Question

in iOS, does any UIImage support stretchableImageWithLeftCapWidth: , does that mean autoresize the uimmage?

Answer 1

First, this is deprecated, replaced by the more powerful resizableImageWithCapInsets:. However, that is only supported by iOS 5.0 and above.

stretchableImageWithLeftCapWidth:topCapHeight: does not resize the image you call it on. It returns a new UIImage. All UIImages may be drawn at different sizes, but a capped image responds to resizing by drawing its caps at the corners, and then filling the remaining space.

When is this useful? When we want to make buttons out of an image, as in this tutorial for the iOS 5 version.

The following code is a UIView drawRect method which illustrates the difference between a regular UIImage and a stretchable image with caps. The image used for stretch.png came from http://commons.wikimedia.org/wiki/Main_Page.

- (void) drawRect:(CGRect)rect;
{
    CGRect bounds = self.bounds;

    UIImage *sourceImage = [UIImage imageNamed:@"stretch.png"];
    // Cap sizes should be carefully chosen for an appropriate part of the image.
    UIImage *cappedImage = [sourceImage stretchableImageWithLeftCapWidth:64 topCapHeight:71];

    CGRect leftHalf = CGRectMake(bounds.origin.x, bounds.origin.y, bounds.size.width/2, bounds.size.height);
    CGRect rightHalf = CGRectMake(bounds.origin.x+bounds.size.width/2, bounds.origin.y, bounds.size.width/2, bounds.size.height);
    [sourceImage drawInRect:leftHalf];
    [cappedImage drawInRect:rightHalf];

    UIFont *font = [UIFont systemFontOfSize:[UIFont systemFontSize]];
    [@"Stretching a standard UIImage" drawInRect:leftHalf withFont:font];
    [@"Stretching a capped UIImage" drawInRect:rightHalf withFont:font];
}

Output: