Android检查之前是否购买了应用内购买

我想在我的Android应用程序中进行应用内购买.我使用google示例:http://developer.android.com/training/in-app-billing/preparing-iab-app.html#GetSample

我在我的android studio项目中实现了这个.在开发者控制台中,我将应用内购买和gmail地址设置为测试帐户.

在我的设备(而不是模拟器)上,我使用此测试帐户登录.我启动我的应用程序并点击"购买高级版"并完成此过程.

现在我想显示一个按钮(文本"恢复"),用户可以恢复他的应用内购买,如果他/她之前购买了高级功能.

我有这个代码:

IabHelper.OnIabPurchaseFinishedListener mPurchaseFinishedListener = new IabHelper.OnIabPurchaseFinishedListener() {
        public void onIabPurchaseFinished(IabResult result, Purchase purchase) {
            Log.e("-->", "Purchase finished: " + result);



            if (mHelper == null) return;

            if (result.isFailure()) {
                Log.e("-->","Error purchasing: " + result);
                return;
            }
            if (!verifyDeveloperPayload(purchase)) {
                Log.e("-->","Error purchasing. Authenticity verification failed.");
                return;
            }



            SharedPreferences prefs = this.getSharedPreferences("xxx", Context.MODE_PRIVATE);
            SharedPreferences.Editor editor = prefs.edit();
            editor.putBoolean("Premium", true);
            editor.commit();
            Log.e("-->", "Premium: " + prefs.getBoolean("Premium", false));


}
};

如果我再次按下"购买"按钮并且我之前购买了此按钮,我会在log cat中收到此消息:

- >:购买完成:IabResult:无法购买商品(回复:7:商品已经拥有),购买:null - >:购买错误:IabResult:无法购买商品(回复:7:商品已经拥有)

我的问题是,如何检查应用内购买之前是否购买过？

 mIabHelper.queryInventoryAsync(true, "your_sku", mGotInventoryListener);

// Listener that's called when we finish querying the items and subscriptions we own
    IabHelper.QueryInventoryFinishedListener mGotInventoryListener = new IabHelper.QueryInventoryFinishedListener() {
        public void onQueryInventoryFinished(IabResult result, Inventory inventory) {
            Log.d("PAY", "Query inventory finished.");

            // Have we been disposed of in the meantime? If so, quit.
            if (mIabHelper == null) return;
                    Purchase purchase = inventory.getPurchase("your_sku");
                    if (purchase != null) {
                        //purchased
                    }                    
            }