Various distance sensor fixes and enhancements #617
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About time we had a diagram for the distance sensor. Couldn't find any reference to HC-SR04 in the Adafruit fritzing repo, but there was one over in fritzing's project pages: http://fritzing.org/projects/hc-sr04-project (or rather in the comments on that page)
This fixes the issue with the DistanceSensor class constantly returning the maximum distance value. The issue was caused by cab6cc8 which changed the Pin.when_changed attribute to use weak references to all handlers. DistanceSensor is the only thing that uses a local function (not a bound method) as its pin handler. As the pin no longer kept a reference to it, it disappeared after the constructor was complete and the echo event never got set. In addition, it introduces a class-level lock to prevent multiple distance sensors from interfering with each other (it's the only current "active" sensor, but this pattern should probably be followed with future active sensors), and a brief delay between sensor reads to prevent overlapping echo responses. Note: this does *not* fix the issue with DistanceSensor operating remotely over pigpio (which requires a change in the pin handlers); that is currently scheduled for 1.5 and is why gpiozero#385 is remaining open for now.
Codecov Report
@@ Coverage Diff @@
## master #617 +/- ##
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- Coverage 84.71% 84.7% -0.02%
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Files 36 36
Lines 6695 6700 +5
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+ Hits 5672 5675 +3
- Misses 1023 1025 +2
Continue to review full report at Codecov.
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Ooops - left in a smaller value from oscilloscope testing, but the larger value does permit better accuracy.
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