有关View的frame、Bounds、contentMode、ClipstoBounds的问题

　　实在应该深入学习分析一下。

　　现在只知道contentMode会影响frame、Bounds和绘制的关系，但是原理、机制如何还不清楚。

详情在：View Programming Guide For IOS，地址为：https://developer.apple.com/library/ios/#documentation/WindowsViews/Conceptual/ViewPG_iPhoneOS/WindowsandViews/WindowsandViews.html#//apple_ref/doc/uid/TP40009503-CH2-SW9

The View Drawing Cycle

The UIView class uses an on-demand drawing model for presenting content. When a view first appears on the screen, the system asks it to draw its content. The system captures a snapshot of this content and uses that snapshot as the view’s visual representation. If you never change the view’s content, the view’s drawing code may never be called again. The snapshot image is reused for most operations involving the view. If you do change the content, you notify the system that the view has changed. The view then repeats the process of drawing the view and capturing a snapshot of the new results.

When the contents of your view change, you do not redraw those changes directly. Instead, you invalidate the view using either the setNeedsDisplay or setNeedsDisplayInRect: method. These methods tell the system that the contents of the view changed and need to be redrawn at the next opportunity. The system waits until the end of the current run loop before initiating any drawing operations. This delay gives you a chance to invalidate multiple views, add or remove views from your hierarchy, hide views, resize views, and reposition views all at once. All of the changes you make are then reflected at the same time.

Note: Changing a view’s geometry does not automatically cause the system to redraw the view’s content. The view’s contentMode property determines how changes to the view’s geometry are interpreted. Most content modes stretch or reposition the existing snapshot within the view’s boundaries and do not create a new one. For more information about how content modes affect the drawing cycle of your view, see “Content Modes.”

Content Modes

Each view has a content mode that controls how the view recycles its content in response to changes in the view’s geometry and whether it recycles its content at all. When a view is first displayed, it renders its content as usual and the results are captured in an underlying bitmap. After that, changes to the view’s geometry do not always cause the bitmap to be recreated. Instead, the value in the contentMode property determines whether the bitmap should be scaled to fit the new bounds or simply pinned to one corner or edge of the view.

The content mode of a view is applied whenever you do the following:

• Change the width or height of the view’s frame or bounds rectangles.

• Assign a transform that includes a scaling factor to the view’s transform property.

By default, the contentMode property for most views is set to UIViewContentModeScaleToFill, which causes the view’s contents to be scaled to fit the new frame size. Figure 1-2 shows the results that occur for some content modes that are available. As you can see from the figure, not all content modes result in the view’s bounds being filled entirely, and those that do might distort the view’s content.

Figure 1-2  Content mode comparisons

Content modes are good for recycling the contents of your view, but you can also set the content mode to the UIViewContentModeRedraw value when you specifically want your custom views to redraw themselves during scaling and resizing operations. Setting your view’s content mode to this value forces the system to call your view’s drawRect: method in response to geometry changes. In general, you should avoid using this value whenever possible, and you should certainly not use it with the standard system views.

For more information about the available content modes, see UIView Class Reference.

了解Stretchable Views特性，通过指定contentStretch 属性实现。

Built-In Animation Support

Among the properties you can animate on a UIView object are the following:

• frame—Use this to animate position and size changes for the view.

• bounds—Use this to animate changes to the size of the view.

• center—Use this to animate the position of the view.

• transform—Use this to rotate or scale the view.

• alpha—Use this to change the transparency of the view.

• backgroundColor—Use this to change the background color of the view.

• contentStretch—Use this to change how the view’s contents stretch.

• 更多内容去看Animation文档。。。有好多好多东西要看啊。

• 
  
• The Relationship of the Frame, Bounds, and Center Properties

• A view object tracks its size and location using its frame, bounds, and center properties:

  • The frame property contains the frame rectangle, which specifies the size and location of the view in its superview’s coordinate system.

  • The bounds property contains the bounds rectangle, which specifies the size of the view (and its content origin) in the view’s own local coordinate system.

  • The center property contains the known center point of the view in the superview’s coordinate system.

  You use the center and frame properties primarily for manipulating the geometry of the current view. For example, you use these properties when building your view hierarchy or changing the position or size of a view at runtime. If you are changing only the position of the view (and not its size), the center property is the preferred way to do so. The value in the center property is always valid, even if scaling or rotation factors have been added to the view’s transform. The same is not true for the value in the frame property, which is considered invalid if the view’s transform is not equal to the identity transform.

  You use the bounds property primarily during drawing. The bounds rectangle is expressed in the view’s own local coordinate system. The default origin of this rectangle is (0, 0) and its size matches the size of the frame rectangle. Anything you draw inside this rectangle is part of the view’s visible content. If you change the origin of the bounds rectangle, anything you draw inside the new rectangle becomes part of the view’s visible content.

  Figure 1-5 shows the relationship between the frame and bounds rectangles for an image view. In the figure, the upper-left corner of the image view is located at the point (40, 40) in its superview’s coordinate system and the size of the rectangle is 240 by 380 points. For the bounds rectangle, the origin point is (0, 0) and the size of the rectangle is similarly 240 by 380 points.

  Figure 1-5  Relationship between a view's frame and bounds

  Although you can change the frame, bounds, and center properties independent of the others, changes to one property affect the others in the following ways:

  • When you set the frame property, the size value in the bounds property changes to match the new size of the frame rectangle. The value in the center property similarly changes to match the new center point of the frame rectangle.

  • When you set the center property, the origin value in the frame changes accordingly.

  • When you set the size of the bounds property, the size value in the frame property changes to match the new size of the bounds rectangle.

  By default, a view’s frame is not clipped to its superview’s frame. Thus, any subviews that lie outside of their superview’s frame are rendered in their entirety. You can change this behavior, though, by setting the superview’s clipsToBounds property to YES. Regardless of whether or not subviews are clipped visually, touch events always respect the bounds rectangle of the target view’s superview. In other words, touch events occurring in a part of a view that lies outside of its superview’s bounds rectangle are not delivered to that view.

  
  

  Coordinate System Transformations

  • To modify your entire view, modify the affine transform in the transform property of your view.

  • To modify specific pieces of content in your view’s drawRect: method, modify the affine transform associated with the active graphics context.

  You typically modify the transform property of a view when you want to implement animations. For example, you could use this property to create an animation of your view rotating around its center point. You would not use this property to make permanent changes to your view, such as modifying its position or size a view within its superview’s coordinate space. For that type of change, you should modify the frame rectangle of your view instead.