Android自定义View（二）

前言

魅族手机的闹钟应用中有个倒计时，这个控件还是蛮有趣的。左边是魅族闹钟，右边是我们最终实现的效果，虽然有些细节还需优化，不过基本上已经达到了想要的效果，我们先来就来看看如何实现吧。

分析

确定宽高

对一个Android自定义控件来说，一般都经过三个步骤

• onLayout()

• onMeasure()

• onDraw()

onLayout明确子控件在父控件中的位置（本控件不需要重写），onMeasure是确定控件的大小（宽、高），而onDraw是我们重点关注的方法，我们需要在这个方法中写入显示View的逻辑代码。
对于本控件，控件的高度 应该等于细线的高度(mLineHeight)加上数字的高度（mFontHeight），当然为了好看，中间需要设上一些边距(mPadding)，因此本控件的高度应该为 mFontHeight + mLineHeight + 10 + mPadding，测量代码如下

    private int measureHeight(int heightMeasureSpec) {
        int mode = MeasureSpec.getMode(heightMeasureSpec);
        int size = MeasureSpec.getSize(heightMeasureSpec);
        switch (mode) {
            case MeasureSpec.EXACTLY:
                return size ;
            case MeasureSpec.AT_MOST:
                return Math.min(size, mFontHeight + mLineHeight + 10 + mPadding) ;
        }
        return size ;
    }

同样地，控件的宽度其实就是0~1000的间隔，测量代码如下

    private int measureWidth(int widthMeasureSpec) {
        int mode = MeasureSpec.getMode(widthMeasureSpec);
        int size = MeasureSpec.getSize(widthMeasureSpec);
        switch (mode) {
            case MeasureSpec.EXACTLY:
                return size ;
            case MeasureSpec.AT_MOST:
                int result = getPaddingLeft() + mContentWidth + getPaddingRight() ;
                return Math.min(size, result) ;
        }
        return size ;
    }

画刻度尺

重点在于刻度尺的计算。思路是先draw上头的数字，然后再draw下边的线条，判断位置确定是否需要draw上头的数字即可。其实就是坐标的计算。代码如下

        int startX = mPadding;
        int stopX = mPadding;
        int stopY =mHeight - mPadding;
        for (int i = 0 ; i<=mContentWidth ; i += mFontWidth)
        if (i % (mFontWidth *10) == 0) {
            canvas.drawLine(startX + i, mFontHeight + mPadding + 5 , stopX + i, stopY, mTextPaint);
            canvas.drawText(i + "", startX + i, mFontHeight + mPadding, mTextPaint);
        } else if (i % mFontWidth == 0) {
            canvas.drawLine(startX + i, mFontHeight + mPadding + 10, stopX + i, stopY, mPaint);
        }

让View动起来

Android本身提供了移动View的API，因此让View动起来也是不难的。两种思路

• 监听Touch事件，当Touch坐标变化时，计算坐标位置，不断调用scrollTo(x,0)达到变换坐标的目的

• 监听Touch事件，记录上次横坐标和本次横坐标的差值，然后调用scrollBy(delta， 0) 即可移动

其实两种方法本质上都是一样的。ScrollBy其实也是调用了scrollTo方法。本文采用方法二。其代码如下

@Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mLastX = (int) event.getX();
                break;
            case MotionEvent.ACTION_MOVE:
                int x = (int) event.getX();
                int deltaX = x - mLastX;
                scrollBy(-deltaX, 0);
                mLastX = x;
                break;
        }
        return true;
    }

当然了，我们是不能让View无限移动的，因此需要重写scrollBy方法，限制View不能超过边界。
代码如下

    @Override
    public void scrollBy(int x, int y) {
        super.scrollBy(x, y);
        if (x < 0) { // drag to left
            if (getScrollX() < -getCenter() + mPadding) {
                scrollTo(-getCenter() + mPadding, 0);
            }
        } else
        if (x >0) { // drag to right
           if (mContentWidth - getScrollX() + x < getCenter()) {
                scrollTo(mContentWidth - getCenter() + mFontWidth, 0);
           }
        }
    }

当超过边界时，直接调用scrollTo，让View停留在特定的位置即可。需要注意的一点是，View往左滑动时，ScrollX的值是负的。

完整代码

package com.nancyyihao.demo;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.text.TextPaint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Scroller;

public class RulerView extends View {

    private static final String TAG = RulerView.class.getSimpleName() ;
    private TextPaint mTextPaint;
    private Paint mPaint ;

    private int mWidth;
    private int mHeight;
    private int mPadding = 10;
    private Scroller mScroller  ;
    private int mLastX;
    private int mContentWidth = 1000;
    private int mLineHeight = 50;
    private int mFontHeight;
    private int mFontWidth = 10;

    private onValueChangedListener mValueChangedListener;

    public interface onValueChangedListener {
        void onValueChanged(int newValue);
    }

    public RulerView(Context context) {
        super(context);
        init(context);
    }

    public RulerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    public RulerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init(context);
    }

    public void setOnValueChangedListener(onValueChangedListener listener) {
        this.mValueChangedListener = listener ;
    }

    private void init(Context context) {
        mTextPaint = new TextPaint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG | Paint.LINEAR_TEXT_FLAG);
        mTextPaint.setTextAlign(Paint.Align.CENTER);
        mTextPaint.setColor(Color.parseColor("#FF4081"));
        mTextPaint.setTextSize(30);
        mTextPaint.setStrokeWidth(2f);
        Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
        mFontHeight = Math.round(Math.abs(fontMetrics.top) + Math.abs(fontMetrics.bottom)) ;

        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
        mPaint.setColor(Color.DKGRAY);
        mPaint.setStrokeWidth(2f);
        mPaint.setTextSize(30);
        mPaint.setTextAlign(Paint.Align.CENTER);

        //mScroller = new Scroller(context) ;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        mWidth = measureWidth(widthMeasureSpec);
        mHeight = measureHeight(heightMeasureSpec);
        setMeasuredDimension(mWidth , mHeight );
    }

    private int measureWidth(int widthMeasureSpec) {
        int mode = MeasureSpec.getMode(widthMeasureSpec);
        int size = MeasureSpec.getSize(widthMeasureSpec);
        switch (mode) {
            case MeasureSpec.EXACTLY:
                return size ;
            case MeasureSpec.AT_MOST:
                int result = getPaddingLeft() + mContentWidth + getPaddingRight() ;
                return Math.min(size, result) ;
        }
        return size ;
    }

    private int measureHeight(int heightMeasureSpec) {
        int mode = MeasureSpec.getMode(heightMeasureSpec);
        int size = MeasureSpec.getSize(heightMeasureSpec);
        switch (mode) {
            case MeasureSpec.EXACTLY:
                return size ;
            case MeasureSpec.AT_MOST:
                return Math.min(size, mFontHeight + mLineHeight + 10 + mPadding) ;
        }
        return size ;
    }

//    private void smoothScrollTo(int destX, int destY) {
//        int scrollX = getScrollX() ;
//        int delta = destX - scrollX ;
//        mScroller.startScroll(scrollX, 0, delta, 0 , 1000);
//        invalidate();
//    }
//
//    @Override
//    public void computeScroll() {
//        super.computeScroll();
//        if (mScroller.computeScrollOffset()) {
//            smoothScrollTo(mScroller.getCurrX(), mScroller.getCurrY());
//            postInvalidate();
////            ((View) getParent()).scrollTo(mScroller.getCurrX(),mScroller.getCurrY());
////            invalidate();
//        }
//    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        int startX = mPadding;
        int stopX = mPadding;
        int stopY =mHeight - mPadding;
        for (int i = 0 ; i<=mContentWidth ; i += mFontWidth)
        if (i % (mFontWidth *10) == 0) {
            canvas.drawLine(startX + i, mFontHeight + mPadding + 5 , stopX + i, stopY, mTextPaint);
            canvas.drawText(i + "", startX + i, mFontHeight + mPadding, mTextPaint);
        } else if (i % mFontWidth == 0) {
            canvas.drawLine(startX + i, mFontHeight + mPadding + 10, stopX + i, stopY, mPaint);
        }
    }


    private int calcValue() {
        return  ( getCenter() + getScrollX() - mPadding) ; //minus startX
    }

    private int getCenter() {
        return (getRight() - getLeft()) / 2 ;
    }

    @Override
    public void scrollBy(int x, int y) {
        super.scrollBy(x, y);
        if (x < 0) { // drag to left
            if (getScrollX() < -getCenter() + mPadding) {
                scrollTo(-getCenter() + mPadding, 0);
            }
        } else
        if (x >0) { // drag to right
           if (mContentWidth - getScrollX() + x < getCenter()) {
                scrollTo(mContentWidth - getCenter() + mFontWidth, 0);
           }
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mLastX = (int) event.getX();
                break;
            case MotionEvent.ACTION_MOVE:
                int x = (int) event.getX();
                int deltaX = x - mLastX;
                scrollBy(-deltaX, 0);
                mLastX = x;
                if (mValueChangedListener != null)
                    mValueChangedListener.onValueChanged(calcValue());
                break;
        }
        return true;
    }
}

总结

整体上还是比较粗糙，原形虽然有了，但是还需要优化。

参考

【Android】自定义View —— 滑动的次数选择器
android 滚轮刻度尺的实现
Android View自定义专题二(View滑动的实现)