View Programming Guide goes into details of what you're asking about. If you scroll down to the section called "Content Modes" you'd find what you're looking for.
Basically according to Apple:
"Each view has a content mode that controls how the view recycles its content in response to changes in the view’s geometry [...] the value in the contentMode property determines whether the bitmap should be scaled to fit the new bounds or simply pinned to one corner or edge of the view."
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
