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Gọi showOpenDialog hiển thị hộp thoại và đình chỉ thực hiện chương trình cho đến khi người dùng nhấp chuột "Open" hoặc "Hủy bỏ". Gọi trả về một giá trị chỉ ra cho dù người dùng nhấp vào "Open" hoặc "Hủy bỏ". Giá trị trả về nếu "Open" được nhấp liên quan đến việc | CHAPTER 2 IMPROVING CLIENT DOWNLOAD AND RENDERING PERFORMANCE The Grade Tab We re going to continue using the Google home page. Load the home page www.google.com. Once the Google home page loads click Run Test within the YSlow tool and if the Grade tab is not already selected click it. You will see a screen similar to that shown in Figure 2-8. Figure2-8. Google.com YSlow results Based on the YSlow tool Google s home page has an overall grade of A and a performance score of 94 100. YSlow contains six result filters Content Cookie CSS Images JavaScript and Server. The filter tabs are shown right below the page grade and when clicked each of these filters contains rules that describe the type of optimization to be done why it s beneficial and the overall grade given to the page for the specific filter. Referring back to the results shown in Figure 2-8 Google received a D in implementing a Content Delivery Network and a C in including the Expires header setting which allows a web page to cache images and content in the browser s cache for later use. 37 CHAPTER 2 IMPROVING CLIENT DOWNLOAD AND RENDERING PERFORMANCE Statistics Tab The Statistics tab like the Grade tab provides a bit more insight into how we can improve the rendering performance. The Statistics tab contains information regarding cache usage and provides a very easy way to understand how caching can play an important role in optimizing rendering. Figure 2-9 contains the statistics results for Google.com. Figure 2-9. Google statistics results graphs Note the pair of graphs. One graph describes the total number of resources requested as well as the total size requested when the cache is empty the user initially visits the web page . The second graph on the right displays the same information when the cache is full primed cache . The results are two resource requests fewer and a 65.4KB reduction in size. With a primed cache the browser will download less of the web page from the web server and fetch the cached
