On iOS, a custom view won't animate? - iphone

Question

In a new Single View App, the following code:
- (void)viewDidAppear:(BOOL)animated {
self.fooView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 200, 200)];
self.fooView.backgroundColor = [UIColor yellowColor];
[self.view addSubview:self.fooView];
[UIView animateWithDuration:2 animations:^{
self.fooView.backgroundColor = [UIColor blueColor];
}];
}
will animate the yellow background to blue. The fooView is just:
#property (strong, nonatomic) UIView *fooView;
But if a custom view class FooView is defined, and I change the above code above by replacing UIView with FooView, and also define an empty -drawRect for FooView, then there will be no animation -- the background is just changed to blue immediately.
It made no difference if using [UIView animateWithDuration: ...] or [FooView animateWithDuration: ...] -- it is no animation.
If inside -drawRect, I call [super drawRect:rect];, then it is the same thing: no animation.
If I change drawRect to drawRect2, or if I remove the whole method, then the animation will work.
(and the -drawRect might be needed, for example, what if we need to draw a circle in the custom view FooView?)
Why is that? Besides giving a solution of how to make it work even with a -drawRect, please explain what was going on that made it not animate?

Answer 1

The presence of drawRect: changes the way UIView operates. You should only use drawRect: when absolutely necessary and you should never have an empty drawRect: implementation.
If the -drawRect: method is present in your subclass then UIView must do a number of things before it can call this method. Namely, it must allocate a potentially expensive backing store into which your custom drawing will occur, and it must fill this backing store with the background color.
UIView will then call your drawRect: implementation to do any custom drawing on to the backing store, before handing the backing store to the GPU. Only after all that does your animation block begin to run.
So at the start of your animation the backing store that represents your custom UIView has already been painted a new background color.
If your UIView subclass has no drawRect: method then the view has no backing store. The background color is handled directly by the GPU, which can very efficiently animate transitions between colors.
Empty UIView objects (without a drawRect: implementation) are rather lightweight. Your custom view should be composed of multiple subviews depending on what animation effects you want to achieve.
For example, the UITableViewCell class is composed of many subviews. UITableViewCell itself is a UIView. It contains a background view and a selected background view. On top of that is the content view, and inside the content view are various text labels and image views. All of these views combine to produce the selection and resize animations you see in a regular UITableView.

Answer 2

Using 'CATransition' class .You will have to be do animation.
Using this method
+(CAAnimation *)setAnimation{}

Answer 3

Use following line in animation Block
[self.view addSubview:self.fooView];

Answer 4

Instead of this we have to create a new class and using the CATransition and using this class methods '+(CAAnimation *)setAnimation'.We have to create an object for this and call when will transition happen