Many applications require access to a persistent storage. The two most common storage types are files and databases.
We will cover text files in this tutorial.
The predefined Files object has several utility methods for working with text files which are pretty easy to use.
Files locations - There are several important locations where you can read or write files.
File.DirAssets
The assets folder includes the files that were added with the file manager. These files are read-only. You can not create new files in this folder (which is actually located inside the apk file).
File.DirInternal / File.DirInternalCache
These two folders are stored in the main memory and are private to your application. Other applications cannot access these files.
The cache folder may get deleted by the OS if it needs more space.
File.DirRootExternal
The storage card root folder.
File.DirDefaultExternal
The default folder for your application in the SD card.
The folder is: <storage card>/Android/data/<package>/files/
It will be created if required.
Note that calling any of the two above properties will add the EXTERNAL_STORAGE permission to your application.
Tip: You can check if there is a storage card and whether it is available with File.ExternalReadable and File.ExternalWritable.
The predefined File object (predefined means that you do not need to declare it yourself) includes several methods for writing and reading to files.
You can also use TextReader and TextWriter to do it manually.
Note that TextReader and TextWriter are not limited to files and can work with other streams.
TextReader and TextWriter have an advantage over the File read/write methods when working with large files. The File methods read the file completely and store its content in memory. In many cases this is the most convenient solution, however if you work with large files (more than 1-2mb) you may prefer to work with TextReader or TextWriter.
File.WriteString - Writes the given text to a new file.
File.ReadString - Reads a file and returns it content as a string.
File.WriteList - Writes all values stored in a list to a file. All values are converted to string type if required. Each value will be stored in its own line.
Note that if a value contains the new line character it will saved over more than one line and when you read it, it will be read as multiple items.
File.ReadList - Reads a file and stores each line as an item in a list.
File.WriteMap - Takes a map object which holds pairs of key and value elements and stores it in a text file. The file format is known as Java Properties file: .properties - Wikipedia, the free encyclopedia
The file format is not too important unless the file is supposed to be edited manually. This format makes it easy to edit it manually.
One common usage of File.WriteMap is to save a map of "settings" to a file.
File.ReadMap - Reads a properties file and returns its key/value pairs as a Map object. Note that the order of entries returned might be different than the original order.
Example:
We will cover text files in this tutorial.
The predefined Files object has several utility methods for working with text files which are pretty easy to use.
Files locations - There are several important locations where you can read or write files.
File.DirAssets
The assets folder includes the files that were added with the file manager. These files are read-only. You can not create new files in this folder (which is actually located inside the apk file).
File.DirInternal / File.DirInternalCache
These two folders are stored in the main memory and are private to your application. Other applications cannot access these files.
The cache folder may get deleted by the OS if it needs more space.
File.DirRootExternal
The storage card root folder.
File.DirDefaultExternal
The default folder for your application in the SD card.
The folder is: <storage card>/Android/data/<package>/files/
It will be created if required.
Note that calling any of the two above properties will add the EXTERNAL_STORAGE permission to your application.
Tip: You can check if there is a storage card and whether it is available with File.ExternalReadable and File.ExternalWritable.
The predefined File object (predefined means that you do not need to declare it yourself) includes several methods for writing and reading to files.
You can also use TextReader and TextWriter to do it manually.
Note that TextReader and TextWriter are not limited to files and can work with other streams.
TextReader and TextWriter have an advantage over the File read/write methods when working with large files. The File methods read the file completely and store its content in memory. In many cases this is the most convenient solution, however if you work with large files (more than 1-2mb) you may prefer to work with TextReader or TextWriter.
File.WriteString - Writes the given text to a new file.
File.ReadString - Reads a file and returns it content as a string.
File.WriteList - Writes all values stored in a list to a file. All values are converted to string type if required. Each value will be stored in its own line.
Note that if a value contains the new line character it will saved over more than one line and when you read it, it will be read as multiple items.
File.ReadList - Reads a file and stores each line as an item in a list.
File.WriteMap - Takes a map object which holds pairs of key and value elements and stores it in a text file. The file format is known as Java Properties file: .properties - Wikipedia, the free encyclopedia
The file format is not too important unless the file is supposed to be edited manually. This format makes it easy to edit it manually.
One common usage of File.WriteMap is to save a map of "settings" to a file.
File.ReadMap - Reads a properties file and returns its key/value pairs as a Map object. Note that the order of entries returned might be different than the original order.
Example:
B4X:
Sub Process_Globals
End Sub
Sub Globals
End Sub
Sub Activity_Create(FirstTime As Boolean)
If File.ExternalWritable = False Then
Msgbox("Cannot write on storage card.", "")
Return
End If
SaveStringExample
ReadStringExample
WriteListExample
ReadListExample
WriteMapExample
ReadMapExample
WriteTextWriter
ReadTextReader
End Sub
Sub SaveStringExample
File.WriteString(File.DirRootExternal, "String.txt", _
"This is some string" & CRLF & "and this is another one.")
End Sub
Sub ReadStringExample
Msgbox(File.ReadString(File.DirRootExternal, "String.txt"), "")
End Sub
Sub WriteListExample
Dim List1 As List
List1.Initialize
For i = 1 To 100
List1.Add(i)
Next
File.WriteList(File.DirRootExternal, "List.txt", List1)
End Sub
Sub ReadListExample
Dim List1 As List
'We are not initializing the list because it just holds the list that returns from File.ReadList
List1 = File.ReadList(File.DirRootExternal, "List.txt")
Msgbox("List1.Size = " & List1.Size & CRLF & "The third item is: " & List1.Get(2), "")
End Sub
Sub WriteMapExample
Dim Map1 As Map
Map1.Initialize
For i = 1 To 10
Map1.Put("Key" & i, "Value" & i)
Next
File.WriteMap(File.DirRootExternal, "Map.txt", Map1)
End Sub
Sub ReadMapExample
Dim Map1 As Map
'Again we are not initializing the map.
Map1 = File.ReadMap(File.DirRootExternal, "Map.txt")
'Append all entries to a string builder
Dim sb As StringBuilder
sb.Initialize
sb.Append("The map entries are:").Append(CRLF)
For i = 0 To Map1.Size - 1
sb.Append("Key = ").Append(Map1.GetKeyAt(i)).Append(", Value = ")
sb.Append(Map1.GetValueAt(i)).Append(CRLF)
Next
Msgbox(sb.ToString,"")
End Sub
Sub WriteTextWriter
Dim TextWriter1 As TextWriter
TextWriter1.Initialize(File.OpenOutput(File.DirRootExternal, "Text.txt", False))
For i = 1 To 10
TextWriter1.WriteLine("Line" & i)
Next
TextWriter1.Close
End Sub
Sub ReadTextReader
Dim TextReader1 As TextReader
TextReader1.Initialize(File.OpenInput(File.DirRootExternal, "Text.txt"))
Dim line As String
line = TextReader1.ReadLine
Do While line <> Null
Log(line) 'write the line to LogCat
line = TextReader1.ReadLine
Loop
TextReader1.Close
End Sub
