UIImageView *loginNameFieldLeftView = [[UIImageView alloc]initWithFrame:CGRectMake(5, 0, 15, 15)];
loginNameFieldLeftView.image = [UIImage imageNamed:@"username.png"];
UIView *leftPaddingView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 25, 15)];
[leftPaddingView addSubview:loginNameFieldLeftView];
self.loginNameField.leftView = leftPaddingView;
self.loginNameField.leftViewMode = UITextFieldViewModeAlways;

UITextField的leftView左侧留空隙