AudioWorkletProcessor.parameterDescriptors (static getter)

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只读 parameterDescriptors property of an AudioWorkletProcessor -derived class is a static getter , which returns an iterable of AudioParamDescriptor -based objects.

The property is not a part of the AudioWorkletProcessor interface, but, if defined, it is called internally by the AudioWorkletProcessor constructor to create a list of custom AudioParam objects in the 参数 property of the associated AudioWorkletNode .

Defining the getter is optional.

句法

AudioWorkletProcessorSubclass.parameterDescriptors;

值

An iterable of AudioParamDescriptor -based objects. The properties of these objects are as follows:

名称: DOMString which represents the name of the AudioParam . Under this name the AudioParam will be available in the 参数 property of the node, and under this name the AudioWorkletProcessor.process method will acquire the calculated values of this AudioParam .
automationRate 可选: 要么 "a-rate" ，或 "k-rate" string which represents an automation rate of this AudioParam 。默认为 "a-rate" .
minValue 可选: A float which represents minimum value of the AudioParam 。默认为 -3.4028235e38 .
maxValue 可选: A float which represents maximum value of the AudioParam 。默认为 3.4028235e38 .
defaultValue 可选: A float which represents initial value of the AudioParam 。默认为 0 .

范例

To demonstrate creation and usage of custom AudioParam s, we'll expand the example from AudioWorkletNode page. There we've created a simple node which outputs white noise. Here, additionally, we'll create a custom gain parameter, so we can directly change volume of the output (although you could use GainNode to achieve this as well).

First, we need to define a custom AudioWorkletProcessor , and register it. Note that this should be done in a separate file.

We expand the processor by adding a static parameterDescriptors getter. It will be used internally by the AudioWorkletNode constructor to populate its 参数 with instantiated AudioParam 对象。

// white-noise-processor.js
class WhiteNoiseProcessor extends AudioWorkletProcessor {
  static get parameterDescriptors () {
    return [{
      name: 'customGain',
      defaultValue: 1,
      minValue: 0,
      maxValue: 1,
      automationRate: 'a-rate'
    }]
  }
  process (inputs, outputs, parameters) {
    const output = outputs[0]
    output.forEach(channel => {
      for (let i = 0; i < channel.length; i++) {
        channel[i] = (Math.random() * 2 - 1) *
          (parameters['customGain'].length > 1 ? parameters['customGain'][i] : parameters['customGain'][0])
        // note: a parameter contains an array of 128 values (one value for each of 128 samples),
        // however it may contain a single value which is to be used for all 128 samples
        // if no automation is scheduled for the moment.
      }
    })
    return true
  }
}
registerProcessor('white-noise-processor', WhiteNoiseProcessor)

Next, in our main scripts file we'll load the processor, create an instance of AudioWorkletNode passing it the name of the processor, and connect the node to an audio graph.

const audioContext = new AudioContext()
await audioContext.audioWorklet.addModule('white-noise-processor.js')
const whiteNoiseNode = new AudioWorkletNode(audioContext, 'white-noise-processor')
whiteNoiseNode.connect(audioContext.destination)

Now we can change the gain on the node like this:

const gainParam = whiteNoiseNode.parameters.get('customGain')
gainParam.setValueAtTime(0, audioContext.currentTime)
gainParam.linearRampToValueAtTime(0.5, audioContext.currentTime + 0.5)

规范

规范	状态	注释
Web 音频 API The definition of 'parameterDescriptors' in that specification.	工作草案

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元数据

最后修改： May 1, 2020

Web 音频 API
AudioWorkletProcessor
构造函数
1. AudioWorkletProcessor()
特性
1. parameterDescriptors (static getter)
2. port
方法
1. process
Related pages for Web Audio API

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