除非您编写自己的控件，否则您不能修改 UISwitch 控件，

You can not modify UISwitch control unless and until you write your own control,

但目前***的方法是，您可以使用 UISegmentControl 并处理其上的事件来切换 on.png 和 off.png 图像.

But best way so far, you can used UISegmentControl and handle event on it to switch the on.png and off.png images.

UISegmentedControl* switchView=[[UISegmentedControl alloc] initWithItems:[[[NSMutableArray alloc] initWithObjects:@"On",@"Off",nil] autorelease]];
    [switchView setFrame:CGRectMake(20,365,140,28)];
    switchView.selectedSegmentIndex=0;
    switchView.segmentedControlStyle=UISegmentedControlStyleBar;
    [switchView setImage:[UIImage imageNamed:@"onSelected.png"] forSegmentAtIndex:0];
    [switchView setImage:[UIImage imageNamed:@"off.png"] forSegmentAtIndex:1];
    [switchView addTarget:self action:@selector(checkOnOffState:) forControlEvents:UIControlEventValueChanged];

    self.navigationItem.titleView=switchView;

并像这样编写 checkOnOffState 方法代码-

and write checkOnOffState method code like this-

-(IBAction)checkOnOffState:(id)sender{

    UISegmentedControl* tempSeg=(UISegmentedControl *)sender;
    if(tempSeg.selectedSegmentIndex==0){
        [tempSeg setImage:[UIImage imageNamed:@"onSelected.png"] forSegmentAtIndex:0];
        [tempSeg setImage:[UIImage imageNamed:@"off.png"] forSegmentAtIndex:1];
    }
    else{
        [tempSeg setImage:[UIImage imageNamed:@"on.png"] forSegmentAtIndex:0];
        [tempSeg setImage:[UIImage imageNamed:@"offSelected.png"] forSegmentAtIndex:1];
    }   
}