Android View Focus Summary

Android View focus

Most of the focus-related logic in Android is in the View, ViewGroup and FocusFinder classes.

ViewRoot

View objects have an mParent variable (after being added to ViewGroup), which refers to its parent container. Most Views' mParents are of ViewGroup type, except for the root node. The mParent of a View root node DecorView in a Window is called ViewRoot. After Android 4.0, ViewRoot corresponds to ViewRootImpl, which is not a subclass of View, but a ViewParent. ViewRootImpl is the link between Window and DecorView. The focus, buttons, layout, rendering and other processes of View all start from ViewRoot.

View's focus

The basic process is as follows

View (including ViewGroup) obtains focus through the following three methods

View.java

From the above, you can see that the first two will eventually execute the third method.

*** requestFocusNoSearch first determines whether it can get the focus, and then enters the following *** process:

View.java

The above process is relatively simple: if there is no focus currently, first set the focus flag, then notify the parent, and then refresh the image.

The main process is in requestChildFocus of mParent, which will be analyzed later. There, the focus View will be modified layer by layer and the focus of the original focused View will be cleared.

onFocusChange will trigger invalidate refresh, and then call onFocusChangeListener. By default, each View can only set one onFocusChangeListener, and in development, we often encounter situations where we need to set multiple Listeners. We can rewrite the onFocusChange method to implement the need to call back multiple onFocusChangeListeners.

ViewGroup Focus

ViewGroup getting focus means adding internal focus processing to the View getting focus process

ViewGroup.java

In the above code, descendantFocusability determines whether to process the focus according to the View focus process first (handle the focus yourself) or to give it to the child View first.

FOCUS_BLOCK_DESCENDANTS does not allow child Views to get focus, so follow the View process

FOCUS_BEFORE_DESCENDANTS First follow the View process, if it cannot get the focus, let the child handle it

FOCUS_AFTER_DESCENDANTS first tries to give the child focus. If there is no focus available, it will follow the View process to get the focus itself.

The default value is FOCUS_BEFORE_DESCENDANTS, we can use setDescendantFocusability(int d)

set up

The onRequestFocusInDescendants method is overridden for subclasses to control the focus of subviews. By default, the focus is processed in the order of subviews. If the direction is downward or right, it starts from the first one, and if it is upward or left, it starts from the last one, until a subview gets the focus.

Note that this method is only executed when requestFocus is called on this ViewGroup and its upper View.

Handling parent container focus

In the process of View getting focus, mParent.requestChildFocus will be called to maintain the unique focus on the View tree and save the child View with focus in each layer of ViewGroup

ViewGroup.java

First clear your own focus. If there is focus inside, clear its focus first, save the child with focus, and then call requestChildFocus of the previous layer. As can be seen from the previous call, this method will be called all the way to the root node of the View tree.

In the current ViewGroup, any child that gains focus will execute this method, so this method is also one of the ways for ViewGroup to know the focus change of the child. (Unfortunately, it cannot know that the child loses focus)

Lose focus or clear focus

Getting focus can be active, but losing focus is usually passive (see the code above), so the logic is relatively simple, just clear the focus state.

ViewGroup.java

View.java

Note that the above method has the default package access level and cannot be overridden or called.

You can also clear the focus actively, similar to the process of getting the focus

ViewGroup.java

View.java

ViewGroup.java

The above is the entire process and methods involved in the focus processing of the Android View system. We do not need to pay attention to the requestChildFocus and clearChildFocus implementations of ViewRootImpl.

There are also some auxiliary methods:

boolean isFocusable() Whether the View can get focus

boolean isFocused() Whether the View has the focus

boolean hasFocus() Whether the View/ViewGroup has focus

View findFocus() Get the focus View inside the View/ViewGroup

View getFocusedChild() Get the child View with focus inside the ViewGroup

View getRootView() Get the root node View (usually DecorView or top-level ViewGroup)

Focus Movement

In addition to controlling the focus in the code, the system automatically handles some keys such as the arrow keys that are not processed according to the focus movement, see the code below

ViewRootImpl.java

The code is relatively simple, but it mainly does three steps

If the View does not process the key, convert the up, down, left, right, tab and other keys into the corresponding directions

Use the focusSearch method to find the next View in the corresponding direction on the current focus View.

The found View calls requestFocus, so the main process is in focusSearch

View.java

Normal View search does nothing and leaves it to the parent to complete.

ViewGroup.java

ViewRootImpl

We can rewrite focusSearch to control the focus movement order, while the default focus movement order is determined by FocusFinder

FocusFinder Finds focus

FocusFinder is a public utility class with two main methods. It can find the next focus of a specified View or coordinate in a specified direction within a given View as follows:

The core logic is two steps. First, find the View set by setNextFocusXXId. If it is not found, search it according to the nearest algorithm. The specific algorithm will not be analyzed here. The source code is in the SDK.

Summarize

Based on the above process analysis, we have the following ideas for the special needs of focus when implementing custom View

requestFocus and clearFocus directly clear or transfer focus to View

In addition to onFocusChangeListener, you can also implement some View notifications when they lose/gain focus in the onFocusChange method

For ViewGroup, if you only need to be notified when a child View gets focus, there is a requestChildFocus method.

Overriding the onRequestFocusInDescendants method can control the ViewGroup focus in certain scenarios

To control the focus movement, you can override the focusSearch method

There is also the FocusFinder tool and the auxiliary methods above.