自定义View的实现流程

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();
     }

     /**
     * 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();

          TypedArray a = context.obtainStyledAttributes(attrs,
                    R.styleable.LabelView);

          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().
          setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));

          int textSize = a.getDimensionPixelOffset(
                    R.styleable.LabelView_textSize, 0);
          if (textSize > 0) {
               setTextSize(textSize);
          }

          a.recycle();
     }

     private final void initLabelView() {
          mTextPaint = new Paint();
          mTextPaint.setAntiAlias(true);
          // Must manually scale the desired text size to match screen density
          mTextPaint.setTextSize(16 * getResources().getDisplayMetrics().density);
          mTextPaint.setColor(0xFF000000);
          setPadding(3, 3, 3, 3);
     }

     /**
     * 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();
     }

     /**
     * Sets the text size for this label
     * 
     * @param size
     *            Font size
     */
     public void setTextSize(int size) {
          // This text size has been pre-scaled by the getDimensionPixelOffset
          // method
          mTextPaint.setTextSize(size);
          requestLayout();
          invalidate();
     }

     /**
     * Sets the text color for this label.
     * 
     * @param color
     *            ARGB value for the text
     */
     public void setTextColor(int color) {
          mTextPaint.setColor(color);
          invalidate();
     }

     /**
     * @see android.view.View#measure(int, int)
     */
     @Override
     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
          setMeasuredDimension(measureWidth(widthMeasureSpec),
                    measureHeight(heightMeasureSpec));
     }

     /**
     * Determines the width of this view
     * 
     * @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);

          if (specMode == MeasureSpec.EXACTLY) {
               // We were told how big to be
               result = specSize;
          } else {
               // Measure the text
               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;
     }

     /**
     * Determines the height of this view
     * 
     * @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
     * 
     * @see android.view.View#onDraw(android.graphics.Canvas)
     */
     @Override
     protected void onDraw(Canvas canvas) {
          super.onDraw(canvas);
          canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent,
                    mTextPaint);
     }
}