先上效果图：
示例用法：
SpectrumView * spectrumView = [[SpectrumView alloc] initWithFrame:CGRectMake(CGRectGetMidX(self.view.frame) - 100,180,200, 40.0)];
spectrumView.text = [NSString stringWithFormat:@"%d",0];
__weak SpectrumView * weakWaver = spectrumView;
spectrumView.itemLevelCallback = ^() {
[self.audioRecorder updateMeters];
//取得第一个通道的音频，音频强度范围是-160到0
float power = [self.audioRecorder averagePowerForChannel:0];
weakWaver.level = power;
};
[self.view addSubview:spectrumView];
使用前请注意：
SpectrumView.frame.size.width / SpectrumView.numberOfItems >= 5
默认SpectrumView.numberOfItems = 20 (可修改，必须为偶数)，所以SpectrumView.frame.size.width默认要大于100
例如：SpectrumView * spectrumView = [[SpectrumView alloc] initWithFrame:CGRectMake(0,0,100,40)];