In SAP UI5, library-preload.js is a very important file, which plays a role in improving application loading performance. This file contains all the JavaScript and XML views in a UI5 library, pre-packaged into a single file so that they are cached by the browser the first time they are loaded. When the application needs to use these resources in the subsequent operation, they are already prepared in the browser cache and no longer need to be obtained from the server, which greatly improves the loading speed and performance of the application.
To gain a deeper understanding of what library-preload.js does, we can compare it to the normal way of loading resources. In a traditional web application, each required resource file (such as a JavaScript or CSS file) is fetched from the server through a separate HTTP request. The main problem with this approach is that each HTTP request incurs a certain amount of overhead. If an application needs to load a large number of resource files, then this overhead can add up and cause the application's load time to increase significantly.
In contrast, using library-preload.js can greatly reduce this overhead. Since all resources are packaged in one file, the entire application only needs to initiate one HTTP request to obtain all required resources. This greatly reduces the number of HTTP requests, thus improving the loading speed of the application.
In addition, library-preload.js can also take advantage of the browser's caching mechanism. When a user visits a UI5 application for the first time, the library-preload.js file is downloaded and stored in the browser's cache. When the user accesses the application again, the browser can directly obtain library-preload.js from the cache without downloading it from the server again. This further improves the loading speed of the application.
For example, suppose we have a UI5 application that uses three libraries: sap.m, sap.ui.core and sap.ui.layout. In the normal loading mode, the application may need to initiate dozens or even hundreds of HTTP requests to load all the JavaScript and XML view files in these libraries. However, if we use library-preload.js, then the application only needs to make one HTTP request to load all the resources in these three libraries. This greatly improves the loading speed of the application.
Overall, library-preload.js is a very useful tool that can greatly improve the loading speed and performance of UI5 applications. By prepackaging and caching resources, it reduces the number of HTTP requests, reduces server load, and improves user experience. However, it should be noted that since library-preload.js contains the resources of the entire library, its file size may be relatively large. Therefore, for those applications that only use a part of the resources in the library, using library-preload.js may cause unnecessary waste of resources.