Android：将画布绘制到视图中

Question

我试图绘制一个包含圆圈的画布到之前通过findViewById（）定义的视图

    View v = findViewById(R.id.view1);
    Bitmap b = Bitmap.createBitmap(v.getMeasuredWidth(), v.getMeasuredHeight(), Bitmap.Config.ARGB_8888);
    Canvas c = new Canvas(b);
    Paint paint = new Paint();
    paint.setStyle(Paint.Style.FILL);
    paint.setColor(Color.WHITE);
    c.drawCircle(v.getMeasuredWidth() / 2, v.getMeasuredHeight() / 2, v.getMeasuredWidth() / 4, paint);
    v.draw(c);

但是视图中没有绘制圆圈

我的整个活动：

package com.burrlieexample.test;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class InstructionsActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d("DEBUG", "InstruciotnActivity");
        setContentView(R.layout.activity_instructions);

        Button okay = (Button) findViewById(R.id.okButton);
        okay.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                finish();
            }
        });

    }

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        // TODO Auto-generated method stub
        super.onWindowFocusChanged(hasFocus);
        Log.d("DEBUG", "onFocusChanged");
//im doing this stuff in onWindowfocusChanged because getWidth returns 0 in onCreate
        View v = findViewById(R.id.view1);
        Log.d("DEBUG", "" + v.getMeasuredWidth() + ", " + v.getMeasuredHeight());
        Bitmap b = Bitmap.createBitmap(v.getMeasuredWidth(), v.getMeasuredHeight(), Bitmap.Config.ARGB_8888);
        Canvas c = new Canvas(b);
        Paint paint = new Paint();
        paint.setStyle(Paint.Style.FILL);
        paint.setColor(Color.WHITE);
        c.drawCircle(v.getMeasuredWidth() / 2, v.getMeasuredHeight() / 2, v.getMeasuredWidth() / 4, paint);
        v.draw(c);

    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
    }
}

并且View在我的布局xml文件中就像这样

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#D0F3FF"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.burrlieexample.test.InstructionsActivity" >

    <Button
        android:id="@+id/okButton"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="52dp"
        android:background="#00455F"
        android:hapticFeedbackEnabled="true"
        android:text="Ok!" />

    <View
        android:id="@+id/view1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/okButton"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true" />

</RelativeLayout>

因此，当Activity开始时，我看到的唯一内容是按钮和我的相对布局的背景

Answer 1

这样做很简单：当您的UI不需要在应用程序中进行动态更改时，绘制圆形以查看要查看的绘图是一个更好的选项。这可以通过扩展View类并定义onDraw（）回调方法来实现。在View组件的onDraw（）中，使用各种Canvas.draw ...（）方法（例如：canvas.drawCircle（x / 2，y / 2，radius，paint）），使用为您的所有绘图提供的Canvas; ）。 onDraw（）是最初绘制视图时调用的回调方法。

public class MainActivity extends Activity {
     /** Called when the activity is first created. */
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(new MyView(this));
     }

     public class MyView extends View {
         public MyView(Context context) {
              super(context);
              // TODO Auto-generated constructor stub
         }

         @Override
         protected void onDraw(Canvas canvas) {
            // TODO Auto-generated method stub
            super.onDraw(canvas);
            int x = getWidth();
            int y = getHeight();
            int radius;
            radius = 100;
            Paint paint = new Paint();
            paint.setStyle(Paint.Style.FILL);
            paint.setColor(Color.WHITE);
            canvas.drawPaint(paint);
            // Use Color.parseColor to define HTML colors
            paint.setColor(Color.parseColor("#CD5C5C"));
            canvas.drawCircle(x / 2, y / 2, radius, paint);
        }
     }
}

Answer 2

您将位图提供给垃圾收集器。你从来没有在视图上画任何东西。因此，视图中没有任何内容。