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Listing 10.2 Shake, shake your iPhone
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- (BOOL)didShake:(UIAcceleration *)acceleration { accelX = ((acceleration.x * kFilteringFactor) + (accelX * (1 - kFilteringFactor))); float moveX = acceleration.x - accelX; accelY = ((acceleration.x * kFilteringFactor) + (accelY * (1 - kFilteringFactor))); float moveY = acceleration.x - accelY; if (lasttime && acceleration.timestamp > lasttime + .25) { BOOL result; if (shakecount >= 3 && biggestshake >= 1.25) { result = YES; } else {
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Waits after last shake
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Positioning: accelerometers, location, and the compass
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result = NO; } lasttime = 0; shakecount = 0; biggestshake = 0; return result; Checks x } else { movement if (fabs(moveX) >= fabs(moveY)) { if ((fabs(moveX) > .75) && (moveX * lastX <= 0)) { lasttime = acceleration.timestamp; shakecount++; lastX = moveX; if (fabs(moveX) > biggestshake) biggestshake = fabs(moveX); } Measures y movement } else { if ((fabs(moveY) > .75) && (moveY * lastY <= 0)) { lasttime = acceleration.timestamp; shakecount++; lastY = moveY; if (fabs(moveY) > biggestshake) biggestshake = fabs(moveY); } } return NO; }
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In this code, you generally follow the logic you used when viewing the accelerometer graph, although with increased sensitivity, as promised. The didShake: method registers a shake if it sees three or more movements of at least .75 g, at least one of which is 1.25 g, with movements going in opposite directions. You start by removing gravity from the accelerometer data, as you did in previous examples. This time, you don t worry about the quirk at the beginning of data collection; it doesn t register as a shake, because it s a small fraction of a g. The main work of the function is found in its latter half, which is called whenever movement continues to occur. First, you check whether the strongest movement is along the x-axis C. If so, you register the movement if it s at least .75 g and if it s in the opposite direction of the last x-axis move. You do the latter check by seeing if the product of the last two moves on that axis is negative; if so, one must have been positive and the other negative, which means they were opposite each other. If the strongest move was instead on the y-axis, you check for a sufficiently strong y-axis move that s in the opposite direction as the last y-axis move D. We could have written a more restrictive shake checker that only looked for x-axis movement, or a less restrictive checker that also looked for z-axis movement, but we opted for this middle ground. As long as movement continues without a break of more than a quarter of a second, the shakecount continues to increment, but when movement stops B, the program is ready to determine whether a shake occurred. You check this by seeing if the shake count equals or exceeds 3 and if the largest movement exceeded 1.25 g. Afterward, all of the variables are reset to check for the next shake.
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All about Core Location
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By building this shake checker as a separate method, you could easily integrate it into a list of checks made in the accelerometer:didAccelerate: method. The following code shows a simple use that changes the color of the screen every time a shake occurs. The nextColor method can be changed to do whatever you want:
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- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration { if ([self didShake:(UIAcceleration *)acceleration]) { self.view.backgroundColor = [self nextColor]; } }
We expect that the shake will be the most common three-dimensional gesture programmed into the iPhone or iPad. With this code, you already have it ready to go, though you may choose to change its sensitivity or to make it work in either one or three dimensions. Other gestures, such as a tilt, a fan, a circle, or a swat may be of use, depending on the specifics of your program, but we leave that up to your ingenuity. For now, we ve covered all of the main points of the accelerometers: orientation, gravity, movement, and gestures. We re now ready to dive into the other major positioning-related tool, and one that we find a lot easier to program because the results are less noisy: Core Location.
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