Android自訂UI執行個體

下面開始實現一個閃屏的效果，首先自訂類繼承於View，然後重寫onDraw方法，之前使用onDraw方法可以繪圖，但是只是繪製一次，那如何?迴圈呢。很容易想到使用invalidate()這個方法，因為使用這個方法它就會調用onDraw方法，這樣就形成了一個死迴圈，不斷重新整理繪製介面。當然還有一個postInvalidate().方法，它與invalidate()的區別在於它用於非UI的線程，invalidate()必須在UI線程使用。所以，代碼可以這樣寫：

import java.util.Random;import android.content.Context;import android.graphics.Canvas;import android.view.View;class RenderView extends View {Random rand = new Random();public RenderView(Context context) {super(context);}protected void onDraw(Canvas canvas) {canvas.drawRGB(rand.nextInt(256), rand.nextInt(256), rand.nextInt(256));invalidate();}}

下面對上一篇文章的例子增強一下：

import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.RectF;import android.os.Bundle;import android.view.View;public class Test extends GraphicsActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new SampleView(this));    }    private static class SampleView extends View {    //畫筆         private Paint[] mPaints;        //畫筆        private Paint mFramePaint;        //是否有圓心        private boolean[] mUseCenters;        //矩形        private RectF[] mOvals;        //矩形        private RectF mBigOval;        //開始弧度        private float mStart;        //增量弧度        private float mSweep;        //索引        private int mBigIndex;        //掃描增量        private static final float SWEEP_INC = 2;        private static final float START_INC = 15;        public SampleView(Context context) {            super(context);            mPaints = new Paint[4];            mUseCenters = new boolean[4];            mOvals = new RectF[4];            mPaints[0] = new Paint();            mPaints[0].setAntiAlias(true);            mPaints[0].setStyle(Paint.Style.FILL);            mPaints[0].setColor(0x88FF0000);            mUseCenters[0] = false;            mPaints[1] = new Paint(mPaints[0]);            mPaints[1].setColor(0x8800FF00);            mUseCenters[1] = true;            mPaints[2] = new Paint(mPaints[0]);            mPaints[2].setStyle(Paint.Style.STROKE);            mPaints[2].setStrokeWidth(4);            mPaints[2].setColor(0x880000FF);            mUseCenters[2] = false;            mPaints[3] = new Paint(mPaints[2]);            mPaints[3].setColor(0x88888888);            mUseCenters[3] = true;            mBigOval = new RectF(40, 10, 280, 250);            mOvals[0] = new RectF( 10, 270,  70, 330);            mOvals[1] = new RectF( 90, 270, 150, 330);            mOvals[2] = new RectF(170, 270, 230, 330);            mOvals[3] = new RectF(250, 270, 310, 330);            mFramePaint = new Paint();            mFramePaint.setAntiAlias(true);            mFramePaint.setStyle(Paint.Style.STROKE);            mFramePaint.setStrokeWidth(0);        }        /**         * @category          * @param canvas         * @param oval         * @param useCenter         * @param paint         */        private void drawArcs(Canvas canvas, RectF oval, boolean useCenter,                              Paint paint) {        //畫矩形            canvas.drawRect(oval, mFramePaint);            //畫弧形            canvas.drawArc(oval, mStart, mSweep, useCenter, paint);        }        @Override protected void onDraw(Canvas canvas) {        //設定背景色            canvas.drawColor(Color.WHITE);            //畫大矩形            drawArcs(canvas, mBigOval, mUseCenters[mBigIndex],                     mPaints[mBigIndex]);            //畫四個小矩形            for (int i = 0; i < 4; i++) {                drawArcs(canvas, mOvals[i], mUseCenters[i], mPaints[i]);            }            /**             * 計算弧度             */            mSweep += SWEEP_INC;            if (mSweep > 360) {                mSweep -= 360;                mStart += START_INC;                if (mStart >= 360) {                    mStart -= 360;                }                //變換                mBigIndex = (mBigIndex + 1) % mOvals.length;            }            //重新整理            invalidate();        }    }}

效果如下：

android中關於繪製平面2D圖形的類基本上都在android.graphics這個包中，比如常用的Paint、Path、Canvas、Rect、Bitmap、Color、Matrix、Point等等。這些雖然基礎，但卻是繪圖重要的類，所以先要掌握好。