自定义 UIBarButtonItem

1. UIBarButtonItem*backBarButtonItem=[[UIBarButtonItemalloc]initWithTitle:@"back"style:UIBarButtonSystemItemCanceltarget:selfaction:nil];
2. [backBarButtonItemsetImage:[UIImageimageNamed:@"icon.png"]];
3. [[cardMessageViewControllernavigationItem]setBackBarButtonItem:backBarButtonItem];
4. [backBarButtonItemrelease];