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Call a JavaScript Function Located in Another Frame
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You ve already seen how the contents of a frame (its forms and other HTML components) can be accessed from external frames, so it should be no surprise that JavaScript variables and functions can be accessed externally as well
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To demonstrate, let s assume we have a JavaScript function named sayhello() defined in the menu frame
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function sayhello() { // This function says "hello" to the user alert("Hello!"); }
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We could define a button in the bodyframe frame that will call the sayhello() function from the menu frame when clicked
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<form action="#"> <input type="button" value="Say Hello" onclick="topmenusayhello()"> </form>
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Figure 10-6 shows the result of clicking the Say Hello button in the browser window As long as JavaScript variables are defined outside of any function, they too can be accessed from another frame
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CHAPTER 10: Communicate Between Browser Frames
Cross-Frame Scripting Can Be Risky
Cross-frame scripting occurs when a frame that originates in one domain (like wwwmyservercom) attempts to access a frame that originates in a different domain (like wwwexamplecom) using JavaScript The problem with cross-frame scripting is that a web site could define a frame that points to a secure web site, such as Microsoft s Hotmail The originating web site could then access any cookies or other sensitive user information from the frame, allowing them to use that information in a malicious manner Because this a security risk, all IE browsers from version 4 on do not allow cross-frame scripting
Handle Synchronization Between Frames
In the last section, we saw some JavaScript code that relied on the existence of a JavaScript function in another frame
<form action="#"> <input type="button" value="Say Hello" onclick="topmenusayhello()"> </form>
But what happens if the user clicks the button before the menu frame has even finished loading Most web browsers load frames in no particular order, and it is quite possible (actually, quite probable) that some frames will load slower than others In this case, a JavaScript error will occur, since the sayhello() function could not be found This problem can be described as one of synchronization When dealing with frames that communicate with each other, we need a way to determine if all the frames are fully loaded before proceeding There are several techniques a JavaScript programmer can employ to handle synchronization between frames The first technique is to create a cascading effect between the frames, where each frame is responsible for loading the frames that depend on it Using this method, we would have the menu frame load the contents of the bodyframe frame
topbodyframedocumentlocation = "mainbodyhtml";
We could then be guaranteed that the menu frame has already loaded in the browser window by the time the bodyframe document has finished loading as well The second technique is to create one or more Boolean JavaScript variables in the frameset document Each of the frames can set its respective Boolean variable to true when
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it has completely loaded For example, we can modify our frameset document to the following HTML and JavaScript code:
<html> <head> <title>Framesets</title> <script type="text/javascript" language="JavaScript"> var banner_loaded = false; var menu_loaded = false; var body_loaded = false; </script> </head> <frameset rows="100, *"> <frame src="bannerhtml" name="banner"> <frameset cols="150, *"> <frame src="menuhtml" name="menu"> <frame src="mainbodyhtml" name="bodyframe"> </frameset> </frameset> </html>
We can then add the following JavaScript code to the end of the menu frame in order to set the appropriate variable to true
<script type="text/javascript" language="JavaScript"> var menu_loaded = true; </script>
Inside the code for the bodyframe, we should move the call to the sayhello() function into a separate function, and add an if statement around it for safety
<script type="text/javascript" language="JavaScript"> function callhello() { // Check the variable named menu_loaded first if (topmenu_loaded == true) { topmenusayhello(); } } </script> <form action="#"> <input type="button" value="Say Hello" onclick="callhello()"> </form>
In this example, the Say Hello button will not work until the menu frame has completely loaded
CHAPTER 10: Communicate Between Browser Frames
Lastly, we could use a technique similar to the DOM-based browser sniffing code from 7 We could simply test for the existence of the frame and the sayhello() function before accessing them
<script type="text/javascript" language="JavaScript"> function callhello() { // Check if the frame and function exist first if (topmenu) { if (topmenusayhello) { topmenusayhello(); } } } </script> <form action="#"> <input type="button" value="Say Hello" onclick="callhello()"> </form>
This same technique can be used to test for the existence of JavaScript variables or web form controls In the next chapter, we will investigate the various objects that make up the DOM It is through the methods and properties of these objects that we are able to interact with the web browser
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