@CallSuper public void draw(Canvas canvas) { final int privateFlags = mPrivateFlags; //获取dirty区域是否不透明 final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE && (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState); //将flag22位21位设置为off,并且将PFLAG_DRAWN设置为on mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN; //下面的注释是google对view的draw方法的6步分解注释 /* * 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) */
// 第一步,绘制背景,如果需要的话。 int saveCount; // 如果dirty区域是不透明的,则跳过绘制背景。 if (!dirtyOpaque) { drawBackground(canvas); }
// 大多数情况(不需要绘制边界阴影的情况)都不需要2和5这两部,跳过 final int viewFlags = mViewFlags; // 是否需要绘制横向边界阴影 boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; // 是否需要绘制竖向向边界阴影 boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0; // 不需要绘制横向和竖向阴影,执行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) */ // 需要绘制阴影的话,则执行全部2-6的流程,不过这个流程并不常见,而且性能和速度上也不是很优秀。 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;
// 第二步,保存fading相关的canvas图层信息 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); }
// 第三步,如果不是透明的,则调用onDraw方法进行绘制 if (!dirtyOpaque) onDraw(canvas);
// 第四步,调用dispatchDraw方法,绘制子View dispatchDraw(canvas);
// 第五步,绘制阴影边缘 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);
// 绘制覆盖物,这个覆盖物在前景图的下面(API18引入) if (mOverlay != null && !mOverlay.isEmpty()) { mOverlay.getOverlayView().dispatchDraw(canvas); } // 第六步,绘制前景图 onDrawForeground(canvas);}
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