Tài liệu Creating and Referencing Global Elements docx

< Day Day Up > Creating and Referencing Global Elements There's no denying that even with a thorough understanding of target paths, it's sometimes hard to keep track of them—especially when you have timelines within timelines within timelines. As you might imagine, target paths can become quite long. To help simplify things, ActionScript provides access to a global object. As a global element, this special Flash object exists apart from any specific timeline and is omnipotent in relation to your Flash project: you can reference it from any timeline without a target path and take advantage of its power to reference other timelines and the variables, functions, and other dynamic elements they contain. Let's take a look at how you create a global element and how you convert an element with a target path into a global element. Creating a global element, such as a variable, is as simple as this: _ global.myVariable = "hello"; Because this variable is now a global element, you can reference it from any timeline by its name: _ root.myMovieClip_mc.favoriteGreeting = myVariable; or _ root.favoriteGreeting = myVariable; You can also use the global identifier to create global functions (see Lesson 5 , "Using Functions"): _ global.myFunction = function(){ //actions } To call the function from any timeline, use its name: myFunction(); The same syntax is used to create instances of objects (see Lesson 4 , "Using Object Classes"): _ global.myDateObject_date = new Date(); You can easily convert an element with a target path (such as a movie clip instance) so that it can be referenced globally—that is, without a path. For example, suppose a movie clip instance in your project has a target path of _root.car_mc.engine_mc.piston_mc. If you want to make it easier to reference this instance, you give it a global address: _ global.myPiston_mc = _root.car_mc.engine_mc.piston_mc; To control that instance, you simply reference its global address from any timeline: myPiston_mc.play(); Is there a benefit to using a global element? It's a matter of preference as well as what a given project dictates. In general, though, any element you use frequently or that's used by a number of timelines is a good candidate for becoming a global element. One thing to keep in mind, though, is that naming collisions can occur when you use global elements—that is, an element in a timeline (for example, a variable) may end up with the same name as a global element. For example, suppose you have a global variable named myVariable. Suppose you also have a movie clip instance named myMovieClip_mc, which contains a variable named myVariable. You have a global variable and a variable in a movie clip instance, both with the same name. You would have a problem if myMovieClip_mc attempted to reference the global variable named myVariable using this: myVariable This is because this timeline has its own local variable named myVariable. If you use this syntax, Flash won't be able to tell whether the script is referencing the local variable (within the movie clip instance) or the global one—a conflict it resolves by automatically referencing the closest variable to the timeline itself, which is the local one. An easy way to avoid these naming collisions is to preface global element names with a small g: gMyVariable gFavoriteColor TIP Another thing to remember is that a global element uses memory that can be freed only by deleting the global element. Therefore, using lots of global elements may not be an efficient use of memory. < Day Day Up > . < Day Day Up > Creating and Referencing Global Elements There's no denying that even with a thorough understanding of target paths, it's. look at how you create a global element and how you convert an element with a target path into a global element. Creating a global element, such as a