调整UIPickerView高度

Advantages:

1. Makes setFrame of UIPickerView behave like it should
2. No transform code within your UIViewController
3. Works within viewWillLayoutSubviews to rescale/position the UIPickerView
4. Works on the iPad without UIPopover
5. The superclass always receives a valid height
6. Works with iOS 5

Disadvantages:

1. Requires you to subclass UIPickerView
2. Requires the use of pickerView viewForRow to undo the transformation for the subViews
3. UIAnimations might not work

Solution:

Subclass UIPickerView and overwrite the two methods using the following code. It combines subclassing, fixed height and the transformation approach.

#define FIXED_PICKER_HEIGHT 216.0f
- (void) setFrame:(CGRect)frame
{
    CGFloat targetHeight = frame.size.height;
    CGFloat scaleFactor = targetHeight / FIXED_PICKER_HEIGHT;
    frame.size.height = FIXED_PICKER_HEIGHT;//fake normal conditions for super
    self.transform = CGAffineTransformIdentity;//fake normal conditions for super
    [super setFrame:frame];
    frame.size.height = targetHeight;
    CGFloat dX=self.bounds.size.width/2, dY=self.bounds.size.height/2;
    self.transform = CGAffineTransformTranslate(CGAffineTransformScale(CGAffineTransformMakeTranslation(-dX, -dY), 1, scaleFactor), dX, dY);
}

- (UIView *)pickerView:(UIPickerView *)pickerView viewForRow:(NSInteger)row forComponent:(NSInteger)component reusingView:(UIView *)view
{
    //Your code goes here

    CGFloat inverseScaleFactor = FIXED_PICKER_HEIGHT/self.frame.size.height;
    CGAffineTransform scale = CGAffineTransformMakeScale(1, inverseScaleFactor);
    view.transform = scale;
    return view;
}