iOS – UIToolBar作为UITextView的inputAccessoryView

我为UITextView添加了一个带UIBarButtonItem的UIToolBar作为inputAccessoryView.它工作正常,但UIBarButtonItem可以在它的框架外触摸,也许在右边外面50像素.这没什么大不了的,但它让我烦恼.谁知道为什么？

这是我的代码(ARC)：

UIToolbar *toolBar = [[UIToolbar alloc] initWithFrame:CGRectMake(0,self.view.bounds.size.height,320,44)];
toolBar.barStyle = UIBarStyleBlack;
toolBar.translucent = YES;

UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:self action:@selector(doneWriting:)];
[toolBar setItems:[NSArray arrayWithObjects:doneButton,nil]];

self.messageTextView.inputAccessoryView = toolBar;