);

         // Create and show the dialog.

        final DialogFragment newFragment = AttachmentProgressDialogFragment.newInstance(

                mAttachment);

        newFragment.show(ft, PROGRESS_FRAGMENT_TAG);
    }

在fragment.java中查看此show()方法：

    public int show(FragmentTransaction transaction, String tag) {
        mDismissed = false;
        mShownByMe = true;
        transaction.add(this, tag);
        mViewDestroyed = false;
        mBackStackId = transaction.commit();
        return mBackStackId;
    }

实际上，isAdded()方法这个值是在FragmentManager.java中修改的。

    final public boolean isAdded() {
        return mActivity != null && mAdded;
    }

    public void addFragment(Fragment fragment, boolean moveToStateNow) {        if (mAdded == null) {            mAdded = new ArrayList<Fragment>();        }        if (DEBUG) Log.v(TAG, "add: " + fragment);        makeActive(fragment);        if (!fragment.mDetached) {            if (mAdded.contains(fragment)) {                throw new IllegalStateException("Fragment already added: " + fragment);            }            mAdded.add(fragment);            fragment.mAdded = true;            fragment.mRemoving = false;            if (fragment.mHasMenu && fragment.mMenuVisible) {                mNeedMenuInvalidate = true;            }            if (moveToStateNow) {                moveToState(fragment);            }        }    }

而在Email 的代码中，由于快速多次点击，会多次调用showDownloadingDialog()方法。导致fragment.java中show()方法也被多次调用。然而上面的addFragment()方法没有被及时调用，出现mAdded的值一直为false。因此，正确的方法去避免重复添加fragment，应该重写show和dismiss函数并用一个boolean变量去记录添加情况，通过检查这个值来避免重复添加fragment的情况。

    @Override    public int show(FragmentTransaction transaction, String tag) {        // TODO Auto-generated method stub        Log.i(TAG," AttachmentProgressDialogFragment22 show:"+isFragmentShown);        if(!isFragmentShown){            isFragmentShown = true;            return super.show(transaction, tag);        }        return -1;    }

在dismiss中将这个值修改为false。