Setneedsdisplay and Setneedslayout methods

Setneedsdisplay and Setneedslayout methods of 1,uiview

The first two methods are executed asynchronously. And Setneedsdisplay will call the automatic call DrawRect method, so you can get uigraphicsgetcurrentcontext, you can draw. And Setneedslayout will call layoutsubviews by default,

You can work with some of the data in the child view.

In full, setneedsdisplay convenient drawing, and layoutsubviews convenient data.

The layoutsubviews is called in the following cases:

1. Init initialization does not trigger layoutsubviews.

2, Addsubview will trigger layoutsubviews.

3, set the frame of the view will trigger Layoutsubviews, of course, if the frame value has changed before and after the setting.

4, rolling a uiscrollview will trigger layoutsubviews.

5. Rotating screen will trigger the Layoutsubviews event on the parent UIView.

6, changing the size of a uiview will also trigger the parent UIView on the Layoutsubviews event.

7, call Setlayoutsubviews directly.

The drawrect is called in the following cases:

1. If the rect size is not set at UIView initialization, the drawrect will not be automatically called. The DrawRect call is dropped after the Controller->loadview, Controller->viewdidload two method. So don't worry. In the controller, The drawrect of the view began to draw. This allows you to set some values in the controller to the view (if some variable values are needed for these view draw).

2, this method is called after calling SizeToFit, so you can call SizeToFit to calculate the size first. The system then automatically calls the DrawRect: method.

3, by setting the Contentmode property value to Uiviewcontentmoderedraw. The drawrect will be called automatically each time the frame is set or changed:.

4. Call Setneedsdisplay directly, or setneedsdisplayinrect: Trigger DrawRect:, but there is a precondition that rect cannot be 0.

Above 1, 2 recommended; 3,4 does not advocate

DrawRect methods use note points:

1, if using UIView drawing, only in the DrawRect: method to obtain the corresponding Contextref and drawing. If obtained in other methods, it gets to a invalidate ref and cannot be used for paint. DrawRect: The method cannot manually display the call, and the system must be automatically tuned by calling Setneedsdisplay or Setneedsdisplayinrect.

2, if using Calayer drawing, can only be drawn in Drawincontext: (similar to DrawRect), or in the corresponding method of delegate. Also call Setneeddisplay and so on indirectly call the above method

3, to real-time paint, can not use Gesturerecognizer, can only use Touchbegan and other methods to drop the Setneedsdisplay real-time refresh screen

Setneedsdisplay and Setneedslayout methods