How to read in data from an XML file
This lesson will show you how to load an XML file and access the data for use in your application. XML files are a very useful for things like storing preference settings, working with the web and for situations where you need to share data with other programs.
LiveCode provides a well-featured library for dealing with XML files, which can take a little getting used to but is quite straight forward to use.
Create a stack with a button and a field
We start by creating a stack and dragging a button and a field onto it. The button will contain our LiveCode code for reading the XML file. The field will contain the resulting data that is read from the file, set the name of the field to "information". Save this stack on your desktop.
Create an XML file to use as an example
Use Notepad on Windows or TextEdit on Mac OS X to create the example preferences XML file. The file should be plain text. Save the file on your desktop as "Preferences.xml". The XML data looks like this:
<introMessage size="18">Welcome guest, please click the button below to log in</introMessage>
<recentDocument>C:/Documents and Settings/Administrator/Documents/DraftReport.txt</recentDocument>
<recentDocument>C:/Documents and Settings/Administrator/Desktop/Temp.txt</recentDocument>
Tell the button to load the file when the user clicks
Edit the script of the button by selecting it, then clicking on the "Script" button in the main menu bar. For this example we are going to load an XML file that contains preferences for our application.
Begin the script with the following code:
1. When the button is clicked, load up the preferences and put them into the field
on mouseUp loadPreferences end mouseUp
2. Load the preferences file
There are two parts to loading the preferences file. The first part is reading the file into memory and creating an XML "tree". The second part is to process the tree and extract the data from it.
This function reads the XML file, and returns the tree. The tree is represented as a number, the actual tree structure and data is managed by LiveCode and so we don't need to worry about it.
command loadPreferences local tTree put readPreferencesToXMLTree() into tTree if tTree is empty then exit loadPreferences end if # Read the preferences we require from the tree and display them. processPreferencesTree tTree # Close the XML tree. # This will free up the memory that the tree was using and prevent our application # using more memory than it needs or "leaking" memory by creating multiple trees # without closing any of them. revXMLDeleteTree tTree end loadPreferences
Note that this code doesn't do anything just yet, because we haven't yet implemented the function
readPreferencesToXMLTree and the command
Read the XML file from disk
Next, we implement a function to read the XML. This is done in two steps, first the file is read into a variable like any other text file would be, secondly, an XML "tree" is created from the file. This tree allows us to manipulate the XML data easily.
Add the code to read the XML file as below.
private function readPreferencesToXMLTree # Find the XML file on disk. # This is for now assumed to be in the same location as the stack / application. # Note that we restore the itemDelimiter to comma (its default value) afterwards. # This is not essential but its good practice to avoid tricky bugs # that can arise due to unexpected delimiter values. set the itemDelimiter to slash local tPreferencesFile put item 1 to -2 of the effective filename of this stack & "/Preferences.xml" into tPreferencesFile set the itemDelimiter to comma # Read the preferences data from the file into a variable. # Always check for the result when reading files # as its possible that the file may have been deleted or moved. local tPreferencesData, tResult put url ("file:" & tPreferencesFile) into tPreferencesData put the result into tResult if tResult is not empty then answer error "Failed to read preferences file at location: " & tPreferencesFile return empty end if # Create the XML "tree" from the data, # checking to make sure that the file has loaded properly. # The revCreateXMLTree function will return a number # (the tree's "handle" or "id") if it succeeds, # otherwise it will return a message saying why it failed. local tTree put revXMLCreateTree(tPreferencesData, false, true, false) into tTree if tTree is not an integer then answer error "Failed to process preferences file with error: " & tTree return empty end if return tTree end readPreferencesToXMLTree
Extract the information out of the XML file and display it in a field
Once we have the XML tree, the final step is to use LiveCode's XML library to get the required information out of it. We use a series of calls to the XML library to extract each piece of information from the tree.
private command processPreferencesTree pTree # Extract the text color and text size preferences. # These are simple nodes in the XML file, # we can get what is inside them using the revXMLNodeContents function # This function will return a string beginning with "xmlerr," # if it fails, but we don't check this # here as we created the file and we know it won't fail. local tTextColor put revXMLNodeContents(pTree, "preferences/textColor") into tTextColor local tTextSize put revXMLNodeContents(pTree, "preferences/textSize") into tTextSize # Extract the introductory message preference. # This node has an attribute. We extract the contents and the # attribute in two separate calls. # The function revXMLAttribute allows us to read attributes from XML files, # its exactly the same as revXMLNodeContents, # except that you also need to tell it which attribute you want. local tIntroMessage put revXMLNodeContents(pTree, "preferences/introMessage") into tIntroMessage local tIntroMessageSize put revXMLAttribute(pTree, "preferences/introMessage", "size") into tIntroMessageSize # Extract the recent documents list. # This is a nested list of nodes, which could have any number of items. # First, we get a list of the recent documents, then we can loop # through them and get each one in turn. # The revXMLChildNames function is useful for returning a list of nodes like this. # The last parameter is important as it tells the function to return a unique # specifier for each node, allowing us to access them correctly. This will # look something like: # recentDocument # recentDocument # recentDocument local tRecentDocuments put revXMLChildNames(pTree, "preferences/recentDocuments", return, "recentDocument", true) into tRecentDocuments # To get each document, we just use revXMLNodeContents again. # However here we concatentate the name of each node # with the path that all recent document nodes have in common, # to get the complete path. local tListOfRecentDocuments repeat for each line tRecentDocument in tRecentDocuments put revXMLNodeContents(pTree, "preferences/recentDocuments/" & tRecentDocument) & return after tListOfRecentDocuments end repeat delete the last char of tListOfRecentDocuments # Now we output what we read from the file to see if it worked. local tOutput put "Text Color = " & tTextColor & return after tOutput put "Text Size = " & tTextSize & return after tOutput put return after tOutput put "Introductory Message (size: " & tIntroMessageSize & ") = " & return after tOutput put tIntroMessage & return & return after tOutput put "Recent Documents = " & return after tOutput put tListOfRecentDocuments after tOutput set the text of field "Information" to tOutput end processPreferencesTree
To test the stack switch to Run mode and click the button.