Example: Read-Write File Access
===============================

The current version of regular Blocks (SDK 7.0) does not provide
**read/write access to an external file**, other than automatic Log or
Match Log file entries. File access is a useful capability, available so
far to Java programmers only. Now it can be done with myBlocks!

Here’s an example pair of myBlocks. One myBlock **writes** a numeric
value to a specified filename, and a companion myBlock can later
**read** that value from the same file.

.. image:: images/a0550-file-access-Blocks.png

The file is stored on the Control Hub or RC phone, in the FIRST/settings
folder. It exists separately from the RC app, OpModes, and other 
files.

Write and read actions can happen in the same OpMode or **different
OpModes**, allowing various scenarios:

-  Autonomous passes information to TeleOp. For example, what was the
   latest value of a sensor or encoder?

-  A special **set-up OpMode** allows gamepad input to choose an
   autonomous strategy and adjust key parameters. The robot could then
   be idle for a long time, even turned off. When the match begins, the
   Autonomous OpMode would read those settings and implement the
   chosen/adjusted actions.

-  A **dedicated log file** reports key sensor data in a custom format,
   with optional time-stamps. For program development and debugging,
   this could be more efficient than working with the large standard
   logs or Match Logs.

The Java code for this example is available
below, with **extensive comments** that explain some unfamiliar Java
expressions. The code can be copied and pasted directly into OnBot Java
or Android Studio.

   Programming tip: Instead of memorizing every possible Java command,
   programmers often study and modify existing code for a similar task.
   Unfamiliar commands are explored with an internet search, reference
   book, at the `Javadoc
   reference <https://javadoc.io/doc/org.firstinspires.ftc>`__, or at
   the official `Oracle
   Javadoc <https://docs.oracle.com/javase/7/docs/api/>`__.

This simple example supports only a single numeric value per filename.
Better versions would allow multiple values and data types – a good
programming challenge!

Be careful about placing **myBlocks inside loops**. Expanding on the
current example, your myBlock might read a larger amount of (unchanging)
data from a file. If your OpMode needs that data only once, reading the
file in a loop needlessly adds cycle time and might increase the risk of
a corrupt or interrupted read operation.

.. image:: images/a0570-do-not-read-in-loop-Blocks.png

Instead, read the file once and store the relevant data in a variable or
array. Then process the variable as needed, inside the loop.

.. image:: images/a0560-read-once-Blocks.png

The same suggestion might apply to reading sensors and encoders, if the
data are not changing and are needed only once.

.. dropdown:: Example Code

   :download:`SampleMyBlocks_v05.java <opmodes/SampleMyBlocks_v05.java>`

   .. literalinclude:: opmodes/SampleMyBlocks_v05.java
      :language: java