paint canvas android

问题

I'm developing in android, and I have to do a Paint for android.

I'm using the code below and, when I execute the code, the draw works, but, it seems that there are 2 surfaces to paint, and when you draw in one, the other one disappears.

I was looking for the exact error, but cannot find it.

Here is the code :

import java.util.Random; 

import android.annotation.SuppressLint;
import android.annotation.TargetApi;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
 import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends Activity {
  MySurfaceView mySurfaceView;
  Button Cuadrado;
  Button Circulo;
  Button Color;
  Button Linea;
  private boolean Bcuadrado,Bcirculo,Bcolor=false;
  private boolean Blinea=true;
    Canvas canvas = new Canvas();

  @TargetApi(11)
@SuppressLint("NewApi")
@Override
  public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       RelativeLayout mainLayout =(RelativeLayout)findViewById(R.id.main_layout_id );
       View view =getLayoutInflater().inflate(R.layout.itemlayout, mainLayout,false);
       mainLayout.addView(view);
       mySurfaceView = new MySurfaceView(this);
       Cuadrado=(Button)findViewById(R.id.button1);
       Circulo=(Button)findViewById(R.id.button2);
       Color=(Button)findViewById(R.id.button3 );
       Linea=(Button)findViewById(R.id.button4 );
       int w= view.getWidth();
       int h= view.getHeight();
       float x=view.getX();
       float y= view.getY();
       mySurfaceView.setY(100);
       mainLayout.addView(mySurfaceView);
       Cuadrado.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            if(Bcuadrado==false){
                Bcuadrado=true;
                Bcirculo=false;
                Bcolor=false;
                Blinea=false;
            }
        }
       });

       Circulo.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            if(!Bcirculo){
            Bcuadrado=false;
            Bcirculo=true;
            Bcolor=false;
            Blinea=false;
        }
    }
   });

       Color.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
        if(!Bcolor){
            Bcuadrado=false;
            Bcirculo=false;
            Bcolor=true;
            Blinea=false;
        }
    }
   });

       Linea.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
        if(!Blinea){
            Bcuadrado=false;
            Bcirculo=false;
            Bcolor=false;
            Blinea=true;
        }
    }
   });
}

  class MySurfaceView extends SurfaceView{
   Path path;
   SurfaceHolder surfaceHolder;
   volatile boolean running = false;        
   private Paint paint = new Paint();
   float x0=0;
    float x1=0;
    float y0=0;
    float y1=0;
    Random random = new Random();

   public MySurfaceView(Context context) {
    super(context);
    surfaceHolder = getHolder();
    paint.setStyle(Paint.Style.STROKE);
    paint.setStrokeWidth(3);
    paint.setColor(android.graphics.Color.WHITE);

   }

   @Override
   public boolean onTouchEvent(MotionEvent event) {


       if(Blinea){
            if(event.getAction() == MotionEvent.ACTION_DOWN){
             path = new Path();
             path.moveTo(event.getX(), event.getY());
            }else if(event.getAction() == MotionEvent.ACTION_MOVE){
             path.lineTo(event.getX(), event.getY());
            }else if(event.getAction() == MotionEvent.ACTION_UP){
             path.lineTo(event.getX(), event.getY());
            }
            if(path != null){
                 canvas = surfaceHolder.lockCanvas();
                 canvas.drawPath(path, paint);
                 surfaceHolder.unlockCanvasAndPost(canvas);
                }

       }else if(Bcuadrado){
           if(event.getAction()==MotionEvent.ACTION_DOWN){
               x0=event.getX();
               y0=event.getY();
           }
           else if(event.getAction()==MotionEvent.ACTION_UP){
               x1=event.getX();
               y1=event.getY();
               canvas = surfaceHolder.lockCanvas();
               canvas.drawRect(x0, y0, x1, y1, paint);
               surfaceHolder.unlockCanvasAndPost(canvas);
           }

       }else if(Bcirculo){
           if(event.getAction()==MotionEvent.ACTION_DOWN){
               x0=event.getX();
               y0=event.getY();
           }
           else if(event.getAction()==MotionEvent.ACTION_UP){
               x1=event.getX();
               canvas=surfaceHolder.lockCanvas();
               canvas.drawCircle(x0, y0,(x1-x0), paint);
               surfaceHolder.unlockCanvasAndPost(canvas);
           }
       }else if(Bcolor){
             int r = random.nextInt(255);
             int g = random.nextInt(255);
             int b = random.nextInt(255);
             canvas=surfaceHolder.lockCanvas();
             paint.setColor(0xff000000 + (r << 16) + (g << 8) + b);
             surfaceHolder.unlockCanvasAndPost(canvas);
       }


    return true; 
   }
  }
}

回答1:

it seems that there are 2 surfaces to paint, and when you draw in one, the other one disappears.

That is exactly how SurfaceView works - it's double buffered. You need to redraw whole frame each time.

回答2:

From Android's doc: SurfaceHolder

The content of the Surface is never preserved between unlockCanvas() and lockCanvas(), for this reason, every pixel within the Surface area must be written. The only exception to this rule is when a dirty rectangle is specified, in which case, non-dirty pixels will be preserved.

The canvas does not save what you previously wrote to it. Every time you call unlock(), you must redraw everything all over again.

来源：https://stackoverflow.com/questions/13895642/paint-canvas-android