如何使android画布每5秒重新绘制一次

Question

我正在制作一个需要动态动画的应用。 （玩家动作）我正在使用Canvas 对象来执行此操作。我的第一个问题是“Canvas 真的是处理这些动画的最佳方式吗？”， 我的第二个问题是“如何将玩家重新绘制到Canvas？”这是我的代码：

theGame.java：

package birdprograms.freezetag;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.view.View;

public class theGame extends Activity {
    players[] arr = {
            new player(),
            new player(),
            new player(),
            new player()
    };
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new myView(this));
    }
    public class myView extends View {
        Paint paint = new Paint();
        public myView(Context context) {
            super(context);
            paint.setColor(Color.YELLOW);
        }
        @Override
        public void onDraw(final Canvas canvas) {
            arr[0].update(true, true);
            arr[0].draw(canvas, paint);
        }
    }
}

player.java

package birdprograms.freezetag;

import android.graphics.*;

public class player {
    int y = 0;
    int x = 0;
    int vy = 5;
    int vx = 5;
    int height = y + 15;
    int width = x + 15;
    public void draw(Canvas canvas, Paint paint){
        canvas.drawRect(x,y,width,height,paint);
    }
    public void update(boolean left, boolean top){
        if(left){x += vx; width = x + 15;}
        else{x -= vx; width = x + 15;}
        if(top){y += vy; height = y + 15;}
        else{y -= vy; height = y + 15;}
    }
}

Answer 1

您可以通过制作计时器并设置时间表固定费率来做到这一点。在视图构造函数中调用 init。

private void init(){
    Timer timer = new Timer();
    timer.scheduleAtFixedRate(new TimerTask(){
        public void run() {
             postInvalidate();
        }
    }, 0, 5 * 1000L);//5 seconds
}

Answer 2

您无法真正控制何时调用 onDraw：当视图无效时，将在将来的某个时间调用 onDraw。

您的代码中有一个根本性的设计缺陷：在执行onDraw 期间玩家的位置被修改：您将无法控制它。

每 5 秒移动一次玩家：您可以使用 Handler.postDelayed 每 5 秒重新发布相同的 Runnable。 Runnable 将更新玩家位置，然后使 View 无效。

这里有一些代码来说明这个想法

（免责声明：这是伪代码，它只关心索引为 0 的玩家，还有更多工作要做才能移动所有玩家，...）

public class myView extends View {
    Paint paint = new Paint();
    public myView(Context context) {
        super(context);
        paint.setColor(Color.YELLOW);
        movePlayer0Runnable.run(); //this is the initial call to draw player at index 0
    }

    @Override
    public void onDraw(final Canvas canvas) {
        super.onDraw(canvas);  //IMPORTANT to draw the background
        arr[0].draw(canvas, paint);
    }

    Handler handler = new Handler(Looper.getMainLooper());
    Runnable movePlayer0Runnable = new Runnable(){
        public void run(){
            arr[0].update(true, true);
            invalidate(); //will trigger the onDraw
            handler.postDelayed(this,5000); //in 5 sec player0 will move again
        }
    }   
    ... 
}

Answer 3

在How to animate a path on canvas - android 中所示的onDraw 方法内部调用postInvalidateDelayed(5000) 比生成计时器或可运行对象的现有答案要简单一些。和他们一样，它不是高分辨率计时器。

这是一个最小的完整示例：

布局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:keepScreenOn="true"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
>
    <view
        class="com.example.foo.FooActivity$Drawer"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
    />
</LinearLayout>

活动：

package com.example.foo;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.os.Bundle;
import android.util.AttributeSet;
import android.view.View;

import java.util.Random;

public class FooActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_foo);
    }

    public static class Drawer extends View {
        private static final int delayMS = 5000;
        private static Random rnd;
        private static Paint paint;
        private static int viewWidth;
        private static int viewHeight;

        public Drawer(Context con, AttributeSet attr) {
            super(con, attr);
            rnd = new Random();
            paint = new Paint(Paint.ANTI_ALIAS_FLAG);
            paint.setStyle(Paint.Style.FILL);
        }

        // https://stackoverflow.com/a/9718512/6243352
        @Override
        protected void onSizeChanged(int xNew, int yNew, int xOld, int yOld) {
            super.onSizeChanged(xNew, yNew, xOld, yOld);
            viewWidth = xNew;
            viewHeight = yNew;
        }

        @Override
        protected void onDraw(final Canvas c) {
            super.onDraw(c);

            // https://stackoverflow.com/a/18617993/6243352
            postInvalidateDelayed(delayMS);

            int color = rnd.nextInt(256);
            paint.setARGB(255, color, color, color);
            c.drawCircle(
                viewWidth / 2,
                viewHeight / 2,
                viewWidth / 4,
                paint
            );
        }
    }
}