前言
从前面的几篇文章我们能够知道UI的测量和布局摆放，那么这次我们来关注下UI是怎么具体画出来的，首先要了解的是具体绘制的流程以及Canvas和Paint在这中间所扮演的角色。

绘制流程

从ViewRootImpl的performTraversals方法开始入手：

// Remember if we must report the next draw.
    if ((relayoutResult & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
        reportNextDraw();
    }

    boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;

    if (!cancelDraw && !newSurface) {
        if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
            for (int i = 0; i < mPendingTransitions.size(); ++i) {
                mPendingTransitions.get(i).startChangingAnimations();
            }
            mPendingTransitions.clear();
        }

        performDraw();
    } else {
        if (isViewVisible) {
            // Try again
            scheduleTraversals();
        } else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
            for (int i = 0; i < mPendingTransitions.size(); ++i) {
                mPendingTransitions.get(i).endChangingAnimations();
            }
            mPendingTransitions.clear();
        }
    }

    mIsInTraversal = false;

在上面的代码我们可以看到一个关键点就是在isViewVisble=true（也就是view为显示的状态下，这里会再次发起一次scheduleTraversals,所以，这也是为什么onMeasure方法会调用两次的原因Android7.0以上版本）
接着进入performDraw方法：

private void performDraw() {
    if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
        return;
    } else if (mView == null) {
        return;
    }

    final boolean fullRedrawNeeded = mFullRedrawNeeded;
    mFullRedrawNeeded = false;

    mIsDrawing = true;
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
    try {
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    // For whatever reason we didn't create a HardwareRenderer, end any
    // hardware animations that are now dangling
    if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
        final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
        for (int i = 0; i < count; i++) {
            mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
        }
        mAttachInfo.mPendingAnimatingRenderNodes.clear();
    }

    if (mReportNextDraw) {
        mReportNextDraw = false;

        // if we're using multi-thread renderer, wait for the window frame draws
        if (mWindowDrawCountDown != null) {
            try {
                mWindowDrawCountDown.await();
            } catch (InterruptedException e) {
                Log.e(mTag, "Window redraw count down interruped!");
            }
            mWindowDrawCountDown = null;
        }

        if (mAttachInfo.mThreadedRenderer != null) {
            mAttachInfo.mThreadedRenderer.fence();
            mAttachInfo.mThreadedRenderer.setStopped(mStopped);
        }

        if (LOCAL_LOGV) {
            Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
        }

        if (mSurfaceHolder != null && mSurface.isValid()) {
            SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
            SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();

            sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
        } else {
            pendingDrawFinished();
        }
    }
}

再接着看draw方法：

private void draw(boolean fullRedrawNeeded) {
    Surface surface = mSurface;
    if (!surface.isValid()) {
        return;
    }

    if (DEBUG_FPS) {
        trackFPS();
    }

    if (!sFirstDrawComplete) {
        synchronized (sFirstDrawHandlers) {
            sFirstDrawComplete = true;
            final int count = sFirstDrawHandlers.size();
            for (int i = 0; i< count; i++) {
                mHandler.post(sFirstDrawHandlers.get(i));
            }
        }
    }

    scrollToRectOrFocus(null, false);

    if (mAttachInfo.mViewScrollChanged) {
        mAttachInfo.mViewScrollChanged = false;
        mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
    }

    boolean animating = mScroller != null && mScroller.computeScrollOffset();
    final int curScrollY;
    if (animating) {
        curScrollY = mScroller.getCurrY();
    } else {
        curScrollY = mScrollY;
    }
    if (mCurScrollY != curScrollY) {
        mCurScrollY = curScrollY;
        fullRedrawNeeded = true;
        if (mView instanceof RootViewSurfaceTaker) {
            ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
        }
    }

    final float appScale = mAttachInfo.mApplicationScale;
    final boolean scalingRequired = mAttachInfo.mScalingRequired;

    int resizeAlpha = 0;
    //获取mDirty，该值表示需要重绘的区域，就是之前我们最先做的那个定位
    final Rect dirty = mDirty;
    if (mSurfaceHolder != null) {
        // The app owns the surface, we won't draw.
        dirty.setEmpty();
        if (animating && mScroller != null) {
            mScroller.abortAnimation();
        }
        return;
    }
    //如果fullRedrawNeeded为真，则把dirty区域置为整个屏幕，表示整个视图都需要绘制
    //第一次绘制流程，需要绘制所有视图
    if (fullRedrawNeeded) {
        mAttachInfo.mIgnoreDirtyState = true;
        dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
    }

    if (DEBUG_ORIENTATION || DEBUG_DRAW) {
        Log.v(mTag, "Draw " + mView + "/"
                + mWindowAttributes.getTitle()
                + ": dirty={" + dirty.left + "," + dirty.top
                + "," + dirty.right + "," + dirty.bottom + "} surface="
                + surface + " surface.isValid()=" + surface.isValid() + ", appScale:" +
                appScale + ", width=" + mWidth + ", height=" + mHeight);
    }

    mAttachInfo.mTreeObserver.dispatchOnDraw();

    int xOffset = -mCanvasOffsetX;
    int yOffset = -mCanvasOffsetY + curScrollY;
    final WindowManager.LayoutParams params = mWindowAttributes;
    final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
    if (surfaceInsets != null) {
        xOffset -= surfaceInsets.left;
        yOffset -= surfaceInsets.top;

        // Offset dirty rect for surface insets.
        dirty.offset(surfaceInsets.left, surfaceInsets.right);
    }

    boolean accessibilityFocusDirty = false;
    final Drawable drawable = mAttachInfo.mAccessibilityFocusDrawable;
    if (drawable != null) {
        final Rect bounds = mAttachInfo.mTmpInvalRect;
        final boolean hasFocus = getAccessibilityFocusedRect(bounds);
        if (!hasFocus) {
            bounds.setEmpty();
        }
        if (!bounds.equals(drawable.getBounds())) {
            accessibilityFocusDirty = true;
        }
    }

    mAttachInfo.mDrawingTime =
            mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;

    if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
        if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
            // If accessibility focus moved, always invalidate the root.
            boolean invalidateRoot = accessibilityFocusDirty || mInvalidateRootRequested;
            mInvalidateRootRequested = false;

            // Draw with hardware renderer.
            mIsAnimating = false;

            if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
                mHardwareYOffset = yOffset;
                mHardwareXOffset = xOffset;
                invalidateRoot = true;
            }

            if (invalidateRoot) {
                mAttachInfo.mThreadedRenderer.invalidateRoot();
            }

            dirty.setEmpty();

            // Stage the content drawn size now. It will be transferred to the renderer
            // shortly before the draw commands get send to the renderer.
            final boolean updated = updateContentDrawBounds();

            if (mReportNextDraw) {
                // report next draw overrides setStopped()
                // This value is re-sync'd to the value of mStopped
                // in the handling of mReportNextDraw post-draw.
                mAttachInfo.mThreadedRenderer.setStopped(false);
            }

            if (updated) {
                requestDrawWindow();
            }

            mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this);
        } else {
            // If we get here with a disabled & requested hardware renderer, something went
            // wrong (an invalidate posted right before we destroyed the hardware surface
            // for instance) so we should just bail out. Locking the surface with software
            // rendering at this point would lock it forever and prevent hardware renderer
            // from doing its job when it comes back.
            // Before we request a new frame we must however attempt to reinitiliaze the
            // hardware renderer if it's in requested state. This would happen after an
            // eglTerminate() for instance.
            if (mAttachInfo.mThreadedRenderer != null &&
                    !mAttachInfo.mThreadedRenderer.isEnabled() &&
                    mAttachInfo.mThreadedRenderer.isRequested()) {

                try {
                    mAttachInfo.mThreadedRenderer.initializeIfNeeded(
                            mWidth, mHeight, mAttachInfo, mSurface, surfaceInsets);
                } catch (OutOfResourcesException e) {
                    handleOutOfResourcesException(e);
                    return;
                }

                mFullRedrawNeeded = true;
                scheduleTraversals();
                return;
            }

            if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                return;
            }
        }
    }

    if (animating) {
        mFullRedrawNeeded = true;
        scheduleTraversals();
    }
}

上面代码中先获取了mDirty值，这里保存了需要重绘的区域的信息，最后调用了ViewRootImpl#drawSoftware方法：

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty) {

    // Draw with software renderer.
    final Canvas canvas;
    try {
        final int left = dirty.left;
        final int top = dirty.top;
        final int right = dirty.right;
        final int bottom = dirty.bottom;
        //锁定canvas区域，由dirty区域决定
        canvas = mSurface.lockCanvas(dirty);

        // The dirty rectangle can be modified by Surface.lockCanvas()(这个巨型区域被锁定了修改)
        //noinspection ConstantConditions（没有检查恒定条件）
        if (left != dirty.left || top != dirty.top || right != dirty.right
                || bottom != dirty.bottom) {
            attachInfo.mIgnoreDirtyState = true;
        }

        // TODO: Do this in native （在本地设置密度）
        canvas.setDensity(mDensity);
    } catch (Surface.OutOfResourcesException e) {
        handleOutOfResourcesException(e);
        return false;
    } catch (IllegalArgumentException e) {
        Log.e(mTag, "Could not lock surface", e);
        // Don't assume this is due to out of memory, it could be
        // something else, and if it is something else then we could
        // kill stuff (or ourself) for no reason.
        mLayoutRequested = true;    // ask wm for a new surface next time.
        return false;
    }

    try {
        if (DEBUG_ORIENTATION || DEBUG_DRAW) {
            Log.v(mTag, "Surface " + surface + " drawing to bitmap w="
                    + canvas.getWidth() + ", h=" + canvas.getHeight());
            //canvas.drawARGB(255, 255, 0, 0);
        }

        // If this bitmap's format includes an alpha channel, we
        // need to clear it before drawing so that the child will
        // properly re-composite its drawing on a transparent
        // background. This automatically respects the clip/dirty region
        // or
        // If we are applying an offset, we need to clear the area
        // where the offset doesn't appear to avoid having garbage
        // left in the blank areas.
      对于上面的翻译是：
      如果位图的格式包含alpha通道，我们
      需要在画之前清除它，以便子控件重新组合其图纸上的透明背景
      这将自动地对剪辑区域进行关联。
      或
      如果我们申请抵销，我们需要清理这个地区。
      在没有出现偏移的情况下避免垃圾
      留在空白区域。

        if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
            canvas.drawColor(0, PorterDuff.Mode.CLEAR);
        }

        dirty.setEmpty();
        mIsAnimating = false;
        mView.mPrivateFlags |= View.PFLAG_DRAWN;

        if (DEBUG_DRAW) {
            Context cxt = mView.getContext();
            Log.i(mTag, "Drawing: package:" + cxt.getPackageName() +
                    ", metrics=" + cxt.getResources().getDisplayMetrics() +
                    ", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());
        }
        try {
            canvas.translate(-xoff, -yoff);
            if (mTranslator != null) {
                mTranslator.translateCanvas(canvas);
            }
            canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
            attachInfo.mSetIgnoreDirtyState = false;

            mView.draw(canvas);

            drawAccessibilityFocusedDrawableIfNeeded(canvas);
        } finally {
            if (!attachInfo.mSetIgnoreDirtyState) {
                // Only clear the flag if it was not set during the mView.draw() call
                attachInfo.mIgnoreDirtyState = false;
            }
        }
    } finally {
        try {
            surface.unlockCanvasAndPost(canvas);
        } catch (IllegalArgumentException e) {
            Log.e(mTag, "Could not unlock surface", e);
            mLayoutRequested = true;    // ask wm for a new surface next time.
            //noinspection ReturnInsideFinallyBlock
            return false;
        }

        if (LOCAL_LOGV) {
            Log.v(mTag, "Surface " + surface + " unlockCanvasAndPost");
        }
    }
    return true;
}

上面的代码可以看出，首先实例化了Canvas对象，最后锁定该canvas的区域，由dirty区域决定，接着对canvas进行一系列的属性赋值，最后调用了mView.draw(canvas)方法，那么之前就讲过这里的mView就是我们的DectorView，所以是从DectorView顶层开始绘制，那么之前的一切都是在进行准备一块画板，具体的绘制是在mView.draw当中，这里将画板给入，而现在则是正式开始绘制流程。

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（绘制遍历执行必须执行的几个绘图步骤）
     * in the appropriate order:（按下列顺序）
     *
     *      1. Draw the background（画背景）
     *      2. If necessary, save the canvas' layers to prepare for fading（必要时，保存画布的层以准备渐变。）
     *      3. Draw view's content（绘制视图内容）
     *      4. Draw children（画子view）
     *      5. If necessary, draw the fading edges and restore layers（如果需要，绘制渐变边缘并恢复图层）
     *      6. Draw decorations (scrollbars for instance)（绘制装饰（例如滚动条））
     */

    // 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);

        drawAutofilledHighlight(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);

        // Step 7, draw the default focus highlight
        drawDefaultFocusHighlight(canvas);

        if (debugDraw()) {
            debugDrawFocus(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);

    drawAutofilledHighlight(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);

    if (debugDraw()) {
        debugDrawFocus(canvas);
    }
}

上面的代码可以看到，draw过程比较复杂，但是逻辑十分清晰，官方注释也清楚的说明每一步的做法。我们首先来看一开始的标记位dirtyOpaque,该标记位的作用是判断当前View是否是透明的，如果View是透明的，那么根据下面的逻辑可以看出，将不会执行一些步骤，比如绘制背景、绘制内容等。这样很容易理解，因为一个View既然是透明的，那么就没必要绘制它了，接着是绘制流程的六个步骤：
1.对View 的背景进行绘制；
2.保存当前的图层信息；
3.绘制View的内容；
4.对View的子View进行绘制（如果有子View）；
5.绘制View的褪色的边缘，类似于阴影效果；
6.绘制View的装饰。
我们只要关注3和4这两点，通过源码发现，View也是暴露了onDraw方法让子类去实现。
由此同理与我们之前的测量和摆放，父控件取得所有子控件开始遍历，调用子控件让子控件自己调用自己的draw开始绘制自己。
逻辑很清晰，都是先设定绘制区域，然后利用canvas进行绘制。

那么，到目前为止，view的绘制流程也讲述完毕了。
其实测量、布局摆放、绘制这三个流程都是调用到onMeasure/onLayout/onDraw让子控件自己去完成的，只不过系统组件（FrameLayout或者ViewGroup）它帮我们按照规则实现了自己本身的效果，那么我们也同样可以根据顺序、原理去施加自己的业务，完成自己想要的自定义控件。