AVFoundation-缩放

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iOS7以前使用AVCaptureConnection

通过 AVCaptureConnection 的属性对摄像头缩放进行有限制的支持，开发者可以通过调整连接缩放的值从默认的1.0增加到 videoMaxScaleAndCropFactor 属性定义的最大值

需要用到AVCaptureConnection的两个属性

  • @property(nonatomic) CGFloat videoScaleAndCropFactor
  • @property(nonatomic, readonly) CGFloat videoMaxScaleAndCropFactor

videoScaleAndCropFactor这个属性取值范围是1.0-videoMaxScaleAndCropFactor

示例：

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AVCaptureStillImageOutput* output = (AVCaptureStillImageOutput*)[self.captureSession.outputs objectAtIndex:0];
 AVCaptureConnection *videoConnection = [output connectionWithMediaType:AVMediaTypeVideo];
 CGFloat maxScale = videoConnection.videoMaxScaleAndCropFactor;
 CGFloat zoom = maxScale / 50;
 if (zoom < 1.0f || zoom > maxScale)
 {
     return;
 }
 videoConnection.videoScaleAndCropFactor += zoom;
 self.preVideoView.transform = CGAffineTransformScale(self.preVideoView.transform, zoom, zoom);

iOS7以后 使用 AVCaptureDevice

AVCaptureDevice 提供了属性 videoZoomFactor用于控制捕捉设备的缩放等级，这个值最小为1.0，最大值由捕捉设备的 activeFormat 值确定

示例：

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//videoMaxZoomFactor值大于1.0，则捕捉设备支持缩放功能
- (BOOL)cameraSupportsZoom {
	return self.activeCamera.activeFormat.videoMaxZoomFactor > 1.0f;        // 1
}
//确定最大缩放因子，4.0f是随意设置的
- (CGFloat)maxZoomFactor {
	return MIN(self.activeCamera.activeFormat.videoMaxZoomFactor, 4.0f);    // 2
}

- (void)setZoomValue:(CGFloat)zoomValue {                                   // 3
	if (!self.activeCamera.isRampingVideoZoom) {

        NSError *error;
        if ([self.activeCamera lockForConfiguration:&error]) {              // 4

            // Provide linear feel to zoom slider
			CGFloat zoomFactor = pow([self maxZoomFactor], zoomValue);      // 5
            //应用程序提供的缩放范围1X到4X 是指数形式的，所以要提供范围线性增长的感觉
            self.activeCamera.videoZoomFactor = zoomFactor;

            [self.activeCamera unlockForConfiguration];                     // 6

		} else {
            [self.delegate deviceConfigurationFailedWithError:error];
        }
	}
}

添加监听缩放

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- (BOOL)setupSessionInputs:(NSError **)error {
	BOOL success = [super setupSessionInputs:error];                        // 1
	if (success) {
		[self.activeCamera addObserver:self                                 // 2
							forKeyPath:@"videoZoomFactor"
							   options:0
							   context:&THRampingVideoZoomFactorContext];
		[self.activeCamera addObserver:self                                 // 3
							forKeyPath:@"rampingVideoZoom"
							   options:0
							   context:&THRampingVideoZoomContext];

	}
	return success;
}

- (void)observeValueForKeyPath:(NSString *)keyPath
					  ofObject:(id)object
						change:(NSDictionary *)change
					   context:(void *)context {

	if (context == &THRampingVideoZoomContext) {
        [self updateZoomingDelegate];                                       // 4
	} else if (context == &THRampingVideoZoomFactorContext) {
		if (self.activeCamera.isRampingVideoZoom) {
			[self updateZoomingDelegate];                                   // 5
		}
	} else {
		[super observeValueForKeyPath:keyPath
							 ofObject:object
							   change:change
							  context:context];
	}
}