关于Android画图程序中的Canvas类使用的疑问

Question

Canvas网上很多文章将它解释成画布,但是在使用
Canvas canvas=new 
Canvas(bitmap);后应该是设置画布内容为该bitmap,然后再使用,canvas.drawLine(startx,starty,stopx,stopy,paint);在上面作画,程序直接会报错关闭,想问这是为什么,只有canvas.drawBitmap(bitmap,matrix,null);后再使用drawLine才能在bitmap上面画图.

Answer 1

package com.example.drawdemo011;import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class Draw extends View {

int startX=0,startY=0,stopX=0,stopY=0;
Path path=new Path();
Paint paint=new Paint();
Matrix matrix=new Matrix();
private float mX=0,mY=0;

Bitmap cacheBitmap=null;
Bitmap bitmap=null;
Canvas cacheCanvas=null; public Draw(Context context, AttributeSet attrs) {
super(context, attrs);
paint.setColor(Color.RED);
paint.setStyle(Paint.Style.STROKE);
bitmap=BitmapFactory.decodeResource(getResources(), R.drawable.koala);
cacheCanvas=new Canvas(bitmap); }
public void onDraw(Canvas canvas){ Paint mpaint=new Paint();
canvas.drawBitmap(bitmap, matrix, null);
canvas.drawPath(path, paint);
}
public boolean onTouchEvent(MotionEvent event){
float x=event.getX(),y=event.getY();
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:
touch_down(x,y);
break; case MotionEvent.ACTION_MOVE:
touch_move(x,y);
break;
case MotionEvent.ACTION_UP:
touch_up(x,y);
break;
}

return true;

}private void touch_down(float x, float y) {
path.moveTo(x,y);
mX=x;mY=y;
this.postInvalidate();
}private void touch_move(float x, float y) {
int distx,disty;
distx=(int)Math.abs(mX-x);
disty=(int)Math.abs(mY-y); path.quadTo(mX,mY,x,y);
mX=x;mY=y;
this.postInvalidate();
}
private void touch_up(float x, float y) { cacheCanvas.drawPath(path,paint);
path.reset();
this.postInvalidate();
}}
这个代码是改了好多遍之后的,还是卡在设置画布为bitmap那,不知道为什么就是过不去.
如果新建立bitmap=Bitmap.created(w,h,Config_ARGB_8888);
cacheCanvas=new Canvas();
cacheCanvas.setIamge(bitmap);
这样就能运行了.....

Answer 2

能告诉我是哪里错了,为什么不能把cacheCanvas= new Canvas(bitmap); ?

Answer 3

还是卡在设置画布为bitmap那,不知道为什么就是过不去.
你说的卡在那里是什么意思？一直在那里运行么

Answer 4

不是,是一到那就直接程序报错强行关闭了,一运行就报错.这段代码改成其他的以后就能顺利运行也可以作画,但是我就是想要在引入的bitmap上面作画

Answer 5

06-21 09:56:39.465: D/dalvikvm(1329): GC_EXTERNAL_ALLOC freed 45K, 53% free 2543K/5379K, external 1625K/2137K, paused 85ms
06-21 09:56:39.965: D/AndroidRuntime(1329): Shutting down VM
06-21 09:56:39.975: W/dalvikvm(1329): threadid=1: thread exiting with uncaught exception (group=0x40015560)
06-21 09:56:39.995: E/AndroidRuntime(1329): FATAL EXCEPTION: main
06-21 09:56:39.995: E/AndroidRuntime(1329): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.drawdemo011/com.example.drawdemo011.MainActivity}: android.view.InflateException: Binary XML file line #7: Error inflating class com.example.drawdemo011.Draw
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.os.Handler.dispatchMessage(Handler.java:99)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.os.Looper.loop(Looper.java:123)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread.main(ActivityThread.java:3683)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at java.lang.reflect.Method.invokeNative(Native Method)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at java.lang.reflect.Method.invoke(Method.java:507)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at dalvik.system.NativeStart.main(Native Method)
06-21 09:56:39.995: E/AndroidRuntime(1329): Caused by: android.view.InflateException: Binary XML file line #7: Error inflating class com.example.drawdemo011.Draw
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.createView(LayoutInflater.java:518)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:570)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.rInflate(LayoutInflater.java:623)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.inflate(LayoutInflater.java:408)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.inflate(LayoutInflater.java:320)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.inflate(LayoutInflater.java:276)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:207)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.Activity.setContentView(Activity.java:1657)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at com.example.drawdemo011.MainActivity.onCreate(MainActivity.java:21)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-21 09:56:39.995: E/AndroidRuntime(1329):  ... 11 more
06-21 09:56:39.995: E/AndroidRuntime(1329): Caused by: java.lang.reflect.InvocationTargetException
06-21 09:56:39.995: E/AndroidRuntime(1329):  at java.lang.reflect.Constructor.constructNative(Native Method)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at java.lang.reflect.Constructor.newInstance(Constructor.java:415)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.view.LayoutInflater.createView(LayoutInflater.java:505)
06-21 09:56:39.995: E/AndroidRuntime(1329):  ... 21 more
06-21 09:56:39.995: E/AndroidRuntime(1329): Caused by: java.lang.IllegalStateException: Immutable bitmap passed to Canvas constructor
06-21 09:56:39.995: E/AndroidRuntime(1329):  at android.graphics.Canvas.<init>(Canvas.java:83)
06-21 09:56:39.995: E/AndroidRuntime(1329):  at com.example.drawdemo011.Draw.<init>(Draw.java:36)
06-21 09:56:39.995: E/AndroidRuntime(1329):  ... 24 more

Answer 6

是不是因为xml出错而导致的呢？

Answer 7

xml<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
   tools:context=".MainActivity" ><com.example.drawdemo011.Draw
    android:id="@+id/ondraw"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
/>
</RelativeLayout>应该没问题的吧...