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辨析API Demos中的LabelView

www.myexceptions.net  网友分享于:2013-04-10  浏览:7次
剖析API Demos中的LabelView

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

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


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