How does Java Graphics.drawImage() work and what is the role of ImageObserver

Question

How should Java's drawImage() be used? I do not find the JDK documentation very forthcoming. For example all drawImage signatures require an ImageObserver but the documentation for this is not very helpful for new users.

Answer 1

You can get away with Graphics.drawImage(img, x, y, null) [or similar]. The ImageObserver parameter is a callback to inform you of the progress of the draw operation; and is really only useful if you're fetching the Image parameter asynchronously.

To be clearer, if you call drawImage with an incompletely loaded Image it will:

1. return false (immediately)
2. draw as much of the Image as possible (all that is loaded)
3. and, at some future point, call into the ImageObserver when more of the Image is available

Basically, if you're working with in memory Images (either loaded from the file system, or constructed by your program) don't worry about the ImageObserver parameter. If you're loading Images across the network and not explicitly waiting for them to load, you'll need to employ an ImageObserver to make sure "completely" draw an Image.