Android自定义View基础之onDraw详解

短命女 2022-07-14 09:38 445阅读 0赞

> 前文先后介绍了onMeasure和onLayout的流程，那么接下来就一起看一下onDraw方法吧。

Draw过程比较简单，主要作用是将View绘制到屏幕上面，先看一下draw方法的源码：

/** * Manually render this view (and all of its children) to the given Canvas. * The view must have already done a full layout before this function is * called. When implementing a view, implement * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method. * If you do need to override this method, call the superclass version. * * @param canvas The Canvas to which the View is rendered. */
        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);
    
                // Step 6, draw decorations (scrollbars)
                onDrawScrollBars(canvas);
    
                if (mOverlay != null && !mOverlay.isEmpty()) {
                    mOverlay.getOverlayView().dispatchDraw(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);
    
            // Step 6, draw decorations (scrollbars)
            onDrawScrollBars(canvas);
    
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }
        }

由源码可以看出，View的绘制过程遵循如下几步：  
1. 绘制背景background.draw(canvas)  
2. 绘制自己(onDraw)  
3. 绘制children(dispathDraw)  
4. 绘制装饰(onDrawScrollBars)

View绘制过程的传递是通过dispatchDraw来实现的，dispatchDraw会便利调用所有子元素的draw方法，如此draw事件就被一层层的传递下去了。

View有一个特殊的方法setWillNotDraw，先看一下源码：

/**
         * 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);
        }

从注释中可以看出，如果一个View不需要绘制任何内容，那么设置这个标记位为true后，系统会进行相应的优化。默认情况下，View没有启用这个优化标记位，但是ViewGroup会默认启用这个优化标记位。

这个标记位对实际开发的意义：当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。

当我们明确知道一个ViewGroup需要通过onDraw来绘制内容时，我们需要显示的关闭WILL\_NOT\_DRAW这个标记位。