APP中會有一些要求，就是要給按鈕加陰影達到懸浮的效果
eg:
UIButton *btn = [UIButton buttomWithType:UIButtonTypeCustom];//設置圓角btn.layer.CornerRadius = 20;//加陰影btn.layer.shadowOffset = CGSizeMake(1, 1);btn.layer.shadowOpacity = 0.8;//陰影顏色btn.layer.shadowColor = [UIColor blackColor].CGColor;新聞熱點
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