Android自定义view绘制顺序及相关原理

[size=medium]Android系统中要自定义view，首先需要了解Android的view加载机制。主要有三个方法：

1、onMeasure() //计算出view自身大小
2、onLayout() //仅在ViewGroup中，用来为子view指定位置（left,top）
3、onDraw() //view绘制内容

那么系统能让我们在onDraw()能够绘制些什么呢，查看View.draw()源码发现
/*         * 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) {            final Drawable background = mBGDrawable;            if (background != null) {                final int scrollX = mScrollX;                final int scrollY = mScrollY;                if (mBackgroundSizeChanged) {                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);                    mBackgroundSizeChanged = false;                }                if ((scrollX | scrollY) == 0) {                    background.draw(canvas);                } else {                    canvas.translate(scrollX, scrollY);                    background.draw(canvas);                    canvas.translate(-scrollX, -scrollY);                }            }        }        // 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);            // 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);            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);            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);            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);            canvas.drawRect(right - length, top, right, bottom, p);        }        canvas.restoreToCount(saveCount);        // Step 6, draw decorations (scrollbars)        onDrawScrollBars(canvas);
从代码中的注释和step可以看出view调用draw方法的流程。
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