自定义View的实现流程

1.继承View组件，比如，LabelView继承了View

 

2.重写两个构造方法，比如，对于自定义View LabelView

 

LabelView(Context context)，如果该自定义View是通过代码来实例化的，那么，就需要该构造方法；

LabelView(Context context, AttributeSet attrs)，通过布局文件XML来使用该自定义View，那么，就需要该构造方法。

 

    /**

     * 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);

   ....

     }

 

3.重写onDraw(Canvas canvas)和onMeasure(int measureWidth, int measureHeight)

 

onDraw(Canvas canvas)，在实现自定义View的时候，你自己要创建一个Paint对象，用来设置自定义View的外观，尺寸，颜色。

然后，再在onDraw方法中，将图形绘制在系统传递过来的画布对象Canvas。

也就是说，系统回调onDraw来实现自定义View的渲染。

 

onMeasure(int measureWidth, int measureHeight)，系统回调该方法，告诉你现在自定义View的宽度和高度是怎样设置的。

通过MeasureSpec.getMode(measureWidth),MeasureSpec.getSize(measureWidth)可以得到当前宽度的模式和大小。通过模式，可以知道用户设置的是wrap,match_parent,还是设置一个具体值，还说没有任何限制；通过大小，可以知道，当前系统检测到的当前View组件的宽度。

 

在得到这些信息后，你可以自己自定义，设置你想要的View组件的宽度和高度。

 

 

上面三个，就是自定义View的关键步骤。

 

-----------------------------------------------------------

 

其它方面:

1.Paint对象和Canvas的使用

 

要绘制一个自定义View，首先，你自己要确定出，图形的最终样式。按照最终样式，使用Paint对象，绘制出最终样式。Canvas对象的使用也需要关注。所以，在绘制图形的时候，要知道怎么使用Paint对象和Canvas绘制出最终的图形。

 

这方面的知识，需要补充。

 

 

2.属性设置

用户在使用你实现的自定义View的时候，它想自己定义View的属性，它想通过View提供的属性控制View的外观，尺寸，颜色等特征。那么，你实现的自定义View，就要提供给用户设置属性，并且要接收到用户传递过来的属性值，根据这些属性值构造出Paint对象。

这个的实现步骤：

1).在res目录下的values目录，创建一个attrs.xml，然后，在里面添加属性描述。如下：声明了三个属性。

 

2).然后，在自定义View的类中，声明setter方法，如下：

根据用户传递过来的属性，重新绘制自定义VIew。requestLayout()的调用，会触发系统回调onMeasure(int measureWidth, int measureHeight)方法；invalidate()的调用，会触发系统回调onDraw()方法。

/**

        * 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();

       }

 

 

3).在使用该自定义View的布局文件中，要声明该自定义View的命名空间，如下：

声明了一个命名空间

xmlns:app = "http://schemas.android.com/apk/res/com.example.customview"

红色部分，是该自定义View所在的包。

然后，你就可以使用app:text，这样命名空间加属性名的方式来设置属性。属性名，与attrs.xml中声明名字一样。

< RelativeLayout xmlns:android ="http://schemas.android.com/apk/res/android"

    xmlns:tools= "http://schemas.android.com/tools"

    xmlns:app= "http://schemas.android.com/apk/res/com.example.customview"

    android:layout_width= "match_parent"

    android:layout_height= "match_parent" >

 

    <com.example.customview.LabelView

        android:id ="@+id/oneLabelView"

        android:layout_width ="wrap_content"

        android:layout_height ="wrap_content"

        app:text= "Somethine"

        app:textColor ="#00868B"

        app:textSize ="34sp" />

 

</ RelativeLayout>

 

---------------下面是实现例子--------------------------

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);
     }
}