调整导航条上leftBarButtonItem和rightBarButtonItem与屏幕边界的间距

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  我们发现，在设置navigationItem的leftBarButtonItem或rightBarButtonItem时，用CustomView初始化UIBarButtonItem，不论怎么设置CustomView的frame，添加到导航条上之后总是和屏幕边界有一定的间距（5pix），如何自由调整这个间距呢？

初始化一个用于控制间距的UIBarButtonItem实例negativeSpacer，并设置negativeSpacer的width属性的值，设为-5的时候，正好可以使按钮与屏幕边界值为0，以rightBarButtonItem情况为例 

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1. UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];  
2. /**  
3.  *  设置frame只能控制按钮的大小  
4.  */  
5. btn.frame= CGRectMake(0, 0, 40, 44);  
6. [btn addTarget:self action:@selector(buttonClicked) forControlEvents:UIControlEventTouchUpInside];  
7. UIBarButtonItem *btn_right = [[UIBarButtonItem alloc] initWithCustomView:btn];  
8. UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc]   
9.                                    initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace   
10.                                    target:nil action:nil];  
11. /**  
12.  *  width为负数时，相当于btn向右移动width数值个像素，由于按钮本身和边界间距为5pix，所以width设为-5时，间距正好调整  
13.  *  为0；width为正数时，正好相反，相当于往左移动width数值个像素  
14.  */  
15. negativeSpacer.width = -5;   
16. self.navigationItem.rightBarButtonItems = [NSArray arrayWithObjects:negativeSpacer, btn_right, nil];  
17. [btn_right release];  

再看一个例子：

    UIButton *btnBack = [UIButton buttonWithType:UIButtonTypeCustom];

    btnBack.frame = CGRectMake(0, 0, 23, 23);

    btnBack.backgroundColor = [UIColor clearColor];

    [btnBack setImage:[UIImage imageNamed:@"bar_back_nomal.png"] forState:UIControlStateNormal];

    [btnBack setImage:[UIImage imageNamed:@"bar_back_selected.png"] forState:UIControlStateHighlighted];

    [btnBack setImage:[UIImage imageNamed:@"bar_back_selected.png"] forState:UIControlStateSelected];

    [btnBack addTarget:self action:@selector(btnBackClicked:) forControlEvents:UIControlEventTouchUpInside];

    [self.navigationItem setLeftBarButtonItem:[[UIBarButtonItem alloc]initWithCustomView:btnBack]];

    UIButton * source = [UIButton buttonWithType:UIButtonTypeCustom];

    source.frame = CGRectMake(0, 0, 35, 35);

    source.backgroundColor = [UIColor clearColor];

    [source setImage:[UIImage imageNamed:@"head_normal.png"] forState:UIControlStateNormal];

    [source setImage:[UIImage imageNamed:@"head_press.png"] forState:UIControlStateHighlighted];

    [source setImage:[UIImage imageNamed:@"head_press.png"] forState:UIControlStateSelected];

    [source addTarget:self action:@selector(btnPublicNum:) forControlEvents:UIControlEventTouchUpInside];

    UIBarButtonItem * btn_right = [[UIBarButtonItem alloc]initWithCustomView:source];

    UIBarButtonItem * negativeSpacer = [[UIBarButtonItem alloc]initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];

    negativeSpacer.width = -6;

    self.navigationItem.rightBarButtonItems = [NSArray arrayWithObjects:negativeSpacer, btn_right, nil];

OK：左右边界一模一样

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