要強制重繪View，調用invalidate()

本例子會在onTouchEvent()之內處理ACTION_UP, ACTION_DOWN 和 ACTION_MOVE 事件. 當用戶觸摸(ACTION_MOVE)或移動(ACTION_DOWN), R.drawable.icon會顯示和跟隨移動. 這是通過event.getX()和event.getY()獲得事件位置, 然後通過調用invalidate()強制重繪View.



繼續使用上一篇文章"處理自定義視圖的事件(Event)"的程序代碼, 修改MyView.java

package com.AndroidView;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Toast;

public class MyView extends View {
 
 private Paint myPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
 private Context myContext;
 Bitmap myBitmap;
 
 private int locX, locY;
 private int offX, offY;
 private boolean showImage = false;

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

 public MyView(Context context, AttributeSet attrs) {
  super(context, attrs);
  // TODO Auto-generated constructor stub
  init(context);
 }

 public MyView(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  // TODO Auto-generated constructor stub
  init(context);
 }

 private void init(Context context){
  myContext = context;
  myBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.icon);
  offX = myBitmap.getWidth()/2;
  offY = myBitmap.getHeight()/2;
 }
 
 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  // TODO Auto-generated method stub
  //super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  
  setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), 
    MeasureSpec.getSize(heightMeasureSpec));
 }

 @Override
 protected void onDraw(Canvas canvas) {
  // TODO Auto-generated method stub
  //super.onDraw(canvas);
  
  if (showImage){
   canvas.drawColor(Color.BLACK);
   canvas.drawBitmap(myBitmap, locX-offX, locY-offY, null);
  }
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
  // TODO Auto-generated method stub
  
  int action = event.getAction();
  
  if ((action == MotionEvent.ACTION_DOWN)||
    (action == MotionEvent.ACTION_MOVE)){
   locX = (int)event.getX(); 
   locY = (int)event.getY();
   showImage = true;
   invalidate();
  }
  else if (action == MotionEvent.ACTION_UP){
   showImage = false;
   invalidate();
  }

  
  return true;
 }

}