文本提示框
#import <QuartzCore/QuartzCore.h>
UILabel *label = [[UILabel alloc] init];
[label setTextColor:[UIColor whiteColor]];
[label setBackgroundColor:[UIColor darkGrayColor]];
[[label layer] setBorderWidth:2];
[[label layer] setCornerRadius:15];
[[label layer] setBorderColor:[UIColor whiteColor].CGColor];
[label setAlpha:0.8];
[label setTextAlignment:UITextAlignmentCenter];
[label setFrame:CGRectMake(0, 0, 220, 50)];
[label setText:@"Loading..."];
[label setCenter:window.center];
[window addSubview:label];