剖析API Demos中的LabelView

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对于谷歌给我们提供的ApiDemo里有很多有学习参考价值的实例，今天来学习理解其中的一个实例类LabelView，此类

继承View，并对View中的onMeasure()，Ondraw()方法进行了重写，其中涉及到setMeasuredDimension() , MeasureSpec,Canvas,Paint，以及自定义

属性的一些应用。

像完全自定义控件(也就是继承View自定义控件)，一般会想到覆盖onMeasure(),Ondraw(),

默认onMeasure()会总是设置一个100*100尺寸

关于自定义属性可以参考 Android开发之自定义属性(Define Custom Attributes)

下面是主要实现代码

package com.example.labelview;// Need the following import to get access to the app resources, since this// class is in a sub-package.import android.content.Context;import android.content.res.TypedArray;import android.graphics.Canvas;import android.graphics.Paint;import android.util.AttributeSet;import android.view.View;/** * Example of how to write a custom subclass of View. LabelView * is used to draw simple text views. Note that it does not handle * styled text or right-to-left writing systems. * */public class LabelView extends View {    private Paint mTextPaint;    private String mText;    private int mAscent;        /**     * Constructor.  This version is only needed if you will be instantiating     * the object manually (not from a layout XML file).     * @param context     */    public LabelView(Context context) {        super(context);        initLabelView();  // 初始化    }    /**     * Construct object, initializing with any attributes we understand from a     * layout file. These attributes are defined in     * SDK/assets/res/any/classes.xml.     *      * @see android.view.View#View(android.content.Context, android.util.AttributeSet)     */    public LabelView(Context context, AttributeSet attrs) {        super(context, attrs);        initLabelView();        // 得到TypedArray 后面会利用它来获取自定义属性的值        TypedArray a = context.obtainStyledAttributes(attrs,                R.styleable.LabelView);        // 获取自定义属性text的值        CharSequence s = a.getString(R.styleable.LabelView_text);        if (s != null) {            setText(s.toString());        }        // Retrieve the color(s) to be used for this view and apply them.        // Note, if you only care about supporting a single color, that you        // can instead call a.getColor() and pass that to setTextColor().        // 获取自定义属性textColor的值，并设置文本相应的颜色值        setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));        int textSize = a.getDimensionPixelOffset(R.styleable.LabelView_textSize, 0);        if (textSize > 0) {            setTextSize(textSize);        }        // 注意这里记得要回收 TypedArray        a.recycle();    }    // 初始化 paint，并对其设置相应的属性值    private final void initLabelView() {        mTextPaint = new Paint();        mTextPaint.setAntiAlias(true);        mTextPaint.setTextSize(16);        mTextPaint.setColor(0xFF000000);        setPadding(10, 10, 10, 10);    }    /**     * Sets the text to display in this label     * @param text The text to display. This will be drawn as one line.     */    public void setText(String text) {        mText = text;        requestLayout();        invalidate();    }    /**     * 设置文本大小     * Sets the text size for this label     * @param size Font size     */    public void setTextSize(int size) {        mTextPaint.setTextSize(size);        // view 在layout上发生的改变(大小，位置)，遂调用此方法        requestLayout();        // 使整个View无效，如果该View可见，那么将会系统调用onDraw(...)方法        invalidate();    }    /**     * 设置文本颜色     * Sets the text color for this label.     * @param color ARGB value for the text     */    public void setTextColor(int color) {        mTextPaint.setColor(color);        invalidate();    }    /**     * 测量View和它的内容，并决定测量宽度和测量高度      * 这个方法被 measure(int, int)所调用     */    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    // 覆盖onMeasure时必须调用此方法，否则会抛出measurement 运行时异常        setMeasuredDimension(measureWidth(widthMeasureSpec),                measureHeight(heightMeasureSpec));    }    /**     * 决定这个view的宽度     * @param measureSpec A measureSpec packed into an int     * @return The width of the view, honoring constraints from measureSpec     */    private int measureWidth(int measureSpec) {        int result = 0;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        // 如果view的宽度是确定值，那么直接获取此确定值        if (specMode == MeasureSpec.EXACTLY) {            // We were told how big to be            result = specSize;        } else { // 如果宽度不是确定值，就需要计算下            // 测量text的宽度            result = (int) mTextPaint.measureText(mText) + getPaddingLeft()                    + getPaddingRight();            if (specMode == MeasureSpec.AT_MOST) {                // Respect AT_MOST value if that was what is called for by measureSpec                result = Math.min(result, specSize);            }        }        return result;    }    /**     * 决定这个view的高度     * 注意： 字体的高度＝上坡度＋下坡度＋行间距     * ascent和top都是负数     * @param measureSpec A measureSpec packed into an int     * @return The height of the view, honoring constraints from measureSpec     */    private int measureHeight(int measureSpec) {        int result = 0;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        mAscent = (int) mTextPaint.ascent();        if (specMode == MeasureSpec.EXACTLY) {            // We were told how big to be            result = specSize;        } else {            // Measure the text (beware: ascent is a negative number)            result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()                    + getPaddingBottom();             if (specMode == MeasureSpec.AT_MOST) {                // Respect AT_MOST value if that was what is called for by measureSpec                result = Math.min(result, specSize);            }        }        return result;    }    /**     * Render the text     * 绘制相应的界面，这里是drawText     * @see android.view.View#onDraw(android.graphics.Canvas)     */    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);    }}

上面代码里涉及到MeasureSpec对象，该对象包含了Measure's Mode和Size两个属性:

关于谷歌怎么把两个属性包装在一个int里，怎么在一个int里解包获取Mode和size属性，请看MeasureSpec源码

这样做可以节约空间，可以减少更多的对象的创建。

public static class MeasureSpec {    private static final int MODE_SHIFT = 30;    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;    public static final int UNSPECIFIED = 0 << MODE_SHIFT;    public static final int EXACTLY     = 1 << MODE_SHIFT;    public static final int AT_MOST     = 2 << MODE_SHIFT;    public static int makeMeasureSpec(int size, int mode) {        return size + mode;    }    public static int getMode(int measureSpec) {        return (measureSpec & MODE_MASK);    }    public static int getSize(int measureSpec) {        return (measureSpec & ~MODE_MASK);    }  }

onMeasure(int widthMeasureSpec, int heightMeasureSpec) 方法，系统在绘制对象时，首先得确定对象在屏幕上占用多大的范围，因此在这个方法中，必须得确定好控件的尺寸然后通过一个特定的函数接口（setMeasuredDimension(width, height)）去通知系统有关该控件的尺寸信息。系统传递进来的两个参数是一个约束条件，控件到底占据多大的尺寸由这两个参数决定，每一个参数其实一个MeasureSpec对象，该对象包含了Measure's Mode和Size两个属性:

Mode

UNSPECIFIED 系统对对象的size没进行约束，可以任意设置

EXACTLY 系统对对象的size已经确定，只能为MeasureSpec对象中指定的size

AT_MOST 系统对对象的最大size进行了约束，即该对象的size不能超过MeasureSpec对象中指定的size

参考资料

android中onMeasure初看,深入理解布局之一!

http://zhidao.baidu.com/question/525618489.html

Custom Components

点击下载源码