Question

What is the importance of drawRect, layoutSubViews and setNeedsDisplay methods. In which contexts we should use them?

Answer 1

As in the documentation says:

drawRect: Draws the receiver’s image within the passed-in rectangle. 

Parameters

rect : A rectangle defining the area to restrict drawing to.

You use this method to draw your UIView inside a specific rect of a view.

• (void)setNeedsDisplay

By default, geometry changes to a view
automatically redisplays the view
without needing to invoke the
drawRect: method. Therefore, you need
to request that a view redraw only
when the data or state used for
drawing a view changes. In this case,
send the view the setNeedsDisplay
message. Any UIView objects marked as
needing display are automatically
redisplayed when the application
returns to the run loop.

I think that this method let you control if you want the system automatically redraw your UIView for you or not. If you want to control the process by your own state like: if the user click this button, redraw; otherwise, don’t.

Lays out subviews.

• (void)layoutSubviews

Overridden by subclasses to layout
subviews when layoutIfNeeded is
invoked. The default implementation of
this method does nothing.

I have not much experience with this method. What I can tell you is that you need to override it to layout your subviews