创建一个清除按钮以清除Android画布

Question

I can't get my clear button to work in my program. My circle button works which displays random circles but my clear button does nothing. I'm lost. I know i'm supposed to use drawColor.Color.TRANSPARENT or the mode.clear but nothing is working.

package com.example.randomcircles;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.*;
import android.os.Bundle;
import android.view.View;
import android.widget.*;
import android.content.*;

public class DisplayRandomCircles extends Activity
{
FrameLayout f1, f2;
@Override
public void onCreate(Bundle b)
{
    super.onCreate(b);
    setContentView(R.layout.activity_display_random_circles);
    Button btn1 = (Button) findViewById(R.id.btn1);
    Button btn2 = (Button) findViewById(R.id.btn2);
    f1 = (FrameLayout) findViewById(R.id.frame);


}
@SuppressLint("WrongCall")
public void doit(View v)
{
    int rand = (int) (Math.random() * 60);
    DrawCircle c = new DrawCircle(getApplicationContext());
    Canvas d = new Canvas();
    Paint p = new Paint();
    switch (v.getId())
    {
        case (R.id.btn1):
            f1.addView(c);
            break;

        case (R.id.btn2):
            d.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
            break;
    }
}
}

package com.example.randomcircles;
import java.util.Random;
import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.*;
import android.view.*;

public class DrawCircle extends View
{
Random random = new Random();
int red = (int) (Math.random() * 255);
int green = (int) (Math.random() * 255);
int blue = (int) (Math.random() * 255);
int randX = random.nextInt(700);
int randY = random.nextInt(1000);
int randR = random.nextInt(200);
int color = Color.rgb(red, green, blue);

public DrawCircle(Context con)
{
    super(con);
}
@SuppressLint("DrawAllocation")
@Override
public void onDraw(Canvas c)
{
    super.onDraw(c);
    Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);
    p.setAntiAlias(true);
    p.setStyle(Paint.Style.STROKE);
    p.setStrokeWidth(100);
    p.setColor(color);
    p.setStyle(Paint.Style.FILL);
    c.drawCircle(randX, randY, randR, p);

}

}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<FrameLayout
    android:id="@+id/frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_weight=".75"
    android:orientation="vertical" >


</FrameLayout>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_weight=".25"
    android:gravity="bottom|center"
    android:orientation="horizontal" >

    <Button
        android:id="@+id/btn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="start|bottom"
        android:layout_weight=".50"
        android:onClick="doit"
        android:text="@string/Circle" />

    <Button
        android:id="@+id/btn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight=".50"
        android:layout_gravity="end|bottom"
        android:onClick="doit"
        android:text="@string/Clear" />
</LinearLayout>

</LinearLayout>

Answer 1

You are creating a new instance of DrawCircle every time doIt is called.

The old instance is still there. So you need to either remove the old view from f1, or get the exact same instance of DrawCircle you created before, set the new color, and then call invalidate() on the DrawCircle to force the redraw.