理解android视图
对于android设备我们所示区域事实上和它在底层的绘制有着非常大的关系，非常多时候我们都仅仅关心我们所示，那么在底层一点它究竟是怎么样的一个东西呢？让我们先来看看这个图。
对于整个设备的可见区域而言事实上就是我们中间的那个屏幕，从上面的拿个图能够清晰的看到，除了我们的可见区域在它的上下左右都应该有内容。那么在android系统中是怎么控制显示它的位置呢？以下我们来解答这个问题。
android怎样控制视图的显示位置
我们能够打开view类的源代码找到这两个函数
/**
* Set the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the x position to scroll to
* @param y the y position to scroll to
*/
public void scrollTo(int x, int y) {
if (mScrollX != x || mScrollY != y) {
int oldX = mScrollX;
int oldY = mScrollY;
mScrollX = x;
mScrollY = y;
invalidateParentCaches();
onScrollChanged(mScrollX, mScrollY, oldX, oldY);
if (!awakenScrollBars()) {
postInvalidateOnAnimation();
}
}
}另一个是 /**
* Move the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the amount of pixels to scroll by horizontally
* @param y the amount of pixels to scroll by vertically
*/
public void scrollBy(int x, int y) {
scrollTo(mScrollX + x, mScrollY + y);
}我们细致的来解读一下上面的函数。这个源代码我是摘自5.0的源代码。我们看到ScrollBy这个函数也是调用的ScrollTo我们就来分析一下ScrollTo这个函数究竟做了什么工作？非常easy的几句代码，最重要的一句就是这一句 postInvalidateOnAnimation();这一句代码会去回调我们的ondraw函数，在ondraw函数里面绘制我们的可见区域，然后我们在来看看VIew的draw的方法
* 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) {
if (mClipBounds != null) {
canvas.clipRect(mClipBounds);
}
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) {
final Drawable background = mBackground;
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);
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);
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);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
}能够看到取出我们的mScrollX和mScrollY然后在画图的各种translate画图这样我们就看到了视图里面显示的内容了。大体上的思路就是这样。
显示区域实例
如今结合一个样例来理解一下这个问题。我们直接上上一段非常easy的代码。
LinearLayout linearLayout = new LinearLayout(this);
linearLayout.setLayoutParams(new LayoutParams(2000, 2000));//这里我把宽高设置大点好做測试由于我的手机是1920*1080所以设置得大一点
linearLayout.setOrientation(LinearLayout.HORIZONTAL);
TextView textView1 = new TextView(this);
textView1.setText("hello i am text 1");
textView1.setLayoutParams(new LayoutParams(1000, 2000));
linearLayout.addView(textView1);
TextView textView2 = new TextView(this);
textView2.setText("hello i am text 2");
textView2.setLayoutParams(new LayoutParams(1000, 2000));
linearLayout.addView(textView2);
setContentView(linearLayout);就是在Activity的Oncreate函数里面初始化我们的界面。
我们给它一个非常大的视图，大得都超出了手机屏幕，然后我们跑一下看会有如何的效果。
能够非常清晰的看到我们的TextView2没有显示出来，这里也就非常明了了，由于超出了屏幕，然后我们又一次加一句代码。
LinearLayout linearLayout = new LinearLayout(this);
linearLayout.setLayoutParams(new LayoutParams(2000, 2000));//这里我把宽高设置大点好做測试由于我的手机是1920*1080所以设置得大一点
linearLayout.setOrientation(LinearLayout.HORIZONTAL);
TextView textView1 = new TextView(this);
textView1.setText("hello i am text 1");
textView1.setLayoutParams(new LayoutParams(1000, 2000));
linearLayout.addView(textView1);
TextView textView2 = new TextView(this);
textView2.setText("hello i am text 2");
textView2.setLayoutParams(new LayoutParams(1000, 2000));
linearLayout.addView(textView2);
linearLayout.scrollTo(1000, 0);//我们加了这一句代码
setContentView(linearLayout);
能够看到text2显示全然了。这也非常明显由于我们把视图向左滚动了。在后面讲的UI控件的系列教程这两个scrollTo和scrollBy函数用得非常频繁。所以这里从源头把它给分析了一次，帮助我们后面好理解。