View's onDraw method get called again and again on Android 2.2

I am new on Android development and meet with a tough problem when trying to implement a graphic drawing on a View.

What I want to do is drawing a chart in a View. I put a View in a ScrollView and override the onDraw method of View and do drawing in onDraw. The most important thing is that the chart needs scroll functions. The View's onDraw method I override includes the coordinate calculation and many necessary elements drawing (line, dot, axis, label, etc) in a chart. As you can imagine, I calculate and draw the whole chart not only the visible area but the whole area including offscreen. Now the problem is that onDraw method get called again and again when I scroll the ScrollView, so that it causes a performance issue and the scroll view running very slow. I was tryi开发者_如何学Cng to find a way to prevent the calling of onDraw after first call but no lucky. I have to calculate the same thing again and again once onDraw get called which is not necessary.

Do you guys have any answer? Thanks.

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You can use mScrollX and mScrollY to calculate which visible part of canvas need to be re-draw.

 /**
 * The offset, in pixels, by which the content of this view is scrolled
 * vertically.
 * {@hide}
 */
@ViewDebug.ExportedProperty
protected int mScrollY;

/**
 * The left padding in pixels, that is the distance in pixels between the
 * left edge of this view and the left edge of its content.
 * {@hide}
 */
@ViewDebug.ExportedProperty
protected int mPaddingLeft;
/**
 * The right padding in pixels, that is the distance in pixels between the
 * right edge of this view and the right edge of its content.
 * {@hide}
 */
@ViewDebug.ExportedProperty
protected int mPaddingRight;
/**
 * The top padding in pixels, that is the distance in pixels between the
 * top edge of this view and the top edge of its content.
 * {@hide}
 */
@ViewDebug.ExportedProperty
protected int mPaddingTop;
/**
 * The bottom padding in pixels, that is the distance in pixels between the
 * bottom edge of this view and the bottom edge of its content.
 * {@hide}
 */
@ViewDebug.ExportedProperty
protected int mPaddingBottom;