Android-6步教会你自定义View，自定义View就是这么简单

如果你打算完全定制一个View，那么你需要实现View类（所有的Android View都实现于这个类），并且实现确定尺寸的onMeasure(…))方法和确认绘图的onDraw(…))方法。
自定义View一共分为6步
第一步
public class SmileyView extends View { private Paint mCirclePaint; private Paint mEyeAndMouthPaint; private float mCenterX; private float mCenterY; private float mRadius; private RectF mArcBounds = new RectF(); public SmileyView(Context context) { this(context, null); } public SmileyView(Context context, AttributeSet attrs) { this(context, attrs, 0); } public SmileyView(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); initPaints(); } private void initPaints() {/* ... /} @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {/ ... /} @Override protected void onDraw(Canvas canvas) {/ ... */} }
2.实现画笔paint类

本文一共两只画笔
private void initPaints() {
mCirclePaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mCirclePaint.setStyle(Paint.Style.FILL);
mCirclePaint.setColor(Color.YELLOW);
mEyeAndMouthPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mEyeAndMouthPaint.setStyle(Paint.Style.STROKE);
mEyeAndMouthPaint.setStrokeWidth(16 * getResources().getDisplayMetrics().density);
mEyeAndMouthPaint.setStrokeCap(Paint.Cap.ROUND);
mEyeAndMouthPaint.setColor(Color.BLACK);
}
3.覆写onMeasure(…)方法

实现这个方法告诉了母容器如何放弃自定义View，可以通过提供的measureSpecs来决定你的View的高和宽，以下是一个正方形，确认它的宽和高是一样的。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int w = MeasureSpec.getSize(widthMeasureSpec);
int h = MeasureSpec.getSize(heightMeasureSpec);

int size = Math.min(w, h);
setMeasuredDimension(size, size);

}
注意：
这个方法需要至少保证一个setMeasuredDimension(..)调用，否则会报IllegalStateException错误。
4.实现onSizeChanged(…)方法

这个方法是你获取View现在的宽和高. 这里我们计算的是中心和半径。
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
mCenterX = w / 2f;
mCenterY = h / 2f;
mRadius = Math.min(w, h) / 2f;
}
5.实现onDraw(…)方法
这个方法提供了如何绘制view，它提供的Canvas类可以进行绘制。
@Override
protected void onDraw(Canvas canvas) {
// draw face
canvas.drawCircle(mCenterX, mCenterY, mRadius, mCirclePaint);
// draw eyes
float eyeRadius = mRadius / 5f;
float eyeOffsetX = mRadius / 3f;
float eyeOffsetY = mRadius / 3f; canvas.drawCircle(mCenterX - eyeOffsetX, mCenterY - eyeOffsetY, eyeRadius, mEyeAndMouthPaint); canvas.drawCircle(mCenterX + eyeOffsetX, mCenterY - eyeOffsetY, eyeRadius, mEyeAndMouthPaint);
// draw mouth
float mouthInset = mRadius /3f;
mArcBounds.set(mouthInset, mouthInset, mRadius * 2 - mouthInset, mRadius * 2 - mouthInset);
canvas.drawArc(mArcBounds, 45f, 90f, false, mEyeAndMouthPaint);
}
6.添加你的View

到此就结束了，自定义View没你想的那么难