iOS-居中in view

float viewWidth = weatherView.frame.size.width;//view的宽
float viewHeight = weatherView.frame.size.height;//view的高
float labelWidth = label.frame.size.width;
float labelHeight = label.frame.size.height;

float xpos = (viewWidth/2.0f) - (labelWidth/2.0f);
float ypos = (viewHeight/2.0f) - (labelHeight/2.0f);

[label setFrame:CGRectMake(xpos,ypos,labelWidth,labelHeight)];