Android学习笔记(二)–Android View绘制
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Android学习笔记(二)–Android View绘制
1、LayoutInflater
2、onMeasure()
measure是测量的意思,那么onMeasure()方法顾名思义就是用于测量视图的大小的。
View系统的绘制流程会从ViewRoot的performTraversals()方法中开始,在其内部调用View的measure()方法。measure()方法接收两个参数,widthMeasureSpec和heightMeasureSpec,这两个值分别用于确定视图的宽度和高度的规格和大小。
MeasureSpec的值由specSize和specMode共同组成的,其中specSize记录的是大小,specMode记录的是规格。specMode一共有三种类型,如下所示:
EXACTLY
表示父视图希望子视图的大小应该是由specSize的值来决定的,系统默认会按照这个规则来设置子视图的大小,开发人员当然也可以按照自己的意愿设置成任意的大小。AT_MOST
表示子视图最多只能是specSize中指定的大小,开发人员应该尽可能小得去设置这个视图,并且保证不会超过specSize。系统默认会按照这个规则来设置子视图的大小,开发人员当然也可以按照自己的意愿设置成任意的大小。- UNSPECIFIED
表示开发人员可以将视图按照自己的意愿设置成任意的大小,没有任何限制。这种情况比较少见,不太会用到。
3、onLayout()
measure过程结束后,视图的大小就已经测量好了,接下来就是layout的过程了。 正如其名字所描述的一样,这个方法是用于给视图进行布局的,也就是确定视图的位置。 ViewRoot的performTraversals()方法会在measure结束后继续执行,并调用View的layout()方法来执行此过程
getWidth、getMeasuredWidth区别
首先getMeasureWidth()方法在measure()过程结束后就可以获取到了,而getWidth()方法要在layout()过程结束后才能获取到。另外,getMeasureWidth()方法中的值是通过setMeasuredDimension()方法来进行设置的,而getWidth()方法中的值则是通过视图右边的坐标减去左边的坐标计算出来的。
4、onDraw()
- Draw the background
- If necessary, save the canvas’ layers to prepare for fading
- Draw view’s content
- Draw children
- If necessary, draw the fading edges and restore layers
- Draw decorations (scrollbars for instance)
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; // 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);
参考
Android视图绘制流程完全解析,带你一步步深入了解View
- Android学习笔记(二)–Android View绘制
- Android学习笔记(一) View、ViewGroup的测量绘制
- Android view的绘制流程(二)
- Android View绘制流程(二)
- android 自定义view之绘制(二)
- Android View绘制机制基础(二)
- Android学习笔记(二)自定义View(1)
- Android学习笔记(二)之basic view
- Android View学习笔记(二):View滑动方式总结
- Android学习 - Android中View绘制流程
- Android - View的绘制流程二(layout)
- Android View详解(二) 视图绘制流程
- Android实现机制(二)——View绘制机制
- Android自定义控件(二)View的绘制
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- View的绘制流程源码分析(Android开发艺术探索学习笔记)
- Android学习自定义View(二)——View和ViewGroup绘制流程以及invalidate()
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