[Android]自定義View的概要與Source Code資源

Android Develpers官網提供的範例:

Creating a View Class

http://developer.android.com/training/custom-views/create-view.html
Sample Code URL:
http://developer.android.com/shareables/training/CustomView.zip

自定義一個View的方法:
步驟一:
    開一Java檔extends
        (1)最原生的View
        (2)ViewGroup
        --------Layout類--------
        (3)LinearLayout
        (4)FrameLayout
        --------元件類--------
        (5)Button
   ...

步驟二:
    在res\values\attrs.xml定義屬性
    <resources>
    <declare-styleable name="PieChart">     <!--需與Java檔的ClassName相同 -->
        <attr name="autoCenterPointerInSlice" format="boolean"/>
        <attr name="highlightStrength" format="float"/>
        <attr name="labelColor" format="color"/>
        <attr name="labelHeight" format="dimension"/>
        <attr name="labelPosition" format="enum">
            <enum name="left" value="0"/>
            <enum name="right" value="1"/>
        </attr>
        <attr name="labelWidth" format="dimension"/>
        <attr name="labelY" format="dimension"/>
        <attr name="pieRotation" format="integer"/>
        <attr name="pointerRadius" format="dimension"/>
        <attr name="showText" format="boolean"/>
    </declare-styleable>
</resources>

還有這些屬性可以使用:

        <!-- Identifier for the child that represents the panel's handle. -->
        <attr name="handle" format="reference" />

       <!-- Defines opened handle (drawable/color). -->
        <attr name="openedHandle" format="reference|color" />

        <attr name="transitionTextColorDown" format="color" />

         <attr name="weight" format="fraction" />


步驟三:
    提供2種建構子:
    //Create View object from your JAVA code.
    public PieChart(Context context) {
        super(context);
        init();
    }

    //Call by the layout engine.
    public PieChart(Context context, AttributeSet attrs) {
        super(context, attrs);
        //Get the Attribute from XML layout.
        TypedArray a = context.getTheme().obtainStyledAttributes(
                attrs,
                R.styleable.PieChart,
                0, 0
        );

        try {
            // Retrieve the values from the TypedArray and store into
            // fields of this class.
            //
            // The R.styleable.PieChart_* constants represent the index for
            // each custom attribute in the R.styleable.PieChart array.
            mShowText = a.getBoolean(R.styleable.PieChart_showText, false);
            mTextY = a.getDimension(R.styleable.PieChart_labelY, 0.0f);
            mTextWidth = a.getDimension(R.styleable.PieChart_labelWidth, 0.0f);
            mTextHeight = a.getDimension(R.styleable.PieChart_labelHeight, 0.0f);
            mTextPos = a.getInteger(R.styleable.PieChart_labelPosition, 0);
            mTextColor = a.getColor(R.styleable.PieChart_labelColor, 0xff000000);
            mHighlightStrength = a.getFloat(R.styleable.PieChart_highlightStrength, 1.0f);
            mPieRotation = a.getInt(R.styleable.PieChart_pieRotation, 0);
            mPointerRadius = a.getDimension(R.styleable.PieChart_pointerRadius, 2.0f);
            mAutoCenterInSlice = a.getBoolean(R.styleable.PieChart_autoCenterPointerInSlice, false);
        } finally {
            // release the TypedArray so that it can be reused.
            a.recycle();
        }

        init();
    }

    步驟四:
    系統在開始onDraw()前，會多次呼叫onMeasure()去跟Layout問現在要畫的範圍是多大
    //
    // Measurement functions. This example uses a simple heuristic: it assumes that
    // the pie chart should be at least as wide as its label.
    //
    @Override
    protected int getSuggestedMinimumWidth() {
        return (int) mTextWidth * 2;
    }

    @Override
    protected int getSuggestedMinimumHeight() {
        return (int) mTextWidth;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // Try for a width based on our minimum
        int minw = getPaddingLeft() + getPaddingRight() + getSuggestedMinimumWidth();

        int w = Math.max(minw, MeasureSpec.getSize(widthMeasureSpec));

        // Whatever the width ends up being, ask for a height that would let the pie
        // get as big as it can
        int minh = (w - (int) mTextWidth) + getPaddingBottom() + getPaddingTop();
        int h = Math.min(MeasureSpec.getSize(heightMeasureSpec), minh);

        setMeasuredDimension(w, h);
    }

   相關方法:
    heightMeasureSpec = MeasureSpec.makeMeasureSpec((int) (parent.getHeight() * mWeight), MeasureSpec.EXACTLY);

    int w = Math.max(minw, MeasureSpec.getSize(widthMeasureSpec));

    int specMode = MeasureSpec.getMode(measureSpec);
    if (specMode == MeasureSpec.EXACTLY) ....

    一開始系統傳進來的引數measureSpec是從main.xml裡抓過來的
    在MeasureSpec.getMode()裡會抓到的尺寸模式，有以下3種

Mode	Value	備註	main.xml裡相對應的設定
AT_MOST	-2147483648	子視圖可自行調整尺寸大小	android:layout_width或height 設為"wrap_content"時
EXACTLY	1073741824	父Layout已經很明確的定義該子視圖的大小	android:layout_width或height有自訂大小或為"fill_parent"時
UNSPECIFIED	0	父Layout未指定大小，子視圖可自行調整。

    步驟五:
    幾次的onMeasure()呼叫完後，(如果是繼承自View)就跑去呼叫onDraw()開始繪圖了。
    若是繼承自其他者，請參考以下:
    Here's a summary of some of the other standard methods that the framework calls on views:

Category	Methods	Description
Creation	Constructors	There is a form of the constructor that are called when the view is created from code and a form that is called when the view is inflated from a layout file. The second form should parse and apply any attributes defined in the layout file.
	onFinishInflate()	Called after a view and all of its children has been inflated from XML.
Layout	onMeasure(int, int)	Called to determine the size requirements for this view and all of its children.
	onLayout(boolean, int, int, int, int)	Called when this view should assign a size and position to all of its children.
	onSizeChanged(int, int, int, int)	Called when the size of this view has changed.
Drawing	onDraw(Canvas)	Called when the view should render its content.
Event processing	onKeyDown(int, KeyEvent)	Called when a new key event occurs.
	onKeyUp(int, KeyEvent)	Called when a key up event occurs.
	onTrackballEvent(MotionEvent)	Called when a trackball motion event occurs.
	onTouchEvent(MotionEvent)	Called when a touch screen motion event occurs.
Focus	onFocusChanged(boolean, int, Rect)	Called when the view gains or loses focus.
	onWindowFocusChanged(boolean)	Called when the window containing the view gains or loses focus.
Attaching	onAttachedToWindow()	Called when the view is attached to a window.
	onDetachedFromWindow()	Called when the view is detached from its window.
	onWindowVisibilityChanged(int)	Called when the visibility of the window containing the view has changed.

總結來說︰
1.在res/values/新增一個自訂的attrs.xml2.在main.xml裡使用他們3.繼承View，並在(1)建構式 裡宣告初始值(2)在onMeasure()裡交待清楚這個自創的View應該有多大(3)在onDraw()裡開始繪圖