The Alert class in AS3 is a good example of using bitwise flags to specify a configuration without having to set lots of separate Boolean flags. Its also very easy to create bitflags for yourself, which save time, code and memory! Below is an example of custom bitflags as well as an explanation of how it all works. You can have various buttons enabled in the Flex Alert - “OK”, “NO”, “Cancel” etc. Any combination can be used, and the configuration is all set via one argument. Each value is separated with the bitwise OR operator (the pipe | – not to be confused with the logical OR operator || - two pipes).
The example below would contain OK, NO and CANCEL buttons.
Alert.show(“My Alert message”, “My Alert tittle”, Alert.OK | Alert.NO | Alert.CANCEL );
Below details how this works in terms of bitwise operations, but first here is an example of how to use this in your own program.
Example of using custom bitwise flags in actionscript Create a static class to hold the values and a method to test them.
package
{
public class Colours
{
public static const ULTRA_RED:uint = 1;
public static const RED:uint = 2;
public static const ORANGE:uint = 4;
public static const YELLOW:uint = 8;
public static const GREEN:uint = 16;
public static const BLUE:uint = 32;
public static const INDIGO:uint = 64;
public static const VIOLETE:uint = 128;
public static const ULTRA_VIOLETE:uint = 256;
//convenience const to enable all flags
public static const ALL:uint = ULTRA_RED | RED | ORANGE | YELLOW | GREEN | BLUE | INDIGO | VIOLETE | ULTRA_VIOLETE;
public static function test(flags:uint, testFlag:uint):Boolean
{
return (flags & testFlag) == testFlag;
}
}
}
Now, you can pass a combination of flags around as one single argument. The flags are combined using the bitwise OR operator.
var chosenColours:uint = Colours.RED | Colours.BLUE | Colours.VIOLETE;
Then later test to see if the flag you want is in that single argument.
if ( Colours.test(chosenColours, Colours.RED) )
trace(“We have RED”);
else
trace(“We don’t have RED”);
Note the Colours.ALL const, which is just a convenient way to enable all the flags in one go.
And that's it. Makes life very simple, less method arguments, less var declaration, less database columns if you store a configuration, less memory used. Very simple to implement and very useful!
How it works - creating Each of the constants that define the flags Colours.RED, Colours.YELLOW etc are all integers. The separate flags must be multiples of 2 (which will become clearer later), so they would be 1,2,4,8,16,32 etc. The bitwise OR looks at the binary representation of each integer and analyses each
bit of each integer in turn. Each bit is treated as a Boolean flag. If the bit is 1, its on, if its 0 its off. The bitwise OR will look at all the corresponding bits of each value in the list, starting with the first bit, then the second etc. If any of the bits are 1, the resulting bit for that position is 1, if ALL are 0 the resulting bit for that position is 0.
0001 ( decimal 1 ) 0010 ( decimal 2 ) 1000 ( decimal 8 ) Bitwise OR = 1011 ( decimal 11 ) So, looking down each column of bits, the OR checks if any are 1, if so, the resulting bit is 1, else its 0. So the output of ( 1 | 2 | 8 ) is 1011,which is decimal 11. As the integers are multiples of 2 you don’t get multiple “1” bits in the binary representation of any number so only 1 column is ever marked as “on” by any flag – as in the example above. If you added 0110 ( binary 10 ) to the example above it would end up with 2 columns marked as "on" for one flag, which would break the system as you could not then calculate if the resulting bit flag contained a specific flag, as explained below.
How it works - testing Now you have one value that represents multiple flags. The next step is to check if it contains a particular flag. This is very similar, except you reverse the process and use the bitwise operator AND (the ampersand &, not to be confused with the logical AND &&) This looks at the columns of bits, and if ALL of them are 1, the result is 1, if not its 0. In the same way, as the columns are unique due to multiples of 2, we can check if the combined integer contains our value.
1011 (decimal 11 - our combined flags) 0010 (decimal 2 - the flag to check for) Bitwise AND = 0010 (decimal 2)
As only the 2nd (from right) column contined both “On” bits, the result is 0010, which is our decimal 2, the value we wanted to check! So you can easily check to see if your value is in the combined flags by doing the following.
var hasValue:Boolean = ((flags & value) == value)
Very useful stuff!