Android 如何自定义控件（继承View）

该例子演示的是如何自定义罗盘的控件

activity_mian.xml代码：

<?xml version="1.0" encoding="utf-8"?><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:orientation="vertical"  android:layout_width="match_parent"  android:layout_height="match_parent">  <com.paad.compass.CompassView    android:id="@+id/compassView"    android:layout_width="match_parent"    android:layout_height="match_parent"  /></FrameLayout>

在strings.xml里面添加：

<?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>

在colors.xml代码如下：

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

CompassView.java代码：

package com.paad.compass;import android.content.Context;import android.content.res.Resources;import android.graphics.Canvas;import android.graphics.Paint;import android.util.AttributeSet;import android.view.View;import android.view.accessibility.AccessibilityEvent;public class CompassView extends View {  private float bearing;  public void setBearing(float _bearing) {    bearing = _bearing;    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_TEXT_CHANGED);  }  public float getBearing() {    return bearing;  }  private Paint markerPaint;  private Paint textPaint;  private Paint circlePaint;  private String northString;  private String eastString;  private String southString;  private String westString;  private int textHeight;  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);    Resources r = this.getResources();    circlePaint = new Paint(Paint.ANTI_ALIAS_FLAG);    circlePaint.setColor(r.getColor(R.color.background_color));    circlePaint.setStrokeWidth(1);    circlePaint.setStyle(Paint.Style.FILL_AND_STROKE);    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));  }  @Override  protected 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.         //如果没有指定界限，返回默认大小200      result = 200;    } else {      // As you want to fill the available space      // always return the full available bounds.      result = specSize;    }    return result;  }  @Override  protected void onDraw(Canvas canvas) {    int mMeasuredWidth = getMeasuredWidth();    int mMeasuredHeight = getMeasuredHeight();    int px = mMeasuredWidth / 2;    int py = mMeasuredHeight / 2 ;    int radius = Math.min(px, py);    // Draw the background    canvas.drawCircle(px, py, radius, circlePaint);    // Rotate our perspective so that the 詔op' is    // facing the current bearing.    canvas.save();    canvas.rotate(-bearing, px, py);    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 points      if (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 45deg        String 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();  }  @Override  public boolean dispatchPopulateAccessibilityEvent(final AccessibilityEvent event) {    super.dispatchPopulateAccessibilityEvent(event);    if (isShown()) {      String bearingStr = String.valueOf(bearing);      if (bearingStr.length() > AccessibilityEvent.MAX_TEXT_LENGTH)        bearingStr = bearingStr.substring(0, AccessibilityEvent.MAX_TEXT_LENGTH);      event.getText().add(bearingStr);      return true;    }    else      return false;  }}

主要是重写onDraw方法和onMeasure方法来实现罗盘。

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