最近在研究VAD演算法，但調整引數時無法實時看到效果，於是決定將音訊波形實時繪製出來，並且語音部分和噪音部分用不同顏色的線條顯示，這樣就能立即看到VAD演算法對各種噪音型別的魯棒性

為了簡化問題規模，先研究出怎麼在Android下實時繪製隨機生成波形的功能，後面再加入語音獲取、根據VAD演算法的結果用不同顏色線條顯示等功能

查了下Canvas類，發現drawLines方法跟drawLine方法都是根據4個浮點數繪製一條直線，前者還需要開闢額外的空間，所以選擇了後者，結果繪製速度很慢，在我的小米2s上繪製2s單通道16KHz取樣率16bit位寬的音訊，耗時達到700ms，無奈嘗試前者，其他條件不變的情況下降至20ms（最快1ms）

由此也能推測出，後者開銷這麼大主要是花在drawLine的函式呼叫，而不是座標點的生成。

程式碼

package com.happen23.games.waveviewer;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.ViewTreeObserver;

import java.util.Random;

/**
 * Created by Administrator on 2016/5/14.
 */
public class VadView extends View {
    private static final String TAG = "VadView";
    Paint paint;
    int audioSampleNum = 2 * 16000;
    int widthPixels;
    int heightPixels;
    float points[];
    short audio[];

    public VadView(Context context, AttributeSet attrs) {
        super(context, attrs);
        paint = new Paint();
        paint.setColor(Color.BLUE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
        paint.setStrokeWidth(1);

        getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                widthPixels = getWidth();
                heightPixels = getHeight();
            }
        });
    }
    protected void drawWave2(Canvas canvas, short audio[]) {
        for (int i = 0; i < audioSampleNum-1; i++){
            points[4*i] = (float)i/audioSampleNum * widthPixels;
            points[4*i+1] = heightPixels/2 + (float)audio[i]/32768 * heightPixels/2;
            points[4*i+2] = (float)(i+1)/audioSampleNum * widthPixels;
            points[4*i+3] = heightPixels/2 + (float)audio[i+1]/32768 * heightPixels/2;
        };
        canvas.drawLines(points, paint);
    }
    protected void drawWave1(Canvas canvas, short audio[]){
        float startX, startY, stopX, stopY;
        for (int i = 0; i < audio.length-1; i++){
            startX = (float)i/audioSampleNum * widthPixels;
            startY = heightPixels/2 + (float)audio[i]/32768 * heightPixels/2;
            stopX = (float)(i+1)/audioSampleNum * widthPixels;
            stopY = heightPixels/2 + (float)audio[i+1]/32768 * heightPixels/2;
            canvas.drawLine(startX, startY, stopX, stopY, paint);
        }
    }
    protected void onDraw(Canvas canvas) {
        if (points == null) {
            points = new float[audioSampleNum * 4];
            audio = new short[audioSampleNum];
        }
        Random r = new Random();
        for (int i = 0; i < audioSampleNum; i++){
            audio[i] = (short)(r.nextInt(65536) - 32768);
        }
        Log.w(TAG, "start drawLine");
        drawWave2(canvas, audio);
        Log.w(TAG, "stop drawLine");
        postDelayed(new Runnable() {
            @Override
            public void run() {
                invalidate();
            }
        }, 200);
    }
}