Flex移动应用程序开发的技巧和窍门(五)

This is Part 5 of a multipart series of articles that cover tips and tricks for Flex mobile development. Earlier parts focused on:

• Handling data when switching between views and between application executions
• Styling the ActionBar and tab components in your mobile app
• Controlling the visibility of the ActionBar and tab components, and moving the tab components to the top of your app
• Creating popup alerts

This article covers transitions when switching between views and when switching between tabs.

While navigating through a Flex mobile application from view to view, by default one view slides off the screen as the new one slides on. You can change the animation or effect used in this view transition using one of four different classes that are available in the spark.transitions package in Flex 4.5:

• SlideViewTransition
• CrossFadeViewTransition
• FlipViewTransition
• ZoomViewTransition

Each of these transition classes can be applied with their default properties, or you can set the direction, duration, and mode (for some) to create a different effect. The following code snippets illustrate how each of these classes can be applied:

By default SlideViewTransition (with PUSH mode and LEFT direction) is used for the pushing and popping of all views. You can change the default transition used when a new view is pushed onto the stack by settingnavigator.defaultPushTransition . Likewise, you can setnavigator.defaultPopTransition to change the default transition used when the view is popped off the view stack (so the view beneath it is shown) . In either case, you set the property to an instance of a transition class; for example:

You can set these properties on any navigator within the main ViewNavigatorApplication or TabbedViewNavigatorApplication. You can also set them on the View class itself. However, if you want the change to apply to the entire application, set it in the root application code.

To see how this works, download the sample files for this article and import ViewTransitionsSample.fxp into Flash Builder. The application (see Figure 1) demonstrates the transitions and shows how to use the different modes that are available. For example, with FlipViewTransition you can see the difference betweenCARD and CUBE mode. You can also test how applying an easing function such as Bounce or Elasticmight affect how it plays. Fun stuff!

Below is a short video clip of the application running on an iPod Touch. It‘s nothing fancy, but you can see the different effects played when the four different view transition types are used.

All of the ViewTransition classes extend ViewTransitionBase. The ViewTransitionBase class (and all that extend it) will dispatch FlexEvent.TRANSITION_START and FlexEvent.TRANSITION_END events when the transition starts and ends respectively. You can create your own custom transitions by extending ViewTransitionBase. If you plan to do so though, you should first review the Flex 4.5 ViewTransition specification. Note that this is the original specification and some names of properties, events, and other details have changed.

The ViewTransition classes are great for applying an effect to your view navigation within a ViewNavigator. However, you cannot use the ViewTransition classes when switching between tabs, because you are no longer operating within the same ViewNavigator, but rather between ViewNavigator containers. It is, however, still possible to apply similar effects to those provided by ViewTransition, and I‘ve outlined one approach below.

First, you need to add an IndexChangeEvent handler to the implicit tabbedNavigator property in your TabbedViewNavigatorApplication on applicationComplete ; for example:

The IndexChangeEvent.CHANGE event is dispatched when a tab is selected, so you can perform any necessary handling within the handler. In this case, you want to apply a Spark Effect to the View that is being selected. You can check the newIndex property on the IndexChangeEvent and play different effects for each tab if you want.

You‘ll need to set up some effects to use. For example, you can define the following in the root application MXML:

Note: If you‘re not familiar with Spark Effects, check out Tour de Flex under Flex 4 > Components > Effects to see examples of how to use them all.

In the onChange handler, simply check the event index and set the target of the effect to the matching ViewNavigator container‘s activeView property:

Since you‘re handling the IndexChangeEvent.CHANGE event and not theIndexChangeEvent.CHANGING event, you can be sure that activeView has been set to the ViewNavigator container‘s new active view.

When the effects are played you will see a transition of sorts similar to what you might see when using a ViewTransition. You can see that you could also create more complex transitions using a sequence or parallel set of effects, and even add an easing function if desired. To simply mimic the ViewTransition classes, you would likely use the Spark Fade, Move, Rotate3D, and Scale effects alone to replicate the CrossFadeViewTransition, SlideViewTransition, FlipViewTransition, and ZoomViewTransition classes respectively. The code above will cause the effect to apply to the View itself, similar to how the ViewTransition classes work by default. However, you could also have the effect include the action bar in the animation as well by changing the code as follows:

Here is the full source for the main TabbedViewNavigatorApplication class:

If you want to play an initial effect when the application is run on the first view, you can also add anactivate event handler on the ViewNavigator and play the effect on the active view in the same manner, such as:

Here‘s a short, 28-second video of this running on my iPhone 4:

The full source code for the sample application (see Figure 2) is available with the sample files for this article.