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毕业设计（论文）外文资料原文及译文专 业 计算机科学与技术班 级 08104121学 号 0810411504姓 名 高芸芸指 导 教 师 吴敏毕业设计（论文） 外文资料 原文原文出处：Flash 8 ActionScript Bible, Joey Lott and Robert Reinhardt, Published by Wiley Publishing, Inc.Flash 8 ActionScript BibleJoey Lott and Robert ReinhardtUnderstanding DatatypesWhen we talk about data, were talking about information or values. These values can be of many types. For example, even in a very simple movie you might still have a number, some text, and a MovieClip instance. All three of these examples are data of different types what ActionScript calls datatypes.Flash is actually capable of performing datatype conversions when necessary. However, this can lead to some poor coding practices on the part of ActionScript developers. For this reason, the ActionScript 2.0 standards require that you pay closer attention to the datatypes you are using.In ActionScript, youll work with many different datatypes. However, for the sake of understanding how these datatypes work, you can consider them in two basic categories: primitive types and reference types. The primitive types are called primitive because they are the basic foundational datatypes, not because they lack importance. The reference datatypes are called reference types because they reference the primitive types.Primitive datatypes include strings, numbers, Booleans, undefined, and null. Well examine each of these primitive datatypes a little more closely in this chapter. Reference datatypes are all objects, which is the subject of much of the rest of this book, so well defer the majority of the discussion of reference datatypes to those later chapters.Working with StringsStrings are characters or words. String values must always be enclosed in either single quotes or double quotes. Here are a few examples of strings:“a”b“1”“Joey”123abc“*”Strings are used whenever you want to work with characters or words. For example, you can use strings to populate text fields in your movie, or you can use strings to programmatically create names for new MovieClip instances. Youve also already seen how strings can be used with actions such as trace(). The trace() action requires that you provide it with a message to display in the Output panel. That message must evaluate to a string value.trace(“I know him; Marleys Ghost!”);As already mentioned, you can use either double quotes or single quotes when defining a string value. Which you choose is often purely a matter of personal preference. There are two rules that you must follow, however, if you want your code to work without error. First, you must have a matching closing quote for every opening quote. And whichever type of quote you use to open the string literal must be used to close it. In other words, mismatched quotes are not allowed. Here are two examples of correctly matched quotes on string literals:“here is a string”here is a stringAnd here are three examples of incorrect quotes on string literals:“here is a stringhere is a string”“here is a stringThere are times when more than personal preference might dictate which type of quotes you choose to use. Notice what would happen if you tried the following:trace(I know him; Marleys Ghost!);This line of code would actually cause an error because it would interpret the apostrophe as the closing quote and then fail to know what to do with the remainder of the string. This is easily remedied by using double quotes around the string:trace(“I know him; Marleys Ghost!”);The inverse is true as well, of course if you want to use a double quotation mark as a character in a string literal, you can use single quotes around the entire value. The problem arises when you want to use both single and double quotation marks as characters within a string literal. There is an easy way to accommodate this: by using special characters. To learn more about special characters, see Chapter 11.Working with NumbersIn Flash, all numbers are treated as the number datatype. Positive, negative, floating point, integer, and so forth, are all simply considered numbers, with no further differentiation generally required on your part.To define a number, you need only type the number without any quotes. The following are examples of numbers.612-13.31.8Number datatypes allow you to perform all kinds of mathematical operations, as shown in the following examples:trace(5 + 5); / Displays: 10trace(5 4); / Displays: 1trace(5 / 2); / Displays: 2.5It is important to understand when to use numbers and when to use strings. If you try to use a string when you really want to use a number, you can end up with unexpected results. For example:trace(“5” + “5”); / Displays: 55In the example, the resulting value of addedAges is “55”, not 10 (as you may expect). This is because ActionScript treats the two values as strings and concatenates them, rather than adding the numeric values.However, it should also be understood that there are legitimate reasons for numbers to be treated as strings. For example, even though a phone number is composed of numbers, it is usually treated as a string. And if you want to format the phone number with spaces, parentheses, dashes, and so on, it is necessary that the phone number be treated as a string by enclosing the value in quotation marks.Using BooleansBoolean data is data that can hold only two values: true and false. Boolean variables are used to test conditions within your Flash movie. Boolean values are used commonly in conditional expressions within statements such as the if statement and control structures such as the for and while statements. For more information about conditional statements and control structures, read the section “Using Statements That Control Flow: Control Structures” later in this chapter.Understanding the undefined and null DatatypesActionScript has two additional primitive datatypes undefined and null with which youll want to familiarize yourself. In order to have a better understanding of these datatypes, it is useful to first be familiar with variables. Therefore, well interweave the discussion of these two datatypes with the discussion of variables later in this chapter.Casting DataActionScript allows you to tell Flash to convert a value to a specific datatype by what is known as casting. When you cast a value, you use the following syntax:Datatype(value)For example, you can cast a string to a number as follows:Number(“123”)When you are casting data you have to be careful. Some casting will have unexpected results. For example, the numeric value of 0 can be converted to a Boolean false, and any nonzero value will be converted to true. For example:trace(Boolean(0); / Displays: falsetrace(Boolean(1); / Displays: trueTherefore, if you convert the string values of true and false, they will both convert to the Boolean true value because both string values are nonzero values.trace(Boolean(“true”); / Displays: truetrace(Boolean(“false”); / Displays: trueUsing VariablesIn Flash movies youre going to be using a lot of data. Now, with all that data floating around, youre going to want some way to keep track of it. This is where variables become very useful.A variable is a named container that you can use to hold or reference some particular data. Once you have created a variable, you can store and retrieve data in the variable. Although a variable can contain only one value at a time, you can use the same variable to contain or reference different data at different times. For example, if you create a variable named nYear, it may contain the value of 2005 at one point, but at another point it may contain the value 2006.Consider a rental storage space as a metaphor for a variable. You can rent a space (declaring the variable), and when you do so, you can begin to place things in that space (assigning data to the variable). At a later time, you may want to view the things from the storage space (retrieving the value from the variable), or you may decide to place other contents in the storage space (assigning a new value to the same variable). And when you are done with the storage space, you can stop renting it (deleting the variable).Declaring VariablesBefore you can meaningfully use a variable, you must first bring it into being by declaring it. Flash 8 and ActionScript 2.0 let you declare variables using strong typing. Strong typing means that when you create the variable, you also specify the type of data that it can hold. When you export your movie, Flash then makes sure that you consistently tried to store the correct type of data in that variable. If Flash detects that you mismatched the datatype with the variable at any point, an error message is generated, alerting you to the fact. This is helpful for ensuring that your Flash applications are well planned and designed using good coding practices.To declare a variable using strong typing, use the following pattern:var variableName:Datatype;The var keyword lets Flash know that you are declaring a variable. The variable name is up to your choosing, but it should follow the rules for variable naming (see the next section, “Naming Variables”). A colon separates the name of the variable and the name of the datatype, and there should be no space between the name, colon, or datatype. If you have code hinting turned on, you should get a drop-down list of built-in datatypes from which you can select. Alternatively, if you have defined your own custom datatype (see Chapter 5), you can declare a variable of that type as well. Youll also see that the line ends with a semicolon.Here is an example in which you declare a variable named nQuantity as a number:var nQuantity:Number;Now, youve declared a variable with the name nQuantity, and youve told Flash that all values you assign to the variable must be numbers. The variable has been created, but you have not defined any value for it yet. If you use trace() to display the value of the variable, you can see that this is so:trace(nQuantity);When you test this, you should see the value undefined appear in the Output panel. The value undefined is a special value that Flash uses for any variable that has not yet been assigned any value.Once a variable has been declared, you can assign a value to it using a simple assignment statement with an equals sign:var nQuantity:Number;nQuantity = 6;You can also declare the variable and assign a value, or initialize the variable, all on one line:var nQuantity:Number = 6;In addition to the undefined value, there is another special value, null, that you can assign to a variable that indicates that the variable does not contain any other, specific value. While undefined is used to indicate no value has been assigned to a variable, you can use null to indicate that a variable has intentionally been left without any other value. It is often a good practice to initialize your variables to some value other than undefined. And because null allows you to quickly distinguish between values intentionally or unintentionally left without another value, it is a good practice to initialize your variables to null when you dont have any other specific value to assign to them:var nQuantity:Number = null;You can use a variable in any situation in which you could use the value the variable contains. Youve already seen an example of this with the trace() actions. You can use a variable to tell Flash what message to output:var sMessage:String = “Welcome!”;trace(sMessage);You can also perform other kinds of operations using variables, just as you would on the actual values themselves. For example:var nQuantity:Number = 6;var nPrice:Number = 9.99;trace(nQuantity * nPrice); / Displays: 59.94Naming VariablesNow that youve looked at declaring and defining variables, the next thing to examine is how to name the variables. There are two main parts to this discussion. First, there is the matter of using valid variable names that Flash will understand, so well look at the rules for naming variables. Second, well examine some additional guidelines for naming variables that, while not strictly enforced by Flash, will aid you in creating more readable code.The following are the rules that you must follow for Flash to be able to understand your variable names:u The first character must be an underscore (_), a dollar sign ($), or a letter. The first character cannot be a number. Although underscores and dollar signs are allowable as the first character, in practical application, you will almost always start the variable name with a letter.u The subsequent characters must be underscores (_), dollar signs ($), letters, or numbers.u Variable names can have no spaces.u The name cannot be a keyword or other special value recognized by Flash. For example, the names MovieClip, true, String, and undefined are not allowable variable names because they already have other meanings in ActionScript.u The name must be unique (within its scope). If you create two variables with the same name in the same scope (more on scope in Chapters 4 and 5), the latter will overwrite the former.Next, lets talk about some good naming conventions that you can use. First, because youll want to be using strong typing with all your variables, it is a good idea to have a convenient way to be reminded of what type of value a variable can hold. A system named Hungarian notation has been devised that can assist in this. For our purposes, well use a modification of Hungarian notation specifically designed for ActionScript. With this system, you can prefix each variable name with a character (or, in some cases, several characters) that can help you to remember what type of datatype the variable can hold. You may have already seen this in the previous examples in this chapter. When we define a variable named nQuantity, the variable name is prefixed with the character n. This tells us that the variable holds a number value. Table 3-1 shows a list of other recommended prefixes.Table 3-1: Modified Hungarian Notation ActionScript Prefixes for Common ClassesPrefixDatatypeaArra ybmpBitmapDatabBooleanbtButtoncColorcamCameracmContextMenucmiContextMenuItemdDatelcLocalConnectionlvLoadVarsmMovieClipmclMovieClipLoadermicMicrophonenNumberncNetConnectionnsNetStreamoObjectpjPrintJobrsRecordSetsStringsndSoundsoSharedObjecttTextFieldtfTextFormatvidVideoxmlXMLxmlsXMLSocketThis modified Hungarian notation convention is completely optional, but it can be very useful. It helps not only you, but also others who may read your code. By adding the appropriate prefix to the variable name, it makes it immediately clear what type of datatype the variable can hold.Its also important when naming your variables to make the names as descriptive as possible. For example, the variable name nQuantity is much more descriptive than nQ. Of course, the level of descriptiveness required depends on the context. For example, if your Flash application deals with quantities of widgets as well as cogs, nQuantity might not be sufficiently clear. It would be better, in such a case, to have variables named nQuantityCog and nQuantityWidget, for example. The more descriptive the variable name, the better, in most situations. Just remember, though, that most likely youll be typing the same variable name multiple times, so it is important to achieve the correct balance between descriptiveness and name length. You can always use abbreviations in the variable names if appropriate. For example, rather than defining a variable named nQuantityWidget you might find it easier to define a variable named nQntyWidget.Remember that you cannot use spaces in your variable names. However, when you want to make your variable names descriptive, the names will often consist of more than one word. There are two conventions that are commonly used when naming variables with multiple words. The first of the two conventions is to use the underscore (_) to separate your words in the variable name. An example of this method follows:var sFirst_name:String = “Joey”;The second of these conventions is what is known as the interCap method (also known as studlyCaps or camelCaps). The word “interCap” refers to the capitalization of the first letter of each word subsequent to the first, using no spaces or underscoresinternal capitalization. An example of this method is the following:var sFirstName:String = “Joey”;It would behoove you to use one of these conventions. Pick one that you like, and if you decide you prefer the other one later on, switch to it. In this book we tend to prefer the interCap method, so you see a preference for it in the examples. But neither convention is more correct or offers any advantages over the other one.It is also important to remember that ActionScript is case-sensitive. This means that sFirstName, sFirstname, SfirstName, and so on are all different variables. If you accidentally type the name of a variable with incorrect capitalization, the result will be that Flash will not recognize that the variable is defined. Here is an example:var sFirstName:String = “Joey”;trace(sFirstName); / Displays: Joeytrace(sfirstName); / Displays: undefinedUsing ExpressionsAnyone who has taken even very basic arithmetic (using addition and subtraction and the like) has worked with expressions. Expressions are simply those parts of statements that evaluate to be equal to something. Here are some very simple examples:1“abcd”nQuantityEven though these examples are all either simple values or variables, they all eva