Android 提供了AnimationDrawable用于實(shí)現(xiàn)幀動(dòng)畫纬纪。在動(dòng)畫開始之前，所有幀的圖片都被解析并占用內(nèi)存次和，一旦動(dòng)畫較復(fù)雜幀數(shù)較多，在低配置手機(jī)上容易發(fā)生 OOM那伐。即使不發(fā)生 OOM踏施，也會(huì)對內(nèi)存造成不小的壓力。下面代碼展示了一個(gè)幀數(shù)為4的幀動(dòng)畫：

AnimationDrawable drawable = new AnimationDrawable();
drawable.addFrame(getDrawable(R.drawable.frame1), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame2), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame3), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame4), frameDuration);
drawable.setOneShot(true);

ImageView ivFrameAnim = ((ImageView) findViewById(R.id.frame_anim));
ivFrameAnim.setImageDrawable(drawable);
drawable.start(); 

有沒有什么辦法讓幀動(dòng)畫的數(shù)據(jù)逐幀加載罕邀，而不是一次性全部加載到內(nèi)存畅形？SurfaceView就提供了這種能力。

SurfaceView

屏幕的顯示機(jī)制和幀動(dòng)畫類似诉探，也是一幀一幀的連環(huán)畫日熬，只不過刷新頻率很高，感覺像連續(xù)的肾胯。為了顯示一幀竖席，需要經(jīng)歷計(jì)算和渲染兩個(gè)過程，CPU 先計(jì)算出這一幀的圖像數(shù)據(jù)并寫入內(nèi)存敬肚，然后調(diào)用 OpenGL 命令將內(nèi)存中數(shù)據(jù)渲染成圖像存放在 GPU Buffer 中毕荐，顯示設(shè)備每隔一定時(shí)間從 Buffer 中獲取圖像并顯示。

上述過程中的計(jì)算艳馒，對于View來說东跪，就好比在主線程遍歷 View樹 以決定視圖畫多大（measure），畫在哪（layout）鹰溜，畫些啥（draw），計(jì)算結(jié)果存放在內(nèi)存中丁恭，SurfaceFlinger 會(huì)調(diào)用 OpenGL 命令將內(nèi)存中的數(shù)據(jù)渲染成圖像存放在 GPU Buffer 中曹动。每隔16.6ms，顯示器從 Buffer 中取出幀并顯示牲览。所以自定義 View 可以通過重載onMeasure()墓陈、onLayout()、onDraw()來定義幀內(nèi)容，但不能定義幀刷新頻率贡必。

SurfaceView可以突破這個(gè)限制兔港。而且它可以將計(jì)算幀數(shù)據(jù)放到獨(dú)立的線程中進(jìn)行。下面是自定義SurfaceView的模版代碼：

public abstract class BaseSurfaceView extends SurfaceView implements SurfaceHolder.Callback {
public static final int DEFAULT_FRAME_DURATION_MILLISECOND = 50;
//用于計(jì)算幀數(shù)據(jù)的線程
private HandlerThread handlerThread;
private Handler handler;
//幀刷新頻率
private int frameDuration = DEFAULT_FRAME_DURATION_MILLISECOND;
//用于繪制幀的畫布
private Canvas canvas;
private boolean isAlive;

public BaseSurfaceView(Context context) {
super(context);
init();
}

protected void init() {
getHolder().addCallback(this);
//設(shè)置透明背景仔拟，否則SurfaceView背景是黑的
setBackgroundTransparent();
}

private void setBackgroundTransparent() {
getHolder().setFormat(PixelFormat.TRANSLUCENT);
setZOrderOnTop(true);
}

@Override
public void surfaceCreated(SurfaceHolder holder) {
isAlive = true;
startDrawThread();
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
stopDrawThread();
isAlive = false;
}

//停止幀繪制線程
private void stopDrawThread() {
handlerThread.quit();
handler = null;
}

//啟動(dòng)幀繪制線程
private void startDrawThread() {
handlerThread = new HandlerThread(“SurfaceViewThread”);
handlerThread.start();
handler = new Handler(handlerThread.getLooper());
handler.post(new DrawRunnable());
}

private class DrawRunnable implements Runnable {

@Override
public void run() {
if (!isAlive) {
return;
}
try {
//1.獲取畫布
canvas = getHolder().lockCanvas();
//2.繪制一幀
onFrameDraw(canvas);
} catch (Exception e) {
e.printStackTrace();
} finally {
//3.將幀數(shù)據(jù)提交
getHolder().unlockCanvasAndPost(canvas);
//4.一幀繪制結(jié)束
onFrameDrawFinish();
}
//不停的將自己推送到繪制線程的消息隊(duì)列以實(shí)現(xiàn)幀刷新
handler.postDelayed(this, frameDuration);
}
}

protected abstract void onFrameDrawFinish();

protected abstract void onFrameDraw(Canvas canvas);
} 

• 用HandlerThread作為獨(dú)立幀繪制線程衫樊，好處是可以通過與其綁定的Handler方便地實(shí)現(xiàn)“每隔一段時(shí)間刷新”，而且在Surface被銷毀的時(shí)候可以方便的調(diào)用HandlerThread.quit()來結(jié)束線程執(zhí)行的邏輯利花。

• DrawRunnable.run()運(yùn)用模版方法模式定義了繪制算法框架科侈，其中幀繪制邏輯的具體實(shí)現(xiàn)被定義成兩個(gè)抽象方法，推遲到子類中實(shí)現(xiàn)炒事，因?yàn)槔L制的東西是多樣的臀栈，對于本文來說，繪制的就是一張張圖片挠乳，所以新建BaseSurfaceView的子類FrameSurfaceView权薯。

逐幀解析 & 及時(shí)回收

public class FrameSurfaceView extends BaseSurfaceView {
    public static final int INVALID_BITMAP_INDEX = Integer.MAX_VALUE;
    private List bitmaps = new ArrayList<>();
    //幀圖片
    private Bitmap frameBitmap;
    //幀索引
    private int bitmapIndex = INVALID_BITMAP_INDEX;
    private Paint paint = new Paint();
    private BitmapFactory.Options options = new BitmapFactory.Options();
    //幀圖片原始大小
    private Rect srcRect;
    //幀圖片目標(biāo)大小
    private Rect dstRect = new Rect();
    private int defaultWidth;
    private int defaultHeight;

    public void setDuration(int duration) {
        int frameDuration = duration / bitmaps.size();
        setFrameDuration(frameDuration);
    }

    public void setBitmaps(List bitmaps) {
      if (bitmaps == null || bitmaps.size() == 0) {
          return;
      }
      this.bitmaps = bitmaps;
      //默認(rèn)情況下，計(jì)算第一幀圖片的原始大小
      getBitmapDimension(bitmaps.get(0));
    }

    private void getBitmapDimension(Integer integer) {
      final BitmapFactory.Options options = new BitmapFactory.Options();
      options.inJustDecodeBounds = true;
      BitmapFactory.decodeResource(this.getResources(), integer, options);
      defaultWidth = options.outWidth;
      defaultHeight = options.outHeight;
      srcRect = new Rect(0, 0, defaultWidth, defaultHeight);
      requestLayout();
    }

     public FrameSurfaceView(Context context) {
        super(context);
      }

      @Override
      protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        dstRect.set(0, 0, getWidth(), getHeight());
      }

    @Override
    protected void onFrameDrawFinish() {
        //在一幀繪制完后睡扬，直接回收它
        recycleOneFrame();
    }

    //回收幀
    private void recycleOneFrame() {
        if (frameBitmap != null) {
            frameBitmap.recycle();
            frameBitmap = null;
        }
    }

    @Override
    protected void onFrameDraw(Canvas canvas) {
        //繪制一幀前需要先清畫布盟蚣，否則所有幀都疊在一起同時(shí)顯示
        clearCanvas(canvas);
        if (!isStart()) {
            return;
        }
      if (!isFinish()) {
          drawOneFrame(canvas);
      } else {
          onFrameAnimationEnd();
      }
    }

    //繪制一幀，是張Bitmap
    private void drawOneFrame(Canvas canvas) {
        frameBitmap = BitmapUtil.decodeOriginBitmap(getResources(),    
         bitmaps.get(bitmapIndex), options);
        canvas.drawBitmap(frameBitmap, srcRect, dstRect, paint);
        bitmapIndex++;
    }

      private void onFrameAnimationEnd() {
        reset();
      }

      private void reset() {
          bitmapIndex = INVALID_BITMAP_INDEX;
      }

//幀動(dòng)畫是否結(jié)束
private boolean isFinish() {
return bitmapInde
x >= bitmaps.size();
}

//幀動(dòng)畫是否開始
private boolean isStart() {
return bitmapIndex != INVALID_BITMAP_INDEX;
_BITMAP_INDEX;
}

//幀動(dòng)畫是否結(jié)束
private boolean isFinish() {
return bitmapInde[外鏈圖片轉(zhuǎn)存中…(img-Eug6Luq9-1642137464297)]
x >= bitmaps.size();
}

//幀動(dòng)畫是否開始
private boolean isStart() {
return bitmapIndex != INVALID_BITMAP_INDEX;
}