public class MyTextView extends View {
    //字型顏色
    private int MyTextColor = Color.parseColor("#000000");
    //要繪製的文字
    private String MyTextString = "";
    //預設文字大小
    private int MyTextSize = 15;
    //畫筆
    private Paint mPaint;

    public MyTextView(Context context) {
        this(context, null);
    }

    public MyTextView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MyTextView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        //獲取自定義屬性
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.MyTextView);
        MyTextColor = a.getColor(R.styleable.MyTextView_myTextColor, MyTextColor);
        MyTextString = a.getString(R.styleable.MyTextView_myText);
        MyTextSize = a.getDimensionPixelSize(R.styleable.MyTextView_myTextSize, sp2px(MyTextSize));
        a.recycle();
        //例項化畫筆
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setColor(MyTextColor);
        mPaint.setTextSize(MyTextSize);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        //獲取寬高模式
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        //獲取寬高
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        if (widthMode == MeasureSpec.AT_MOST) {
            //測量文字寬度
            Rect rect = new Rect();
            mPaint.getTextBounds(MyTextString, 0, MyTextString.length(), rect);
            widthSize = rect.width() + getPaddingLeft() + getPaddingRight();
        }
        if (heightMode == MeasureSpec.AT_MOST) {
            //測量文字的高度
            Rect rect = new Rect();
            mPaint.getTextBounds(MyTextString, 0, MyTextString.length(), rect);
            heightSize = rect.height() + getPaddingTop() + getPaddingBottom();
        }
        setMeasuredDimension(widthSize, heightSize);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        int x = getPaddingLeft();
        //計算文字基線
        Paint.FontMetricsInt metricsInt = mPaint.getFontMetricsInt();
        Log.e("MyTextView-bottom",metricsInt.bottom+"");
        Log.e("MyTextView-top",metricsInt.top+"");
        //需要注意metricsInt.bottom是正值  metricsInt.top是負值
        int dy = (metricsInt.bottom - metricsInt.top) / 2 - metricsInt.bottom;
        int y = getHeight() / 2 + dy;
        canvas.drawText(MyTextString, x, y, mPaint);
    }

    private int sp2px(int sp) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, sp, getResources().getDisplayMetrics());
    }
}
 

效果：

上面是一個簡單的自定義textview效果，主要是說明下自定義view的大致實現流程，就是extend View，重寫onMeasure、onDraw方法等，在onMeasure測試模式中介紹了系統的三種模式，在自定義view中如果給的AT_MOST模式，就需要通過Paint畫筆進行測量，重新設定繪製寬高；在通過canvas.drawText()方法繪製文字時需要注意繪製文字的y，這裡的y指的是文字的基線，而不是文字高度的一半；

需要注意的是metricsInt.bottom得到的是正值，metricsInt.top得到的是負值；這個是extend view實現的效果，其實有時候不注意的時候會extend ViewGroup，這個時候並沒有出現想要的效果；這就涉及到view中的draw()方法，在onDraw中進行繪製，其實回去呼叫view中的draw方法；

//這裡是View中draw方法的原始碼
public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // we're done...
            return;
        }

        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
    }
 

上面是View中draw的原始碼，其中採用了模板設計模式，在其中有這樣一段程式碼：

// Step 3, draw the content
 if (!dirtyOpaque) onDraw(canvas);

很明顯dirtyOpaque這個標識為false的時候才會去呼叫onDraw方法，extend ViewGroup沒有效果，那就說明dirtyOpaque標識不為false而沒有呼叫onDraw方法；

final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

dirtyOpaque標識在已進入draw方法的時候通過privateFlags，最終又是通過mPrivateFlags來設定的，mPrivateFlags的話則是在View的建構函式中通過computeOpaqueFlags()方法來設定的；

protected void computeOpaqueFlags() {
        // Opaque if:
        //   - Has a background 
        //   - Background is opaque
        //   - Doesn't have scrollbars or scrollbars overlay

        if (mBackground != null && mBackground.getOpacity() == PixelFormat.OPAQUE) {
            mPrivateFlags |= PFLAG_OPAQUE_BACKGROUND;
        } else {
            mPrivateFlags &= ~PFLAG_OPAQUE_BACKGROUND;
        }

        final int flags = mViewFlags;
        if (((flags & SCROLLBARS_VERTICAL) == 0 && (flags & SCROLLBARS_HORIZONTAL) == 0) ||
                (flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_INSIDE_OVERLAY ||
                (flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_OUTSIDE_OVERLAY) {
            mPrivateFlags |= PFLAG_OPAQUE_SCROLLBARS;
        } else {
            mPrivateFlags &= ~PFLAG_OPAQUE_SCROLLBARS;
        }
    }

這樣的話，那就去看看是不是ViewGoup中通過方法將這一標識進行了修改；在ViewGroup的構造方法中有一個initViewGroup()方法；

private void initViewGroup() {
        // ViewGroup doesn't draw by default
        if (!debugDraw()) {
            //這裡呼叫的是View中的setFlags方法
            setFlags(WILL_NOT_DRAW, DRAW_MASK);
        }
        mGroupFlags |= FLAG_CLIP_CHILDREN;
        mGroupFlags |= FLAG_CLIP_TO_PADDING;
        mGroupFlags |= FLAG_ANIMATION_DONE;
        mGroupFlags |= FLAG_ANIMATION_CACHE;
        mGroupFlags |= FLAG_ALWAYS_DRAWN_WITH_CACHE;

        if (mContext.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB) {
            mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
        }

        setDescendantFocusability(FOCUS_BEFORE_DESCENDANTS);

        mChildren = new View[ARRAY_INITIAL_CAPACITY];
        mChildrenCount = 0;

        mPersistentDrawingCache = PERSISTENT_SCROLLING_CACHE;
    }

在ViewGroup中的initViewGroup方法中呼叫了View中的setFlags方法將標識修改成了WILL_NOT_DRAW,就是不去呼叫onDraw方法；到這裡差不多就明白了為什麼extend ViewGroup會沒有效果了，那就是說只要改變這一標識extend ViewGroup就會和extend View一樣有效果咯；方法有很多種；

方法一：

其實會發現在draw方法中的dispatchDraw()就沒有標識來判斷，就是不管extend ViewGroup還是extend View都會被呼叫，那就在extend ViewGroup的時候不重寫onDraw方法而去重寫dispatchDraw()方法，在dispatchDraw()方法中進行繪製；

方法二：

還是extend ViewGroup，重寫的也是onDraw方法，但是給自定義view設定setBackground也會實現同樣的效果；設定背景的時候回去呼叫View中的setBackground方法；

//view中setBackground原始碼
public void setBackground(Drawable background) {
        //noinspection deprecation
        setBackgroundDrawable(background);
    }

接著呼叫了setBackgroundDrawable()方法；一進入setBackgroundDrawable()方法就看到呼叫了computeOpaqueFlags()方法，在上面可以看到，computeOpaqueFlags()裡面會對mPrivateFlags進行賦值，所以就看到效果了；

方法三：

既然ViewGroup可以通過setFlags方法設定不呼叫onDraw方法，那可以去找下是否可以通過呼叫方法來設定mPrivateFlags標識；

/**
     * If this view doesn't do any drawing on its own, set this flag to
     * allow further optimizations. By default, this flag is not set on
     * View, but could be set on some View subclasses such as ViewGroup.
     *
     * Typically, if you override {@link #onDraw(android.graphics.Canvas)}
     * you should clear this flag.
     *
     * @param willNotDraw whether or not this View draw on its own
     */
    public void setWillNotDraw(boolean willNotDraw) {
        setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
    }

會看到這個方法，傳入一個false就可以改變mPrivateFlags標識，從而可以實現效果，其實呼叫setFocusable(false)；等方法都可以實現；