View工作原理六:draw流程

2018-08-29  本文已影响18人  水言

View框架在经过了measure过程和layout过程之后,就已经确定了每一个View的尺寸和位置,那么接下来的工作就是将具体的View绘制到屏幕上,这就是draw的主要工作。

View的draw流程:

fading edge使用可参考https://blog.csdn.net/qq1263292336/article/details/78675635

draw的入口也是在ViewRootImpl中,执行ViewRootImpl#performTraversals中会执行ViewRootIml#performDraw:

private void performDraw() {
...
//fullRedrawNeeded,它的作用是判断是否需要重新绘制全部视图
draw(fullRedrawNeeded);
...
}

然后会执行到ViewRootImpl#draw:

private void draw(boolean fullRedrawNeeded) {
 ...
 //获取mDirty,该值表示需要重绘的区域
 final Rect dirty = mDirty;
 if (mSurfaceHolder != null) {
  // The app owns the surface, we won't draw.
  dirty.setEmpty();
  if (animating) {
   if (mScroller != null) {
    mScroller.abortAnimation();
   }
   disposeResizeBuffer();
  }
  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 (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
    return;
  }
}

接着会执行到ViewRootIml#drawSoftware,然后在ViewRootIml#drawSoftware会执行到 mView.draw(canvas)。

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
   boolean scalingRequired, Rect dirty) {
 final Canvas canvas;
  //锁定canvas区域,由dirty区域决定
  //这个canvas就是我们想在上面绘制东西的画布
  canvas = mSurface.lockCanvas(dirty);
  ...
 //画布支持位图的密度,和手机分辨率相关
  canvas.setDensity(mDensity);
 ...
   if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }
   ...
      canvas.translate(-xoff, -yoff);
   ...
   //正式开始绘制
   mView.draw(canvas);
  ...
 //提交需要绘制的东西
  surface.unlockCanvasAndPost(canvas);
}

mView.draw(canvas)开始真正的绘制。
View#draw:

    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;

        //绘制背景
        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // 如果可以跳过2和5步
        final int viewFlags = mViewFlags;
      //判断是否有绘制衰退边缘的标示
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
     // 如果没有绘制衰退边缘只需要3,4,6步
        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);
    }

1.绘制背景

View#drawBackground

    private void drawBackground(Canvas canvas) {
       //获取背景的Drawable,没有就不需要绘制
        final Drawable background = mBackground;
        if (background == null) {
            return;
        }
       //确定背景Drawable边界
        setBackgroundBounds();
        ...

       //如果有偏移量先偏移画布再将drawable绘制上去
        final int scrollX = mScrollX;
        final int scrollY = mScrollY;
        if ((scrollX | scrollY) == 0) {
            background.draw(canvas);
        } else {
            canvas.translate(scrollX, scrollY);
            //此处会执行各种Drawable对应的draw方法
            background.draw(canvas);
            //把画布的原点移回去,drawable在屏幕上的位置不动
            canvas.translate(-scrollX, -scrollY);
        }
    }

3.绘制View的内容

先跳过第2步,是因为不是所有的View都需绘制褪色边缘。
View#onDraw:

  protected void onDraw(Canvas canvas) {
    }

没有具体的实现,不同的View需要不同的绘制,自定义View需要自己去实现。
看看ImageView的ImageVIew#onDraw

@Override
protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //mDrawable空退出绘制
        if (mDrawable == null) {
            return; 
        }
        if (mDrawableWidth == 0 || mDrawableHeight == 0) {
            return;    
        }
       //没有Pad和Matrix时候直接绘制
        if (mDrawMatrix == null && mPaddingTop == 0 && mPaddingLeft == 0) {
            mDrawable.draw(canvas);
        } else {
       // 返回栈中 matrix/clip 状态的数量,值等于 save()调用次数减去 restore()调用次数 
            int saveCount = canvas.getSaveCount();
            canvas.save();
           // android:cropToPadding="true"
            if (mCropToPadding) {
                final int scrollX = mScrollX;
                final int scrollY = mScrollY;
                canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,
                        scrollX + mRight - mLeft - mPaddingRight,
                        scrollY + mBottom - mTop - mPaddingBottom);
            }
            //设置pad偏移
            canvas.translate(mPaddingLeft, mPaddingTop);
            //设置矩阵
            if (mDrawMatrix != null) {
                canvas.concat(mDrawMatrix);
            }
            mDrawable.draw(canvas);
           //回到上一个save()方法调用之前的状态 
            canvas.restoreToCount(saveCount);
        }
}

4.绘制子View

View没有子View,所以在View中该方法是空的。ViewGroup中可以放入子View,所以ViewGroup的绘制需要绘制子View,直接看看ViewGroup#dispatchDraw:

protected void dispatchDraw(Canvas canvas) {
       boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
       final int childrenCount = mChildrenCount;
       final View[] children = mChildren;
       int flags = mGroupFlags;

//ViewGroup是否有设置子View入场动画,如果有绑定到View
// 启动动画控制器
      ...

//指定修改区域
       int clipSaveCount = 0;
       final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
       if (clipToPadding) {
           clipSaveCount = canvas.save();
           canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
                   mScrollX + mRight - mLeft - mPaddingRight,
                   mScrollY + mBottom - mTop - mPaddingBottom);
       }

     ...

       for (int i = 0; i < childrenCount; i++) {
//先取mTransientViews中的View,mTransientViews中的View通过addTransientView添加,它们只是容器渲染的一个item
           while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
               final View transientChild = mTransientViews.get(transientIndex);
               if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                       transientChild.getAnimation() != null) {
                   more |= drawChild(canvas, transientChild, drawingTime);
               }
               transientIndex++;
               if (transientIndex >= transientCount) {
                   transientIndex = -1;
               }
           }
           int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
           final View child = (preorderedList == null)
                   ? children[childIndex] : preorderedList.get(childIndex);
           if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
               more |= drawChild(canvas, child, drawingTime);
           }
       }
    ...
   }

ViewGroup#dispatchDraw的流程是先启动第一次加到布局中的动画,然后确定绘制区域,遍历绘制View,遍历View的时候优先绘制渲染的mTransientViews,绘制View调用到ViewGroup#drawChild:

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
       //View中有两个draw方法
      //这个多参数的draw用于view绘制自身内容
        return child.draw(canvas, this, drawingTime);
    }

View#draw(canvas, this, drawingTime)

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
 
       boolean drawingWithRenderNode = mAttachInfo != null
                && mAttachInfo.mHardwareAccelerated
                && hardwareAcceleratedCanvas;
      ...

//主要判断是否有绘制缓存,如果有,直接使用缓存,如果没有,调用 draw(canvas)方法
        if (!drawingWithDrawingCache) {
            if (drawingWithRenderNode) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
            } else {
                // Fast path for layouts with no backgrounds
                if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                    mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                    dispatchDraw(canvas);
                } else {
                    draw(canvas);
                }
            }
        } else if (cache != null) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            if (layerType == LAYER_TYPE_NONE) {
                // no layer paint, use temporary paint to draw bitmap
                Paint cachePaint = parent.mCachePaint;
                if (cachePaint == null) {
                    cachePaint = new Paint();
                    cachePaint.setDither(false);
                    parent.mCachePaint = cachePaint;
                }
                cachePaint.setAlpha((int) (alpha * 255));
                canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
            } else {
                // use layer paint to draw the bitmap, merging the two alphas, but also restore
                int layerPaintAlpha = mLayerPaint.getAlpha();
                mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
                canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
                mLayerPaint.setAlpha(layerPaintAlpha);
            }
      }
}

6.绘制装饰

    public void onDrawForeground(Canvas canvas) {
//绘制滑动指示
        onDrawScrollIndicators(canvas);
//绘制ScrollBar
        onDrawScrollBars(canvas);
//获取前景色的Drawable,绘制到canvas上
        final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
        if (foreground != null) {
            if (mForegroundInfo.mBoundsChanged) {
                mForegroundInfo.mBoundsChanged = false;
                final Rect selfBounds = mForegroundInfo.mSelfBounds;
                final Rect overlayBounds = mForegroundInfo.mOverlayBounds;
                if (mForegroundInfo.mInsidePadding) {
                    selfBounds.set(0, 0, getWidth(), getHeight());
                } else {
                    selfBounds.set(getPaddingLeft(), getPaddingTop(),
                            getWidth() - getPaddingRight(), getHeight() - getPaddingBottom());
                }
                final int ld = getLayoutDirection();
                Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(),
                        foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld);
                foreground.setBounds(overlayBounds);
            }
            foreground.draw(canvas);
        }
    }

2和5.绘制View的褪色边缘

当horizontalEdges或者verticalEdges有一个true的时候,表示需要绘制View的褪色边缘:

     boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
     boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;

这时候先计算出是否需要绘制上下左右的褪色边缘和它的参数,然后保存视图层:

        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;
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }
        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);
        }

绘制褪色边缘,恢复视图层

        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);

参考:《Android 进阶之光》

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