label同时设置sizeToFit，NSTextAlignmentCenter不起作用

问题：label要多行显示，按照这样子设置，iOS9以上work，iOS8无用

self.bookNameLabel.lineBreakMode = NSLineBreakByCharWrapping;
    self.bookNameLabel.numberOfLines = 0;
    self.bookNameLabel.textAlignment = NSTextAlignmentCenter;

然后据说要加 sizeToFit：好吧，依然然并卵

然后据说要同时加font，好吧，多行出来了，但是不居中

然后stackOverFlow：https://stackoverflow.com/questions/28579776/nstextalignmentcenter-for-uilabel-not-working/35066864#35066864?newreg=725179871da349d698d31b72bdbb3565

work！

最终：

self.bookNameLabel.font =[UIFont systemFontOfSize:18];
    self.bookNameLabel.lineBreakMode = NSLineBreakByCharWrapping;
    self.bookNameLabel.numberOfLines = 0;
    self.bookNameLabel.textAlignment = NSTextAlignmentCenter;
    [self.bookNameLabel sizeToFit];
    self.bookNameLabel.frame = CGRectMake((self.frame.size.width-self.bookNameLabel.frame.size.width)/2, self.bookNameLabel.frame.origin.y, self.bookNameLabel.frame.size.width, self.bookNameLabel.frame.size.height);