画图

//mainactivity

package com.example.my;

import android.app.Activity;
import android.content.ContentResolver;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.PaintFlagsDrawFilter;
import android.net.Uri;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.SeekBar;
import android.widget.Toast;

import java.io.FileNotFoundException;

import static android.graphics.Bitmap.Config.ARGB_8888;

public class MainActivity extends Activity implements View.OnClickListener {

    private View red,green,blue;
    private SeekBar seek;
    private ImageView iv;
    private Bitmap baseBitmap,copyBitmap;
    private Canvas canvas;
    private Paint paint;
    private int startX,startY;
    private int width,height;
    private Button button;
    private float b;
    public void click(View view){
        paint.setColor(Color.WHITE);
        paint.setStrokeWidth(40);
    }
    public void clear(View view){
        canvas.drawColor(Color.WHITE);
        paint.setColor(Color.RED);
      /*baseBitmap=BitmapFactory.decodeResource(getResources(), R.drawable.bg);
      iv.setImageBitmap(baseBitmap);*/
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        red=(View)findViewById(R.id.red);
        green=(View)findViewById(R.id.green);
        blue=(View)findViewById(R.id.blue);
        iv=(ImageView) findViewById(R.id.iv);
        seek=(SeekBar) findViewById(R.id.seek);
        button = findViewById(R.id.tianjia);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(Intent.ACTION_PICK, android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
                startActivityForResult(intent, 1);
            }
        });

        iv.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                iv.getViewTreeObserver().removeGlobalOnLayoutListener(this);
                width =  iv.getWidth();
                height = iv.getHeight();
                copyBitmap=Bitmap.createBitmap(width,height, ARGB_8888);
                canvas=new Canvas(copyBitmap);
                canvas.setDrawFilter(new PaintFlagsDrawFilter(0, Paint.ANTI_ALIAS_FLAG|Paint.FILTER_BITMAP_FLAG));
            }
        });


        red.setOnClickListener(this);
        blue.setOnClickListener(this);
        green.setOnClickListener(this);

        //画板的颜色
        //canvas.drawColor(Color.BLACK);
        paint=new Paint();
        paint.setStrokeWidth((float)20);
        //画笔的颜色
        paint.setColor(Color.RED);
        paint.setAntiAlias(true);
        iv.setOnTouchListener(new View.OnTouchListener() {

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                // TODO Auto-generated method stub
                switch (event.getAction()) {
                    case MotionEvent.ACTION_DOWN://落下
                        startX=(int)event.getX();
                        startY=(int)event.getY();
                        break;
                    case MotionEvent.ACTION_MOVE://移动
                        int stopX=(int)event.getX();
                        int stopY=(int)event.getY();
                        canvas.drawLine(startX, startY, stopX, stopY, paint);
                        startX=(int)event.getX();
                        startY=(int)event.getY();
                        iv.setImageBitmap(copyBitmap);
                        break;
                    case MotionEvent.ACTION_UP://离开

                    default:
                        break;

                }
                return true;
            }
        });
        seek.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {
                // TODO Auto-generated method stub
                int progress=seekBar.getProgress();
                //使拖动条的取值为0f-2f，满足我们的取值要求
                float count=progress/50f;
                paint.setStrokeWidth(progress);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                // TODO Auto-generated method stub

            }
        });
    }


    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        switch (v.getId()) {
            case R.id.red:
                paint.setColor(Color.RED);
                break;
            case R.id.green:
                paint.setColor(Color.GREEN);
                break;
            case R.id.blue:
                paint.setColor(Color.BLUE);
                break;

            default:
                break;
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (resultCode == RESULT_OK) {
            Uri uri = data.getData();
            ContentResolver cr = this.getContentResolver();
            try {
                Bitmap bitmap = BitmapFactory.decodeStream(cr.openInputStream(uri));
                Bitmap newbitmap = zoomImg(bitmap,width,height);
                canvas.drawBitmap(newbitmap,0,b,paint);
            } catch (FileNotFoundException e) {
                Toast.makeText(MainActivity.this, "找不到图片", Toast.LENGTH_SHORT).show();
            }
        }
        super.onActivityResult(requestCode, resultCode, data);
    }
    public Bitmap zoomImg(Bitmap bm, int newWidth, int newHeight) {
        // 获得图片的宽高
        int width = bm.getWidth();
        int height = bm.getHeight();
        // 计算缩放比例
        float scaleWidth = ((float) newWidth) / width;
        float scaleHeight = ((float) newHeight) / height;
        float scale;
        if(scaleHeight >= scaleWidth){
            scale = scaleWidth;
        }else{
            scale = scaleHeight;
        }

        // 取得想要缩放的matrix参数
        Matrix matrix = new Matrix();
        matrix.postScale(scale, scale);
        float a = (float)height*scale;
        b = newHeight/2-a/2+10;
        // 得到新的图片
        Bitmap newbm = Bitmap.createBitmap(bm, 0, 0, width, height, matrix, true);
        return newbm;
    }
}

//activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <View
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="#ff0000"
            android:id="@+id/red"/>
        <View
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="#00ff00"
            android:id="@+id/green"/>
        <View
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="#0000ff"
            android:id="@+id/blue"/>
        <Button android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="橡皮擦"
            android:onClick="click"/>
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="清空"
            android:onClick="clear"/>
        <Button
            android:id="@+id/tianjia"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="+" />
    </LinearLayout>
    <SeekBar
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/seek"/>
    <ImageView
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:id="@+id/iv"
        />
</LinearLayout>