Android音视频技术入门之绘制一张图片

Android 的音视频入门学习，首先了解一下绘制图片。在 Android 平台绘制一张图片，使用至少 3 种不同的 API，ImageView，SurfaceView，自定义 View作绘制图片。下面我以SurfaceView作重点来讲，为什么不用其他的来作例子，分析完SurfaceView就是知道为什么要用SurfaceView作例子。

SurfaceView

我们以下面几个点来了解SurfaceView

• SurfaceView 有那些相关类。
• SurfaceView 有那些特点。
• 如何使用SurfaceView呢。
• SurfaceView的优缺。
• SurfaceView 在视频开发中应用在那里。

SurfaceView 其实是继承了View ，但与View又有一些区别，View是通过 onDraw(Canvas canvas)方法中的Canvas去绘制自身显示有界面上，而SurfaceView则不需要onDraw方法，有人会有些疑问，如果SurfaceView不需要实现onDraw方法怎么去绘制自身呢？其实View是在UI线程中绘制的，SurfaceView是在子线程中绘制的（即在一个子线程中对自己进行绘制）。在子线程中绘制怎么拿到canvas呢？下面我们去了解SurfaceView 有那些相关类。

SurfaceView 有那些相关类。

有三个重要的类，分别如下：

• Surface
• SurfaceHolder
• SurfaceView

Surface

我们看看Surface的源码

/**
 * Handle onto a raw buffer that is being managed by the screen compositor.
 *
 * <p>A Surface is generally created by or from a consumer of image buffers (such as a
 * {@link android.graphics.SurfaceTexture}, {@link android.media.MediaRecorder}, or
 * {@link android.renderscript.Allocation}), and is handed to some kind of producer (such as
 * {@link android.opengl.EGL14#eglCreateWindowSurface(android.opengl.EGLDisplay,android.opengl.EGLConfig,java.lang.Object,int[],int) OpenGL},
 * {@link android.media.MediaPlayer#setSurface MediaPlayer}, or
 * {@link android.hardware.camera2.CameraDevice#createCaptureSession CameraDevice}) to draw
 * into.</p>
 *
 * <p><strong>Note:</strong> A Surface acts like a
 * {@link java.lang.ref.WeakReference weak reference} to the consumer it is associated with. By
 * itself it will not keep its parent consumer from being reclaimed.</p>
 */
public class Surface implements Parcelable {
    private static final String TAG = "Surface";

    private static native long nativeCreateFromSurfaceTexture(SurfaceTexture surfaceTexture)
            throws OutOfResourcesException;
    private static native long nativeCreateFromSurfaceControl(long surfaceControlNativeObject);

    private static native long nativeLockCanvas(long nativeObject, Canvas canvas, Rect dirty)
            throws OutOfResourcesException;
    private static native void nativeUnlockCanvasAndPost(long nativeObject, Canvas canvas);

    private static native void nativeRelease(long nativeObject);
    private static native boolean nativeIsValid(long nativeObject);
    .......

   /**
     * Create Surface from a {@link SurfaceTexture}.
     *
     * Images drawn to the Surface will be made available to the {@link
     * SurfaceTexture}, which can attach them to an OpenGL ES texture via {@link
     * SurfaceTexture#updateTexImage}.
     *
     * @param surfaceTexture The {@link SurfaceTexture} that is updated by this
     * Surface.
     * @throws OutOfResourcesException if the surface could not be created.
     */
    public Surface(SurfaceTexture surfaceTexture) {
        if (surfaceTexture == null) {
            throw new IllegalArgumentException("surfaceTexture must not be null");
        }
        mIsSingleBuffered = surfaceTexture.isSingleBuffered();
        synchronized (mLock) {
            mName = surfaceTexture.toString();
            setNativeObjectLocked(nativeCreateFromSurfaceTexture(surfaceTexture));
        }
    }

    /* called from android_view_Surface_createFromIGraphicBufferProducer() */
    private Surface(long nativeObject) {
        synchronized (mLock) {
            setNativeObjectLocked(nativeObject);
        }
    }

    ........

}

也不难看出，其实Surface就充当着Model层，也是一个原始数据的缓冲区，表面通常是由图像缓冲区的使用者创建的。

SurfaceHolder

看看SurfaceHolder的源码

/**
 * Abstract interface to someone holding a display surface.  Allows you to
 * control the surface size and format, edit the pixels in the surface, and
 * monitor changes to the surface.  This interface is typically available
 * through the {@link SurfaceView} class.
 * 
 * <p>When using this interface from a thread other than the one running
 * its {@link SurfaceView}, you will want to carefully read the
 * methods
 * {@link #lockCanvas} and {@link Callback#surfaceCreated Callback.surfaceCreated()}.
 */
public interface SurfaceHolder {

    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_NORMAL = 0;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_HARDWARE = 1;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_GPU = 2;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_PUSH_BUFFERS = 3;

    /**
     * Exception that is thrown from {@link #lockCanvas} when called on a Surface
     * whose type is SURFACE_TYPE_PUSH_BUFFERS.
     */
    public static class BadSurfaceTypeException extends RuntimeException {
        public BadSurfaceTypeException() {
        }

        public BadSurfaceTypeException(String name) {
            super(name);
        }
    }

    /**
     * A client may implement this interface to receive information about
     * changes to the surface.  When used with a {@link SurfaceView}, the
     * Surface being held is only available between calls to
     * {@link #surfaceCreated(SurfaceHolder)} and
     * {@link #surfaceDestroyed(SurfaceHolder)}.  The Callback is set with
     * {@link SurfaceHolder#addCallback SurfaceHolder.addCallback} method.
     */
    public interface Callback {
        /**
         * This is called immediately after the surface is first created.
         * Implementations of this should start up whatever rendering code
         * they desire.  Note that only one thread can ever draw into
         * a {@link Surface}, so you should not draw into the Surface here
         * if your normal rendering will be in another thread.
         * 
         * @param holder The SurfaceHolder whose surface is being created.
         */
        public void surfaceCreated(SurfaceHolder holder);

        /**
         * This is called immediately after any structural changes (format or
         * size) have been made to the surface.  You should at this point update
         * the imagery in the surface.  This method is always called at least
         * once, after {@link #surfaceCreated}.
         * 
         * @param holder The SurfaceHolder whose surface has changed.
         * @param format The new PixelFormat of the surface.
         * @param width The new width of the surface.
         * @param height The new height of the surface.
         */
        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                int height);

        /**
         * This is called immediately before a surface is being destroyed. After
         * returning from this call, you should no longer try to access this
         * surface.  If you have a rendering thread that directly accesses
         * the surface, you must ensure that thread is no longer touching the 
         * Surface before returning from this function.
         * 
         * @param holder The SurfaceHolder whose surface is being destroyed.
         */
        public void surfaceDestroyed(SurfaceHolder holder);
    }

    /**
     * Additional callbacks that can be received for {@link Callback}.
     */
    public interface Callback2 extends Callback {
        /**
         * Called when the application needs to redraw the content of its
         * surface, after it is resized or for some other reason.  By not
         * returning from here until the redraw is complete, you can ensure that
         * the user will not see your surface in a bad state (at its new
         * size before it has been correctly drawn that way).  This will
         * typically be preceeded by a call to {@link #surfaceChanged}.
         *
         * @param holder The SurfaceHolder whose surface has changed.
         */
        public void surfaceRedrawNeeded(SurfaceHolder holder);
    }

    /**
     * Add a Callback interface for this holder.  There can several Callback
     * interfaces associated with a holder.
     * 
     * @param callback The new Callback interface.
     */
    public void addCallback(Callback callback);

    .........
}

从源码有可以看出，SurfaceHolder是以接口的形式给持有显示表面使用，允许你控制表面尺寸和格式，编辑表面的像素。监视对表面的更改。我们可以理解为SurfaceHolder充当控制层，管理Surface的生命周期，让SurfaceView来绘制Surface的数据。

SurfaceView

SurfaceView就是视图层，SurfaceView 中包含一个专门用于绘制的Surface ，Surface中包含了一个Canvas。如果细心的一点，也不难发现Surface、SurfaceHolder、SurfaceView其实就是一个MVC模式。

那么问题不了，那么如何获取到Canvas?
在SurfaceView中有一个getHolder() -> SurfaceHolder。那么Holder包含了Canvas（Canvas+管理SurfaceView的生命周期）。所以Canvas = holder.lockCanvas()。调用生命周期的holder.addCallback(Callback callback)。
SurfaceView的生命周期管理有三个方法：

1. SurfaceCreated
2. SurfaceChanged
3. SurfaceDestoryed

如何使用SurfaceView呢？

1. 获取SurfaceHolder对象，其是SurfaceView的内部类。
2. 监听Surface生命周期。
   • 只有当native层的Surface创建完毕之后，才可以调用lockCanvas()，否则失败。
   • holder.Callback。
3. 调用holder.lockCanvas()。
4. 绘制
5. 调用SurfaceHolder.unlockCanvasAndPost，将绘制内容post到Surface中

注意:第3、4、5步是在子线程中执行的。

SurfaceView的特点有那些

• 具有独立的绘图表面Surface。

• 需要在宿主窗口上挖一个洞来显示自己，z轴比普通的window要小。

• 它的UI绘制可以在独立的线程中进行，这样就可以进行复杂的UI绘制，并且不会影响应用程序的主线程响应用户输入。

SurfaceView的优缺点

优点

1. 在一个子线程中对自己进行绘制，避免造成UI线程阻塞。
2. 高效复杂的UI效果。
3. 独立Surface，独立的Window。
4. 使用双缓冲机制，播放视频时画面更流畅。

缺点

1. 每次绘制都会优先绘制黑色背景，更新不及时会出现黑边现象。
2. Surface不在View hierachy中，它的显示也不受View的属性控制，平移，缩放等变换。

SurfaceView的基本知道了解得差不多了，那么我们写一个SurfaceView绘制图片的一个公共View的实现。

public class CommonSurfaceView extends SurfaceView implements SurfaceHolder.Callback,Runnable {

    private SurfaceHolder mHolder;
    private Canvas mCanvas;
    //用于绘制的线程
    private Thread mThread;
    //线程状态的标记(线程的控制开关)
    private boolean isRunning;

    public CommonSurfaceView(Context context) {
        this(context,null);
    }

    public CommonSurfaceView(Context context, AttributeSet attrs) {
        this(context, attrs,0);
    }

    public CommonSurfaceView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        //初始化
        mHolder = getHolder();
        mHolder.addCallback(this);//管理生命周期
        //获取焦点
        setFocusable(true);
        setFocusableInTouchMode(true);
        //设置常量
        setKeepScreenOn(true);
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {

        isRunning = true;
        mThread = new Thread(this);
        mThread.start();//开启线程
    }

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

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

    @Override
    public void run() {
        //不断地进行绘制
        while (isRunning){
            draw();
        }
    }

    private void draw() {

        //为什么要try catch 因为当view在主界面时有可能按Home键或Back键时回到界面，Surface被销毁了。
        //这时有可能已经进入到draw() ，这时候获取的mCanvas可能为null。
        // 还有一种可能，就是界面被销毁的，我们的线程还没有销毁，mCanvas可能为null。

        try{
            //获取Canvas
            mCanvas = mHolder.lockCanvas();
            if(mCanvas !=null){
                //do something
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            if(mCanvas !=null){
                //释放Canvas
                mHolder.unlockCanvasAndPost(mCanvas);
            }
        }

    }
}

总结一下有那些问题、疑虑

• 不绘制任何东西，SurfaceView显示的是黑色？
• SurfaceView 能绘制什么东西？
• SurfaceVeiw双缓冲区
• SurfaceView 和 SurfaceHolder 怎么交互？
• SurfaceHolder与Surface的交互
• SurfaceView 怎么进行旋转，透明操作的？
• 一般视频播放器可以横竖屏切换，是如何实现的？
• SurfaceView 和普通的View的区别？
• SurfaceView 挖洞原理
• SurfaceView 生命周期
• 横屏录制横屏播放，竖屏录制竖屏播放

那么我们来解答一下上面的一些疑虑和问题，就浅析一下，有做得不好的请多多指出，谢谢。

不绘制任何东西，SurfaceView显示的是黑色？

每次更新视图时都会先将背景绘制成黑色。所以在移动或者缩放过程，会更新不及时时就会看黑边。

@Override
public void draw(Canvas canvas) {
    if (mDrawFinished && !isAboveParent()) {
        // draw() is not called when SKIP_DRAW is set
        if ((mPrivateFlags & PFLAG_SKIP_DRAW) == 0) {
            // punch a whole in the view-hierarchy below us
            canvas.drawColor(0, PorterDuff.Mode.CLEAR);
        }
    }
    super.draw(canvas);
}

//这句话表示PorterDuff.Mode.CLEAR会将像素设置为0，也就是黑色
//Destination pixels covered by the source are cleared to 0.
public enum Mode {
    // these value must match their native equivalents. See SkXfermode.h
    /**
     * <p>
     *     <img src="{@docRoot}reference/android/images/graphics/composite_CLEAR.png" />
     *     <figcaption>Destination pixels covered by the source are cleared to 0.</figcaption>
     * </p>
     * <p>\(\alpha_{out} = 0\)</p>
     * <p>\(C_{out} = 0\)</p>
     */
    CLEAR       (0),   
}

SurfaceView 能绘制什么东西？

从下面代码可以看到，SurfaceView 的绘制也是使用 Canvas 进行绘制的，绘制应该跟普通的 View 绘制差不多

/**
 * 绘制
 */
private void draw() {
    if (radius > getWidth()) {
        return;
    }
    Canvas canvas = mHolder.lockCanvas();
    if (canvas != null) {
        canvas.drawCircle(300, 300, radius += 10, mPaint);
        mHolder.unlockCanvasAndPost(canvas);
    }
}

SurfaceVeiw双缓冲区

双缓冲：在运用时可以理解为：SurfaceView在更新视图时用到了两张 Canvas，一张 frontCanvas 和一张 backCanvas ，每次实际显示的是 frontCanvas ，backCanvas 存储的是上一次更改前的视图。当你在播放这一帧的时候，它已经提前帮你加载好后面一帧了，所以播放起视频很流畅。
当使用lockCanvas（）获取画布时，得到的实际上是backCanvas 而不是正在显示的 frontCanvas ，之后你在获取到的 backCanvas 上绘制新视图，再 unlockCanvasAndPost（canvas）此视图，那么上传的这张 canvas 将替换原来的 frontCanvas 作为新的frontCanvas ，原来的 frontCanvas 将切换到后台作为 backCanvas 。例如，如果你已经先后两次绘制了视图A和B，那么你再调用 lockCanvas（）获取视图，获得的将是A而不是正在显示的B，之后你将重绘的 A 视图上传，那么 A 将取代 B 作为新的 frontCanvas 显示在SurfaceView 上，原来的B则转换为backCanvas。

相当与多个线程，交替解析和渲染每一帧视频数据。

surfaceholder.lockCanvas--surfaceholder.unlockCanvasAndPost

SurfaceView 和 SurfaceHolder 怎么交互？

SurfaceHolder 是 SurfaceView 内部类，可以通过 SurfaceView.getHolder() 即可获取对应的 SurfaceHolder 对象。

通过 getHolder() 就可以将 SurfaceHolder ，然后将其传递给 MediaPlayer 或者 Camera 显示出来。实际上就是通过 SurfaceHolder 去控制 SurfaceView 的显示。

/**
 * Return the SurfaceHolder providing access and control over this
 * SurfaceView's underlying surface.
 *
 * @return SurfaceHolder The holder of the surface.
 */
public SurfaceHolder getHolder() {
    return mSurfaceHolder;
}

SurfaceHolder与Surface的交互

SurfaceHolder 是一个接口，它具体的实现在 SurfaceView 中定义的一个内部类。对于 SurfaceHolder 的操作，实际上是操作Surface 的相关接口。

因为 Surface 会牵扯到 native 层的 Surface ，只有 Native 层的 Surface 创建成功之后，我们才能真正开始去绘制我们的视图。

那么如何去捕获到这个 Surface 的创建生命周期呢？

注册 SurfaceHolder.Callback 接口，监听这个接口的回调:

surfaceCreated
播放视频

surfaceDestroy
停止视频播放

Canvas canvas = mHolder.lockCanvas();
if (canvas != null) {
    canvas.drawBitmap(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher, null), 0, 0, mPaint);
    mHolder.unlockCanvasAndPost(canvas);
}

• mHolder.lockCanvas(); 实际获取的是 Surface 中的 Canvas。

/**
 * Gets a {@link Canvas} for drawing into the SurfaceView's Surface
 *
 * After drawing into the provided {@link Canvas}, the caller must
 * invoke {@link #unlockCanvasAndPost} to post the new contents to the surface.
 *
 * The caller must redraw the entire surface.
 * @return A canvas for drawing into the surface.
 */
@Override
public Canvas lockCanvas() {
    return internalLockCanvas(null);
}


private final Canvas internalLockCanvas(Rect dirty) {
    mSurfaceLock.lock();
    Canvas c = null;
    if (!mDrawingStopped && mWindow != null) {
        try {
            //实际调用的是 surface.lockCancas()
            c = mSurface.lockCanvas(dirty);
        } catch (Exception e) {
            Log.e(LOG_TAG, "Exception locking surface", e);
        }
    }
    ...
    return null;
}

• canvas.drawXXX();

  在 Canvas 中绘制内容。

• mHolder.unlockCanvasAndPost(canvas);将 绘制在 Canvas 中的内容刷新到 Surface 中。

//将 backcanvas 中的内容刷新到 surface 中并且释放这个 canvas
/**
 * Posts the new contents of the {@link Canvas} to the surface and
 * releases the {@link Canvas}.
 *
 * @param canvas The canvas previously obtained from {@link #lockCanvas}.
 */
@Override
public void unlockCanvasAndPost(Canvas canvas) {
    mSurface.unlockCanvasAndPost(canvas);
    mSurfaceLock.unlock();
}

SurfaceView 怎么进行旋转，透明操作的？

• 普通View旋转后,View的内容也跟着同步做了旋转.
• SurfaceView在旋转之后，其显示内容并没有跟着一起旋转.

比喻：这就好比在墙上开了一个窗(Surface)，通过窗口可以看外面的花花世界，但窗口无论怎样变化，窗外面的世界是不会跟着窗口一同变化。

一般视频播放器可以横竖屏切换，是如何实现的？

在 Activity 中覆写 onConfigurationChanged 方法就可以。根据横竖屏切换，修改 SurfaceView 的 Parameter 的宽高参数即可。

android:configChanges="orientation|keyboardHidden|screenSize"

@Override  
public void onConfigurationChanged(Configuration newConfig) {  

    super.onConfigurationChanged(newConfig);  
    
    if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {  
            //变成横屏了    
    }   else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) {  
            //变成竖屏了 
    }  
}  

SurfaceView 挖洞原理

挖洞原理了解之后再补上吧

SurfaceView 和普通的View的区别？

surfaceView是在一个新起的单独线程中可以重新绘制画面。

View必须在UI的主线程中更新画面。

那么在UI的主线程中更新画面可能会引发问题，比如你更新画面的时间过长，那么你的主UI线程会被你正在画的函数阻塞。那么将无法响应按键，触屏等消息。

当使用surfaceView 由于是在新的线程中更新画面所以不会阻塞你的UI主线程。

SurfaceView 生命周期

使用：双缓冲
导致：需要更多的内存开销
为了节约系统内存开销：

SurfaceView 可见时 -> 创建 SurfaceHolder
SurfaecView 不可见时 -> 摧毁 SurfaceHolder

1、程序打开
Activity 调用顺序:onCreate()->onStart()->onResume()
SurfaceView 调用顺序: surfaceCreated()->surfaceChanged()

2、程序关闭（按 BACK 键）
Activity 调用顺序:onPause()->onStop()->onDestory()
SurfaceView 调用顺序: surfaceDestroyed()

3、程序切到后台（按 HOME 键）
Activity 调用顺序:onPause()->onStop()
SurfaceView 调用顺序: surfaceDestroyed()

4、程序切到前台
Activity 调用顺序: onRestart()->onStart()->onResume()
SurfaceView 调用顺序: surfaceChanged()->surfaceCreated()

5、屏幕锁定（挂断键或锁定屏幕）
Activity 调用顺序: onPause()
SurfaceView 什么方法都不调用

6、屏幕解锁 
Activity 调用顺序: onResume()
SurfaceView 什么方法都不调用

横屏录制横屏播放，竖屏录制竖屏播放

通过以下方法可以获取到视频的宽高，根据视频的宽高就可以知道该视频是横屏还是竖屏录制的。

MediaPlayer# public void onVideoSizeChanged(MediaPlayer mp, int width, int height)

横屏判断：

width>height
旋转屏幕：
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

竖屏录制
height>width
旋转屏幕：
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

mediaPlayer.setOnVideoSizeChangedListener(new MediaPlayer.OnVideoSizeChangedListener() {
    @Override
    public void onVideoSizeChanged(MediaPlayer mp, int width, int height) {
        Log.e(TAG, "onVideoSizeChanged:WIDTH>>" + width);
        Log.e(TAG, "onVideoSizeChanged:HEIGHT>>" + height);
        
        if (width > height) {
            //横屏录制
            if (getRequestedOrientation() != ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE) {
                setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
            }
        } else {
            //竖屏录制
            if (getRequestedOrientation() != ActivityInfo.SCREEN_ORIENTATION_PORTRAIT) {
                setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
            }
        }
    }
});

View的绘制要知道的知识

View的绘制其实是在UI线程（实现onCanvas方法进行绘制）。如果进行绘制高效复杂的UI，最好不用自定义View。要用SurfaceView进行绘制。

View的绘画三要素

1. Canvas (画布，绘制BitMap操作)
2. Paint (绘制的画笔Paint，颜色、样式)
3. Path （路径）

一、Canvas

1. 如果直接extends View 可以重写onDraw(Canvas canvas)方法，直接用里面的canvas进行绘制。
2. 可以直接利用Activity的绘制机制，用lockCanvas()方法来获得当前的Activity的Canvas。
3. 在SurfaceView中，同2可以利用SurfaceHolder的对象的lockCanvas()方法来Canvas。

二、Paint

1. 直接通过new关键字来实例化，然后通过Paint对象来对画笔进行相应的设置：
   如：
   1.1 去锯齿setAntiAlia(true)
   1.2 去抖动setDither(true)
   1.3 设置图层混合模式setXfermode(Xfermode,xfermode)

三、 path

1、Path路径 直接用new来实例化
2、通过path对象设置想要画图的轨迹或路线
如：矩形 、三角形 、圆、曲线等

实现一个自定义View，代码如下：

//自定义绘图类
public class BallView extends View {
    private Paint paint;        //定义画笔
    private float cx = 150;      //圆点默认X坐标
    private float cy = 250;      //圆点默认Y坐标
    private int radius = 60;    // 半径
    //定义颜色数组
    private int colorArray[] = {Color.BLACK,Color.BLACK,Color.GREEN,Color.YELLOW, Color.RED};
    private int paintColor = colorArray[0]; //定义画笔默认颜色

    private int screenW;        //屏幕宽度
    private int screenH;        //屏幕高度

    public BallView(Context context,int screenW,int screenH) {
        super(context);
        this.screenW=screenW;
        this.screenH=screenH;
        //初始化画笔
        initPaint();
    }
    private void initPaint(){
        paint = new Paint();
        //设置消除锯齿
        paint.setAntiAlias(true);
        //设置画笔颜色
        paint.setColor(paintColor);
    }

    //重写onDraw方法实现绘图操作
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //将屏幕设置为白色
        canvas.drawColor(Color.WHITE);
        //修正圆点坐标
        revise();
        //随机设置画笔颜色
        setPaintRandomColor();
        //绘制小圆作为小球
        canvas.drawCircle(cx, cy, radius, paint);
    }

    //为画笔设置随机颜色
    private void setPaintRandomColor(){
        Random rand = new Random();
        int randomIndex = rand.nextInt(colorArray.length);
        paint.setColor(colorArray[randomIndex]);
    }

    //修正圆点坐标
    private void revise(){
        if(cx <= radius){
            cx = radius;
        }else if(cx >= (screenW-radius)){//防止出边界
            cx = screenW-radius;
        }
        if(cy <= radius){
            cy = radius;
        }else if(cy >= (screenH-radius)){//防止出边界
            cy = screenH-radius;
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                // 按下
                cx = (int) event.getX();
                cy = (int) event.getY();
                // 通知重绘
                postInvalidate();   //该方法会调用onDraw方法，重新绘图
                break;
            case MotionEvent.ACTION_MOVE:
                // 移动
                cx = (int) event.getX();
                cy = (int) event.getY();
                // 通知重绘
                postInvalidate();
                break;
            case MotionEvent.ACTION_UP:
                // 抬起
                cx = (int) event.getX();
                cy = (int) event.getY();
                // 通知重绘
                postInvalidate();
                break;
        }

            /*
             * 备注1：此处一定要将return super.onTouchEvent(event)修改为return true，原因是：
             * 1）父类的onTouchEvent(event)方法可能没有做任何处理，但是返回了false。
             * 2)一旦返回false，在该方法中再也不会收到MotionEvent.ACTION_MOVE及MotionEvent.ACTION_UP事件。
             */
        //return super.onTouchEvent(event);
        return true;
    }
}

懂得运用View的绘画三要素，画出自己想要的图也不难。

ImageView 绘制图片就不多说了，看一下例子吧

public class RoundImageView extends ImageView {

    private Bitmap mBitmap;
    private Rect mRect = new Rect();
    private PaintFlagsDrawFilter pdf = new PaintFlagsDrawFilter(0, Paint.ANTI_ALIAS_FLAG);
    private Paint mPaint = new Paint();
    private Path mPath=new Path();
    public RoundImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }


    //传入一个Bitmap对象
    public void setBitmap(Bitmap bitmap) {
        this.mBitmap = bitmap;
    }


    private void init() {
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setFlags(Paint.ANTI_ALIAS_FLAG);
        mPaint.setAntiAlias(true);// 抗锯尺
    }


    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(mBitmap == null)
        {
            return;
        }
        mRect.set(0,0,getWidth(),getHeight());
        canvas.save();
        canvas.setDrawFilter(pdf);
        mPath.addCircle(getWidth() / 2, getWidth() / 2, getHeight() / 2, Path.Direction.CCW);
        canvas.clipPath(mPath, Region.Op.REPLACE);
        canvas.drawBitmap(mBitmap, null, mRect, mPaint);
        canvas.restore();
    }
}

综上所述，为什么视频技术入门要先了解图片绘制，那么图片绘制的API也有多种，为什么选择用SurfaceView这个API，因为其一，绘制是在子线程中进行绘制的，其二，可能绘制出高效复杂的UI效果，其三，使用双缓冲机制，播放视频时画面更流畅。

所以我也把我了解到的，学习到的基础知识分享给大家。希望对大家有帮助。