View’s twin brother-SurfaceView

Overview

The Android system provides View for drawing processing. View can meet most of the drawing needs, but it is not enough at some times, especially when some development is carried out.

General View redraws the view by refreshing. The Android system redraws the screen by sending out the VSYNC signal. The refresh interval is 16ms.

If the View completes the operation that needs to be performed within 16ms, then the user will not feel stuck visually; and if the execution logic is too much, especially the interface that needs to be refreshed frequently, Such as the game interface, then it will continue to block the main thread, resulting in interface freezes.

We can often see in the log

SKipped 47 frames! The application may be doing too much work on its main Thread

Many reasons for this kind of alarm are caused by too much processing logic in the drawing process.

In order to avoid this problem, Android provides SurfaceView to solve this problem.

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The difference between SurfaceView and View

The difference between SurfaceView and View:

• View is mainly applicable In the case of active update, SurfaceView is suitable for passive update, such as frequent refresh.
• View performs the screen in the main thread, while SurfaceView usually refreshes the page in a sub-thread
• View does not use double buffering mechanism when drawing, while SurfaceView is at the bottom A double buffering mechanism is implemented

In short, if your custom View needs to be refreshed frequently, or the amount of data processed during refresh is relatively large, you can consider using SurfaceView instead of View.

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Use of SurfaceView

Although the use of SurfaceView is more complicated than that of View, there is a set of template codes used when SurfaceView is used. Most All SurfaceView drawing operations can be written using such template code.

Usually, use the following steps to create a SurfaceView template

Create SurfaceView

Create a custom SurfaceView inherited from SurfaceView , And implement two interfaces: SurfaceHolder.Callback and Runnable.

The code is as follows:

For the SurfaceHolder.Callback interface

public < span class="hljs-class">class SurfaceViewTemplate extends  SurfaceView implements SurfaceHolder. Callback ,Runnable { .......}

To implement the SurfaceHolder.Callback and Runnable interfaces, the following methods need to be rewritten

/** * SurfaceView creation* @param holder */ @Override public void surfaceCreated(SurfaceHolder holder) {} /** * SurfaceView change*  @param holder * @param format * @param width * @param height */ @Override public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {} /* * * SurfaceView destruction* @param holder */ @Override public void surfaceDestroyed(SurfaceHolder holder) {}

< p>For the Runnable interface, there is no need to say more, you need to rewrite the run method

@Override public void run() {}

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< h3 id="Initialize surfaceview">Initialize SurfaceView

In the construction method of custom SurfaceView, SurfaceView needs to be initialized.

Generally, we usually define the following three member variables

//SurfaceHolder private SurfaceHolder mSurfaceHolder; // Canvas for drawing private Canvas mCanvas; // Sub-thread flag bit private boolean span> mIsDrawing;

The initialization method is to initialize the SurfaceHolder, and then use the following code to initialize a SurfaceHolder object and register the SurfaceHolder callback method.

mSurfaceHolder = getHolder(); mSurfaceHolder.addCallback(this);< /span>

The other two member variables mCanvas and flag bits.

Canvas needless to say, it is the same as using Canvas drawing in the onDraw method of View.

Another flag bit is used to control the child thread. SurfaceView usually starts a child thread to draw, and this flag bit can control the child thread.

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Use SurfaceView

Through the lockCanvas() method of the SurfaceHolder object, you can get the current Canvas drawing object.

However, it should be noted that the obtained Canvas object is still the last Canvas object, not a new object. Therefore, the previous drawing operations will be retained. If you need to erase, you can use the drawColor() method to clean the screen before drawing.

When drawing, make full use of the three callback methods of SurfaceView, open the child thread in the surfaceCreated method for drawing,
and the child thread uses a while( mIsDrawing) loop to draw continuously, and submit the canvas content through the unlockCanvasAndPost(mCanvas) method.

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The template code of the entire SurfaceView is as follows:

import android.content.Context; import android.graphics.Canvas;import android.util.AttributeSet;import android.view.SurfaceHolder;import android.view.SurfaceView;/** * MyApp * * @author Mr.Yang on 2016-04-22 15:59. * @version 1.0 *< span class="hljs-javadoctag"> @desc */public class SurfaceViewTemplate extends SurfaceView implements SurfaceHolder.Callback span>, Runnable { //SurfaceHolder private SurfaceHolder mSurfaceHolder; // Canvas for drawing private Canvas mCanvas; // Sub-thread flag private boolean mIsDrawing; /** * Constructor* * @param context */ public SurfaceViewTemplate(Context context) {super(context); initView();} /** * Constructor* * @param context * @param attrs */  public SurfaceViewTemplate(Context context, AttributeSet attrs) {super(context, attrs); initView();} /** * Constructor* *  @param context * @param attrs * @param span> defStyleAttr */ public SurfaceViewTemplate(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr); initView();} /** * Initialization, call in the constructor*/ private void < span class="hljs-title">initView() {mSurfaceHolder = getHolder(); mSurfaceHolder.addCallback(this); setFocusable(true); setFo cusableInTouchMode(true); this.setKeepScreenOn(true span>); //mSurfaceHolder.setFormat(PixelFormat.OPAQUE);} /** * SurfaceView creation* * @param holder */ @Override public span> void surfaceCreated(SurfaceHolder holder) {mIsDrawing = true ; new Thread(this).start();} /** * SurfaceView changes* * @param holder * @param format * @param width * @param he ight */ @Override public void< /span> surfaceChanged(SurfaceHolder holder, int format, int  width, int height) {} /** * SurfaceView destruction* * @param holder */ @Override public void surfaceDestroyed(SurfaceHolder holder) {mIsDrawing = false; } @Override public void run() {while (mIsDrawing) {draw() ;}} /** * */ private void< /span> draw() {try {mCanvas = mSurfaceHolder.lockCanvas(); // draw something} catch (Exception e) {} finally {if (mCanvas != null) {mSurfaceHolder.unlockCanvasAndPost(mCanvas);}} }} 
The above code can basically meet most of the SurfaceView drawing needs. It should be noted that put mSurfaceHolder.unlockCanvasAndPost(mCanvas) in finally to ensure that the content can be submitted every time p> 
 
SurfaceView example
 
sine curve
 
To draw a sine curve, you only need to constantly change the horizontal The value of the ordinate, and let them satisfy the sine function, so a Path object is used to save the coordinate points on the sine function, and the value of the abscissa and ordinate is constantly changed in the sub-thread While loop. 
 
import android.content.Context;import android. graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import< /span> android.view.SurfaceHolder;import android.view.SurfaceView;/** * MyApp * * @author Mr.Yang on 2016-04-22 22:17. * @version 1.0 * @desc */public class SinView extends SurfaceView  implements SurfaceHolder.Callback , Runnable { private SurfaceHolder mHolder; private  Canvas mCanvas; private boolean mIsDrawing; private  int x = 0; private span> int y = 0; private Path mPath; private Paint mPaint; public SinView(Context context) {super(context); initView();} publi c SinView(Context context, AttributeSet attrs) {super(context, attrs); initView ();} public SinView(Context context, AttributeSet attrs, int defStyle) {super(context, attrs, defStyle); initView();} private void initView() {mHolder = getHolder(); mHolder.addCallback(this); setFocusable(true); setFocusableInTouchMode(true); this.setKeepScreenOn(true); mPath = new span> Path(); mPaint = new Paint(Paint.ANTI_ALIAS_FLAG); mPaint.setColor(Color.RED); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeWidth(10); // Set the brush width mPaint.setStrokeCap(Paint.Cap.ROUND); // Set the corner to be rounded mPaint.setStrokeJoin(Paint.Join.ROUND);//The joint is rounded} @Override public void surfaceCreated (SurfaceHolder holder) {mIsDrawing = true; mPath.moveTo(0, 400); new Thread(this).start( );} @Override public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {} @Override public void surfaceDestroyed(SurfaceHolder holder) {mIsDrawing = false;} @Override public void run() {while (mIsDrawing) { draw(); x += 1; y = (int) (100 * Math.sin(x * 2 * Math.PI / 180) + 400); mPath.lineTo(x, y);}} private void draw() {try {mCanvas = mHolder.lockCanvas(); // SurfaceView background mCanvas.drawColor(Color.WHITE); mCanvas.drawPath(mPath , mPaint);} catch (Exception e) {} finally {if (mCanvas != null) mHolder.unlockCanvasAndPost(mCanvas);} }}
< span class="hljs-keyword">import android.support.v7.app.AppCompatActivity;import android.os.Bundle;public class SinViewAct extends< /span> AppCompatActivity { @Override protected  void onCreate(Bundle savedInstanceState) {super .onCreate(savedInstanceState); setContentView(new SinView(this)); }}< /pre> 
 
The final look: 
 
 
 
Drawing board
 h3> 
Record the path of the finger sliding through the Path object for drawing, and record the Path path in the onTouchEvent() of the SurfaceView. 
 
import android.content.Context;import android. graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import< /span> android.view.MotionEvent;import android.view.SurfaceHolder;import android.view.SurfaceView ;/** * MyApp * * @author Mr.Yang on 2016-04-22 22:02. * @version 1.0 * @desc */public< /span> class SimpleDraw extends SurfaceView implements SurfaceHolder.Callback, Runnable { //SurfaceHolder private SurfaceHolder mSurfaceHolder; // Canvas for drawing private Canvas mCanvas; // Child thread flag private  boolean mIsDrawing; private Path mPath; private  Paint mPaint; /** * Constructor* * @param context */ public SimpleDraw(Context context) {super(context); initView();} /** * Constructor* * @param context * @param attrs */ public SimpleDraw(Context context, AttributeSet attrs) {super(context, attrs); initView();} /** * Constructor* * @param context * @param attrs * @param defStyleAttr */ public SimpleDraw(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr); initView(); } /** * Initialization, call in the constructor */ private void initView() {mSurfaceHolder = getHolder( ); mSurfaceHolder.addCallback(this); setFocusable(true); setFocusableInTouchMode(true); this.setKeepScreenOn(true); //mSurfaceHolder.setFormat(PixelFormat.OPAQUE); mPath = new Path(); mPaint = new Paint(); mPaint.setColor(Color.BLUE); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeWidth(10.0f);} /** * SurfaceView creation* * @param holder */ @Override public void surfaceCreated(SurfaceHolder holder) { mIsDrawing = true; new Thread(this ).start();} /** * SurfaceView change* * @param holder * @param format * @param width * @param height */ @Override public void surfaceChanged(SurfaceHolder holder, int format, int width , int height) {} /** * SurfaceView destruction* * @param holder */ @Override public void surfaceDestroyed(SurfaceHolder holder) {mIsDrawing = false< /span>;} @Override public void run() {long start = System.currentTimeMillis(); while (mIsDrawing) {draw();} long end = System.currentTimeMillis(); / / 50-100 if (end-start <100) {try {Thread.sleep(100-(end-start));} catch (InterruptedException e) {e.printStackTrace();}}} /** * */ private void draw() {try {mCanvas = mSurfaceHolder.lockCanvas(); // draw something mCanvas.drawColor(Color.WHITE); mCanvas.drawPath(mPath, mPaint);} catch (Exception e) {} finally {if (mCanvas != null) {mSurfaceHolder. unlockCanvasAndPost(mCanvas);}}} @Override public boolean onTouchEvent(MotionEvent event) {int x = (int) event.getX(); int y = (int) event.getY(); switch (event.getAction()) {case MotionEvent.ACTION_DOWN: mPath.moveTo(x, y); break; case MotionEvent.ACTION_MOVE: mPath.lineTo(x, y); break; case MotionEvent.ACTION_UP: break;} return true; }}
Sometimes the drawing is not so frequent, so we can perform Sleep operation in the sub-thread, in order to save the system as much as possible resource. 
Determine the value of Sleep by judging the logical duration used by the draw method. This is a very general solution. The 100ms in the code is a rough empirical value, generally around 50ms~100ms
 p> 
import android.os.Bundle;import android.support. v7.app.AppCompatActivity;public class SimpleDrawAct extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState); setContentView(new SimpleDraw(this)); }}

By custom view Realizing the drawing board
It can also be realized by customizing View, the code is as follows
 import android.content.Context; import android.graphics.Bitmap; import android.graphics .Canvas; import android.graphics.Color; import android.graphics.Paint; import android.graphics.Path; import android.util.AttributeSet; import android.view.MotionEvent; import android.view.View; /** * MyApp * * @author Mr.Yang on 2016-04-23 10:32. * @version 1.0 * Analysis: * Place the drawing board where? * In View, we customize a View and complete the drawing in onDraw(). In addition, the View also has an onTouchEvent method, we can get the user's gesture operation! * What do I need to prepare? * A paintbrush (Paint), a canvas (Canvas), and a path (Path) to record the user's drawing route; * In addition, when drawing a line, each time it is from the point of the last drag time to the current drag time The point of occurrence! * 那么之前绘制的 就会丢失，为了保存之前绘制的内容， * 我们可以引入所谓的"双缓冲"技术： 其实就是每次不是直接绘制到Canvas上，而是先绘制到Bitmap上，等Bitmap上的绘制完了， 再一次性地绘制到View上！ * 
 * 具体的实现流程？ * 初始化画笔，设置颜色等等一些参数； * 在View的onMeasure()方法中创建一个View大小的Bitmap， * 同时创建一个Canvas；onTouchEvent中获得X,Y坐标，做绘制连线， * 最后invalidate()重绘，即调用 onDraw方法将bitmap的东东画到Canvas上！ */ public class SimpleDrawByCustomView extends View { // 定义画布 private Canvas mCanvas; // 定义画笔 private Paint mPaint; // 记录用户绘制的Path private Path mPath; //缓存绘制的内容 private Bitmap mBitmap; private int mLastX; private int mLastY; /** * @param context */ public S impleDrawByCustomView(Context context) { super(context); init(); } /** * @param context * @param attrs */ public SimpleDrawByCustomView(Context context, AttributeSet attrs) { super(context, attrs); init(); } /** * @param context * @param attrs * @param defStyleAttr */ public SimpleDrawByCustomView(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); init(); } /** * 初始化工作，在构造函数中调用 */ private void init() { mPaint = new Paint(); mPath = new Path(); mPaint.setColor(Color.BLUE); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); //结合处为圆角 mPaint.setStrokeCap(Paint.Cap.ROUND); // 设置转弯处为圆角 mPaint.setStrokeWidth(20); // 设置画笔宽度 } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); int width = getMeasuredWidth(); int height = getMeasuredHeight(); // 初始化bitmap,Canvas mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); mCanvas = new Canvas(mBitmap); } //重写该方法，在这里绘图 @Override protected void onDraw(Canvas canvas) { drawPath(); canvas.drawBitmap(mBitmap, 0, 0, null); } //绘制线条 private void drawPath() { mCanvas.drawPath(mPath, mPaint); }  @Override public boolean onTouchEvent(MotionEvent event) { int action = event.getAction(); int x = (int) event.getX(); int y = (int) event.getY(); switch (action) { case MotionEvent.ACTION_DOWN: mLastX = x; mLastY = y; mPath.moveTo(mLastX, mLastY); break; case MotionEvent.ACTION_MOVE: int dx = Math.abs(x - mLastX); int dy = Math.abs(y - mLastY); if (dx > 3 || dy > 3) mPath.lineTo(x, y); mLastX = x; mLastY = y; break; } invalidate(); return true; } } 

SKipped 47 frames! The application may be doing too much work on its main Thread

public class SurfaceViewTemplate extends SurfaceView implements SurfaceHolder.Callback ,Runnable {    .......}

/** * SurfaceView创建 * @param holder */    @Override    public void surfaceCreated(SurfaceHolder holder) {    }    /** * SurfaceView改变 * @param holder * @param format * @param width * @param height */    @Override    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {    }    /** * SurfaceView销毁 * @param holder */    @Override    public void surfaceDestroyed(SurfaceHolder holder) {    }

@Override    public void run() {    }

//SurfaceHolder    private SurfaceHolder mSurfaceHolder;    // 用于绘图的Canvas    private Canvas mCanvas;    // 子线程标志位    private boolean mIsDrawing;

mSurfaceHolder = getHolder(); mSurfaceHolder.addCallback(this);

import android.content.Context;import android.graphics.Canvas;import android.util.AttributeSet;import android.view.SurfaceHolder;import android.view.SurfaceView;/** * MyApp * * @author Mr.Yang on 2016-04-22 15:59. * @version 1.0 * @desc */public class SurfaceViewTemplate extends SurfaceView implements SurfaceHolder.Callback, Runnable {    //SurfaceHolder    private SurfaceHolder mSurfaceHolder;    // 用于绘图的Canvas    private Canvas mCanvas;    // 子线程标志位    private boolean mIsDrawing;    /** * 构造函数 * * @param context */    public SurfaceViewTemplate(Context context) {        super(context);        initView();    }    /** * 构造函数 * * @param context * @param attrs */    public SurfaceViewTemplate(Context co ntext, AttributeSet attrs) {        super(context, attrs);        initView();    }    /** * 构造函数 * * @param context * @param attrs * @param defStyleAttr */    public SurfaceViewTemplate(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        initView();    }    /** * 初始化，在构造函数中调用 */    private void initView() {        mSurfaceHolder = getHolder();        mSurfaceHolder.addCallback(this);        setFocusable(true);        setFocus ableInTouchMode(true);        this.setKeepScreenOn(true);        //mSurfaceHolder.setFormat(PixelFormat.OPAQUE);    }    /** * SurfaceView创建 * * @param holder */    @Override    public void surfaceCreated(SurfaceHolder holder) {        mIsDrawing = true;        new Thread(this).start();    }    /** * SurfaceView改变 * * @param holder * @param format * @param width * @param heigh t */    @Override    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {    }    /** * SurfaceView销毁 * * @param holder */    @Override    public void surfaceDestroyed(SurfaceHolder holder) {        mIsDrawing = false;    }    @Override    public void run() {        while (mIsDrawing) {            draw();        }    }    /** * */    private void draw() {        try {            mCanvas = mSurfaceHolder.lockCanvas();            // draw something        } catch (Exception e) {        } finally {            if (mCanvas != null) {                mSurfaceHolder.unlockCanvasAndPost(mCanvas);            }        }    }}

import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import android.view.SurfaceHolder;import android.view.SurfaceView;/** * MyApp * * @author Mr.Yang on 2016-04-22 22:17. * @version 1.0 * @desc */public class SinView extends SurfaceView< /span> implements SurfaceHolder.Callback, Runnable {    private SurfaceHolder mHolder;    private Canvas mCanvas;    private boolean mIsDrawing;    private int x = 0;    private int y = 0;    private Path mPath;    private Paint mPaint;    public SinView(Context context) {        super(context);        initView();    }    public  SinView(Context context, AttributeSet attrs) {        super(context, attrs);        initView();    }    public SinView(Context context, AttributeSet attrs, int defStyle) {        super(context, attrs, defStyle);        initView();    }    private void initView() {        mHolder = getHolder();        mHolder.addCallback(this);        setFocusable(true);        setFocusableInTouchMode(true);        this.setKeepScreenOn(true);        mPath = new Path();        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);        mPaint.setColor(Color.RED);        mPaint.setStyle(Paint.Style.STROKE);        mPaint.setStrokeWidth(10); // 设置画笔宽度        mPaint.setStrokeCap(Paint.Cap.ROUND); // 设置转弯处为圆角        mPaint.setStrokeJoin(Paint.Join.ROUND);//结合处为圆角    }    @Override    public void surfaceCreated(SurfaceHolder holder) {        mIsDrawing = true;        mPath.moveTo(0, 400);        new Thread(this).start();    }    @Override    public void surfaceChanged(SurfaceHolder holder,                               int format, int width, int height) {    }    @Override    public void surfaceDestroyed(SurfaceHolder holder) {        mIsDrawing = false;    }    @Override    public void run() {        while (mIsDrawing) {            draw();            x += 1;            y = (int) (100 * Math.sin(x * 2 * Math.PI / 180) + 400);            mPath.lineTo(x, y);        }    }    private void draw() {        try {            mCanvas = mHolder.lockCanvas();            // SurfaceView背景            mCanvas.drawColor(Color.WHITE);            mCanvas.drawPath(mPath, mPaint);        } catch (Exception e) {        } finally {            if (mCanvas != null)                mHolder.unlockCanvasAndPost(mCanvas);        }    }}

import android.support.v7.app.AppCompatActivity;import android.os.Bundle;public class SinViewAct extends AppCompatActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new SinView(this));    }}

import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import android.view.MotionEvent;import android.view.SurfaceHolder;import android.view.SurfaceView;/** * MyApp * * @author Mr.Yang on 2016-04-22 22:02. * @version 1.0 * @desc */public class SimpleDraw extends SurfaceView implements SurfaceHolder.Callback, Runnable {    //SurfaceHolder    private SurfaceHolder mSurfaceHolder;    // 用于绘图的Canvas    private Canvas mCanvas;    // 子线程标志位    private boolean mIsDrawing;    private Path mPath;    private Paint mPaint;    /** * 构造函数 * * @param context */    public SimpleDraw(Context context) {        super(context);        initView();    }    /** * 构造函数 * * @param context * @param attrs */    public SimpleDraw(Context context, AttributeSet attrs) {        super(context, attrs);        initView();    }    /** * 构造函数 * * @param context * @param attrs * @param defStyleAttr */    public SimpleDraw(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        initView();    }    /** * 初始化，在构造函数中调用 */    private void initView() {        mSurfaceHolder = getHolder();        mSurfaceHolder.addCallback(this);        setFocusable(true);        setFocusableInTouchMode(true);        this.setKeepScreenOn(true);        //mSurfaceHolder.setFormat(PixelFormat.OPAQUE);        mPath = new Path();        mPaint = new Paint();        mPaint.setColor(Color.BLUE);        mPaint.setStyle(Paint.Style.STROKE);        mPaint.setStrokeWidth(10.0f);    }    /** * SurfaceView创建 * * @param holder */    @Override    < span class="hljs-keyword">public void surfaceCreated(SurfaceHolder holder) {        mIsDrawing = true;        new Thread(this).start();    }    /** * SurfaceView改变 * * @param holder * @param format * @param width * @param height */    @Override    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {    }    /** * SurfaceView销毁 * * @param holder */    @Override    public void surfaceDestroyed(SurfaceHolder holder) {        mIsDrawing = false;    }    @Override    public void run() {        long start = System.currentTimeMillis();        while (mIsDrawing) {            draw();        }        long end = System.currentTimeMillis();        // 50 - 100        if (end - start < 100) {            try {                Thread.sleep(100 - (end - start));            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }    /** * */    private void draw() {        try {            mCanvas = mSurfaceHolder.lockCanvas();            // draw something            mCanvas.drawColor(Color.WHITE);            mCanvas.drawPath(mPath, mPaint);        } catch (Exception e) {        } finally {            if (mCanvas != null) {                mSurfaceHolder.unlockCanvasAndPost(mCanvas);            }        }    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int x = (int) event.getX();        int y = (int) event.getY();        switch (event.getAction()) {            case MotionEvent.ACTION_DOWN:                mPath.moveTo(x, y);                break;            case MotionEvent.ACTION_MOVE:                mPath.lineTo(x, y);                break;            case MotionEvent.ACTION_UP:                break;        }        return true;    }}

import android.os.Bundle;import android.support.v7.app.AppCompatActivity;public class SimpleDrawAct extends AppCompatActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new SimpleDraw(this));    }}

import android.content.Context; import android.graphics.Bitmap; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Paint; import android.graphics.Path; import android.util.AttributeSet; import android.view.MotionEvent; import android.view.View; /** * MyApp * * @author Mr.Yang on 2016-04-23 10:32. * @version 1.0 * 分析： * 画板放在哪里？ * View里，我们自定义一个View，在onDraw()里完成绘制，另外View还有个onTouchEvent的方法， 我们可以在获取用户的手势操作！ * 需要准备些什么？ * 一只画笔(Paint)，一块画布(Canvas)，一个路径(Path)记录用户绘制路线； * 另外划线的时候，每次都是从上次拖动时间的发生点到本次拖动时间的发生点！ * 那么之前绘制的 就会丢失，为了保存之前绘制的内容， * 我们可以引入所谓的"双缓冲"技术： 其实就是每次不是直接绘制到Canvas上，而是先绘制到Bitmap上，等Bitmap上的绘制完了， 再一次性地绘制到View上！ * 
 * 具体的实现流程？ * 初始化画笔，设置颜色等等一些参数； * 在View的onMeasure()方法中创建一个View大小的Bitmap， * 同时创建一个Canvas；onTouchEvent中获得X,Y坐标，做绘制连线， * 最后invalidate()重绘，即调用 onDraw方法将bitmap的东东画到Canvas上！ */ public class SimpleDrawByCustomView extends View { // 定义画布 private Canvas mCanvas; // 定义画笔 private Paint mPaint; // 记录用户绘制的Path private Path mPath; //缓存绘制的内容 private Bitmap mBitmap; private int mLastX; private int mLastY; /** * @param context */ public S impleDrawByCustomView(Context context) { super(context); init(); } /** * @param context * @param attrs */ public SimpleDrawByCustomView(Context context, AttributeSet attrs) { super(context, attrs); init(); } /** * @param context * @param attrs * @param defStyleAttr */ public SimpleDrawByCustomView(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); init(); } /** * 初始化工作，在构造函数中调用 */ private void init() { mPaint = new Paint(); mPath = new Path(); mPaint.setColor(Color.BLUE); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); //结合处为圆角 mPaint.setStrokeCap(Paint.Cap.ROUND); // 设置转弯处为圆角 mPaint.setStrokeWidth(20); // 设置画笔宽度 } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); int width = getMeasuredWidth(); int height = getMeasuredHeight(); // 初始化bitmap,Canvas mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); mCanvas = new Canvas(mBitmap); } //重写该方法，在这里绘图 @Override protected void onDraw(Canvas canvas) { drawPath(); canvas.drawBitmap(mBitmap, 0, 0, null); } //绘制线条 private void drawPath() { mCanvas.drawPath(mPath, mPaint); }  @Override public boolean onTouchEvent(MotionEvent event) { int action = event.getAction(); int x = (int) event.getX(); int y = (int) event.getY(); switch (action) { case MotionEvent.ACTION_DOWN: mLastX = x; mLastY = y; mPath.moveTo(mLastX, mLastY); break; case MotionEvent.ACTION_MOVE: int dx = Math.abs(x - mLastX); int dy = Math.abs(y - mLastY); if (dx > 3 || dy > 3) mPath.lineTo(x, y); mLastX = x; mLastY = y; break; } invalidate(); return true; } }