支持返回按钮。
count 方法返回当前计数,initialize 方法询问 Back 按钮支持(有些时候也许不想要 Back 按钮支持)并初始化计数值。
The count method returns the current count, and the initialize method asks for Back button support (sometimes you might not want Back button support) and initializes the value of count.
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