创建指南针View的例子 android canvas compass

在接下来的例子里，你将通过扩展View类创建一个指南针View。它使用传统的指南针上升箭头来指示方向。当完成时，应该和图4-3看起来一样。

 

指南针是一个UI控件的例子，它需要完全不同的视觉显示，不同于SDK工具箱中的TextView和Button，让我们从无到有使它成为一个出色的控件。

 

在第10章，你将使用这个指南针View和设备内建的重力加速计来显示用户当前的方向。在11章中，你将学习更高级的Canvas绘制技巧来戏剧性地改进它的外观。

图4-3

 

1. 创建一个新的指南针工程，包含指南针View和拥有它的Activity。现在创建CompassView类来扩展View。创建构造函数来运行View可以在代码中实例化，或者通过资源layout的膨胀。添加一个新的initCompassView方法来初始化控件，并在每个构造函数中调用它。

package com.paad.compass;import android.content.Context;import android.graphics.*;import android.graphics.drawable.*;import android.view.*;import android.util.AttributeSet;import android.content.res.Resources;public class CompassView extends View {public CompassView(Context context) {super(context);initCompassView();}public CompassView(Context context, AttributeSet attrs) {super(context, attrs);initCompassView();}public CompassView(Context context, AttributeSet ats, int defaultStyle) {super(context, ats, defaultStyle);initCompassView();}protected void initCompassView() {setFocusable(true);}}

2. 指南针控件应该总是一个圆的方式占据画布允许的尽可能多的空间。重写onMeasure方法来计算最小的边，使用setMeasuredDimension来设置高度和高度值。

@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {// The compass is a circle that fills as much space as possible.// Set the measured dimensions by figuring out the shortest boundary,// height or width.int measuredWidth = measure(widthMeasureSpec);int measuredHeight = measure(heightMeasureSpec);int d = Math.min(measuredWidth, measuredHeight);setMeasuredDimension(d, d);}private int measure(int measureSpec) {int result = 0;// Decode the measurement specifications.int specMode = MeasureSpec.getMode(measureSpec);int specSize = MeasureSpec.getSize(measureSpec);if (specMode == MeasureSpec.UNSPECIFIED) {// Return a default size of 200 if no bounds are specified.result = 200;} else {// As you want to fill the available space// always return the full available bounds.result = specSize;}return result;}

3. 创建两个你将在绘制指南针时用到的资源文件：颜色和字符串。

3.1. 创建文本字符串资源 /res/values/strings.xml.

<?xml version=”1.0” encoding=”utf-8”?><resources><string name=”app_name”>Compass</string><string name=”cardinal_north”>N</string><string name=”cardinal_east”>E</string><string name=”cardinal_south”>S</string><string name=”cardinal_west”>W</string></resources>

3.2. 创建颜色资源 /res/values/colors.xml.

<?xml version=”1.0” encoding=”utf-8”?><resources><color name=”background_color”>#F555</color><color name=”marker_color”>#AFFF</color><color name=”text_color”>#AF00</color></resources>

4. 现在回到CompassView类中。创建一个用来显示方向的属性并为它创建get和set方法。

private float bearing;public void setBearing(float _bearing) {bearing = _bearing;} public float getBearing() {return bearing;}

5. 接下来，返回到initCompassView方法中，获取第3步中创建的资源的引用。以类作用域的方法存储字符串值和由颜色值创建的Paint对象。你将在下一步中用这些对象来绘制指南针。

private Paint markerPaint;private Paint textPaint;private Paint circlePaint;private String northString;private String eastString;private String southString;private String westString;private int textHeight; protected void initCompassView() {setFocusable(true); circlePaint = new Paint(Paint.ANTI_ALIAS_FLAG);circlePaint.setColor(R.color. background_color);circlePaint.setStrokeWidth(1);circlePaint.setStyle(Paint.Style.FILL_AND_STROKE);Resources r = this.getResources();northString = r.getString(R.string.cardinal_north);eastString = r.getString(R.string.cardinal_east);southString = r.getString(R.string.cardinal_south);westString = r.getString(R.string.cardinal_west);textPaint = new Paint(Paint.ANTI_ALIAS_FLAG);textPaint.setColor(r.getColor(R.color.text_color));textHeight = (int)textPaint.measureText(“yY”);markerPaint = new Paint(Paint.ANTI_ALIAS_FLAG);markerPaint.setColor(r.getColor(R.color.marker_color));}

6. 最后一步就是用第5步中创建的字符串和Paint对象来绘制指南针。接下来的代码片段只给出了有限的提示。你可以在第11章找到更多关于如何在Canvas上绘制和使用高级的Paint效果的细节。

6.1. 首先重写onDraw方法。

@Overrideprotected void onDraw(Canvas canvas) {// TODO Auto-generated method stubsuper.onDraw(canvas);}

6.2. 找到控件的中心，存储最小边的长度作为指南针的半径。

int px = getMeasuredWidth() / 2;int py = getMeasuredHeight() /2 ;int radius = Math.min(px, py);

6.3.  使用drawCircle方法绘制外边框，背景的颜色使用第5步中创建的circlePaint对象。

// Draw the backgroundcanvas.drawCircle(px, py, radius, circlePaint);

6.4. 指南针通过旋转面板来显示当前的指向，所以当前的方向总是在设备的顶端。为了达到这个效果，沿着当前指向的相反方向来旋转画布。

// Rotate our perspective so that the ‘top’ is// facing the current bearing.canvas.save();canvas.rotate(-bearing, px, py);

6.5. 现在剩下来的就是绘制表盘。旋转画布一周，每隔15°绘制一个标记，每隔45°绘制一个方向字符串。

int textWidth = (int)textPaint.measureText(“W”);int cardinalX = px-textWidth/2;int cardinalY = py-radius+textHeight; // Draw the marker every 15 degrees and text every 45.for (int i = 0; i < 24; i++) {// Draw a marker.canvas.drawLine(px, py-radius, px, py-radius+10, markerPaint);canvas.save();canvas.translate(0, textHeight);// Draw the cardinal pointsif (i % 6 == 0) {String dirString = “”;switch (i){case(0) :{dirString = northString;int arrowY = 2*textHeight;canvas.drawLine(px, arrowY, px-5, 3*textHeight, markerPaint);canvas.drawLine(px, arrowY, px+5, 3*textHeight, markerPaint);break;}case(6) : dirString = eastString; break;case(12) : dirString = southString; break;case(18) : dirString = westString; break;}canvas.drawText(dirString, cardinalX, cardinalY, textPaint);} else if (i % 3 == 0) {// Draw the text every alternate 45degString angle = String.valueOf(i*15);float angleTextWidth = textPaint.measureText(angle);int angleTextX = (int)(px-angleTextWidth/2);int angleTextY = py-radius+textHeight;canvas.drawText(angle, angleTextX, angleTextY, textPaint);}canvas.restore();canvas.rotate(15, px, py);}canvas.restore();

7. 为了查看指南针，修改main.xml资源，使用你的CompassView来替换TextView。这个过程将在下个章节中有更加详细的解释。

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical"    android:layout_width="fill_parent"    android:layout_height="fill_parent"    >    <com.pad.compass.view.CompassViewandroid:id="@+id/compass"          android:layout_width="fill_parent"         android:layout_height="fill_parent"     /></LinearLayout>

8. 运行Activity，你会看到指南针显示出来了。第10章中，你将了解如何绑定CompassView到设备的指南针。

Sample Code:
      http://files.cnblogs.com/xirihanlin/[e-mail]

Sample图示：