如果主要类(principal class)为nil，则默认为UIApplication；如果代理类(delegate class)为nil，则程序假设程序的代理来自Main nib文件。
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这样做主要是为了避免名称冲突：如果两个类具有相同的属性，映射为全局名可能会造成冲突。
This is primarily to avoid name clashes: If two classes have the same attribute and are mapped to global names, they may conflict.
葡萄皮和葡萄子是酿酒后的下脚料，主要含有色素、 单宁、酚类及油脂和芳香类物质，如果不及时处理影响环境卫生。
The hull residue of grape after brewing contains pigment, tannin, phenols, grease and spicery. It can impact on the environment if not be disposed in time.
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