Layer Filters - Core Animation - page 100

1. Select the property you want to animate, in this case, we use opacity .

2. Create a new dictionary from the layer's current actions property.

3. Add the transition to the new actions dictionary for the key opacity .

4. Set the layer's actions property to the newly created dictionary.

The code to set the default transition for the opacity field is in Listing 6-12.

LISTING 6-12 Set the Default Transition for Opacity

CATransition *transition=[ CATransition animation ];
[transition setType:kCATransitionMoveIn];
[transition setSubtype:kCATransitionFromTop];
[transition setDuration:1.0f];

// Get the current list of actions
NSMutableDictionary *actions = [NSMutableDictionary
dictionaryWithDictionary:[transitionLayer actions]];
[actions setObject:transition forKey: @”opacity” ];

[transitionLayer setActions :actions];

First, you need to instantiate the CATransition object. Using the default transitions as
specified in the Tables 6-2 and Table 6-3, set the type to kCATransitionMoveIn , which
causes the transition to move in from some direction. Then, in the subtype field, specify
the direction from which the transition should move in; in this example, we move in
from the top ( kCATransitionFromTop ). Next, specify the duration for the transition; in
this case, a full second.

At this point, we need to add the transition to the layer's list of actions. The layer knows
to use this transition rather than the default fade because of the key-path. We specify in
the dictionary that our key-path is opacity . Any time the layer's opacity changes, it uses
the specified transition rather than the default fade transition.

To make this work, we first instantiate a new dictionary using the original actions field
from transitionLayer . We do this just in case some other actions have been specified in
the collection.

We know that there isn't anything else in the dictionary; however, I mention it for clarity.
You just need to understand that if you create a new dictionary and set the layer's actions
to it without first grabbing what might have been there previously, you might be
confused as to why other transitions are not working correctly.

Finally, set the layer's actions dictionary to point to the newly created dictionary that
contains the CATransition object.